The Exhaust System Affects Engine Performance
3-04 Describe how the exhaust system affects engine performance.
An often overlooked cause of engine performance concerns is exhaust backpressure. Despite advancements in computerized engine controls and testing equipment, excessive exhaust backpressure does not have an onboard diagnostics (OBD) code associated with it. Excessive backpressure affects the engine’s ability to produce power, often leading the technician on a variety of tests that include everything but backpressure. Because of how necessary backpressure testing is, these tests need to become part of a technician’s routine diagnostic arsenal.
Backpressure is a result of anything that restricts the flow of exhaust from the engine to the muffler outlet (FIGURE 3-9). Restrictions or obstructions can stem from a mild clog in a catalytic converter or muffler to even a collapsed inner section of double-walled exhaust tubing that is not visible. Diagnosing exhaust restriction is not always as simple as it sounds. The converter substrate can suffer multiple failures that lead to a hard-to-diagnose condition. The substrate can break apart and intermittently block the outlet of the converter. Another failure that is even harder to diagnose happens when contamination coats the honeycomb monolith substrate. The obstructed substrate causes varying degrees of restriction, from no restriction to being fully blocked. A partial restriction may flow enough exhaust gas at idle and under low-demand driving but fail to breathe properly under high-load or high-rpm conditions. In addition, depending on the level of the restriction, it can mimic late valve timing, leading to a misdiagnosis. Verifying an exhaust restriction, therefore, is essential, particularly for a marginal or intermittent concern, to prevent misdiagnosis or incorrectly replacing an expensive catalytic converter.
FIGURE 3-9 Determining where the restriction is will help in repairing the exhaust system. To diagnose this, the technician may need to remove sections of the exhaust to inspect the components’ internals or to eliminate that component as a potential source of the restriction.
Restricted Exhaust System Diagnosis
This section will present options for diagnosing exhaust system faults. There are several ways to diagnose a restricted exhaust system. It starts from a simple visual inspection to a tap test, a vacuum test, and finally a backpressure test. As with most repairs on the vehicle, a simple visual inspection can show the technician the location of the failure, but some vehicles will require a more investigative approach.
Visual Inspection
Before performing more intrusive backpressure testing, always perform a visual inspection first. Spend a few minutes before testing to look for obvious concerns, like impact damage to a catalytic converter, muffler, exhaust, or tail pipes (FIGURE 3-10). A collapsed exhaust or tailpipe is sometimes visually noticeable. Performing a visual inspection can prevent unnecessary testing and save time. Mud or snow can pack into the tailpipe, restricting exhaust, depending on vehicle use. Water is a byproduct of “good” combustion. The water can freeze and turn into ice, developing into a restriction in cold climates.
FIGURE 3-10 A lot of the time the most obvious failures are the cause of the exhaust restriction. Keep it simple.
TECHNICIAN TIPA federally mandated eight-year, 80,000-mile (129,000 km) warranty covers the catalytic converter in the event of a failure, which must be replaced free of charge to the customer. Physical damage voids the warranty. Never replace a catalytic converter without finding the cause of the original failure, to prevent “killing” another catalytic converter. Note: Aftermarket converters do not have to meet the same specifications as original equipment. A difference in catalyst efficiency may exist between original equipment manufacturer (OEM) and aftermarket parts.
Converter Tap Test and Hiss Test
Perform a tap test of the muffler and converter during the visual inspection. When lightly tapping the converter or muffler, it should sound hollow. Use a rubber mallet, not a hammer; even lightly tapping with a hammer may cause damage that was not present. A rattle indicates a loose substrate in the converter or substrate that has made its way into the muffler from a failed converter. A dull thud, if heard, indicates a restriction (FIGURE 3-11).
FIGURE 3-11 Hearing the substrate rattle inside the catalytic converter (CAT) indicates that the CAT failed and needs to be replaced.
Top technicians use their senses as another tool when they perform any diagnostic. With the engine running, a restricted exhaust may produce an audible hiss noise from the tailpipe or from any seam present in the exhaust system. The hiss is pressure attempting to exit past the restriction.
Backpressure Testing—Using a Pressure Gauge
Backpressure can affect drivability without affecting engine vacuum enough to be noticeable when performing an exhaust backpressure vacuum test if a slight restriction exists. Using only engine vacuum as a test to find a restricted exhaust, therefore, makes it possible to overlook the restriction. Several methods are available for monitoring backpressure coming directly from the exhaust. The best and simplest methods selected for testing depend on vehicle condition and configuration.
TECHNICIAN TIPIf a converter melts or breaks apart, pieces will tend to end up downstream, plugging the next exhaust component. Be sure to inspect downstream from the failed converter, including either another converter or muffler.
- Begin by removing the upstream O2 sensor (closest to the exhaust manifold), with the engine off. All oxygen sensors use an 18 mm (0.7") threaded hole; one adapter will work on all vehicles.
- Install the adapter and backpressure tester in place of the O2 sensor and tighten it.
- Start the vehicle and let it reach operating temperature. Some restrictions require time to build as the exhaust heats up and expands.
- If the vehicle doesn’t start, due to a restricted exhaust, perform the test while cranking the engine.
- By raising the rpm to between 2,000 and 2,500 and holding steady for 30 seconds, the pressure should not exceed 2 to 3 psi (13.8 to 20.7 kPa).
- Snap the throttle to wide-open throttle (WOT) and release it quickly; the pressure should stay under 5 to 6 psi (34.5 to 41.4 kPa).
- If backpressure is greater than specification, locate and repair the restriction.
Checking Exhaust Backpressure on Vehicles Equipped with Exhaust Gas Recirculation
Vehicles using an exhaust gas recirculation (EGR) backpressure transducer, or Ford vehicles using a digital pressure feedback EGR sensor (DPFE), provide access to the exhaust (FIGURE 3-12). Disconnect the pressure hose that runs from the transducer/sensor to the exhaust system. Connect the pressure gauge by using high-temperature silicone tubing. The same testing procedures and specifications discussed earlier apply.
FIGURE 3-12 The digital pressure feedback sensor detects flow when the EGR is actuated to determine which is used, which will determine whether the EGR system is working correctly.
Isolating the Cause of the Restriction
Any reading that indicates high backpressure, regardless of the method used to find it, results in pinpointing the restriction as the next step. To isolate the restriction, remove sections of the exhaust system until the restriction has been located. Using the special tool that taps a hole into the exhaust system allows before and after checks of the individual components, preventing the technician from having to “drop” parts of the exhaust system to find the cause. Regardless of the method used, the hole or component removal process, begin at the most likely cause of the restriction—i.e., in front of the catalytic converter—and then move rearward with the testing. Completely removing the converter is normally not required to notice a change in backpressure. Instead, to verify a plugged exhaust, loosen the converter enough to “drop” it a couple of inches. The gap created is normally sufficient for the backpressure to escape.
Using Exhaust Color and Smell during a Diagnosis
During the visual inspection process, the color or odor from the exhaust may provide clues that an internal engine concern exists, leading the technician to select the appropriate testing procedures to isolate the cause.
Exhaust Smoke Color/Odor |
Description |
Possible Causes |
Blue/Gray Smoke |
• Indicator of oil burning in the combustion chamber • Blue/gray smoke only when starting after sitting usually indicates defective valve seals • Burning oil can eventually contaminate and damage the oxygen sensor(s) and the catalytic converter, which will need to be repaired or replaced. |
|
Black Smoke |
• Indicator of a rich (excessive) fuel condition • Incomplete burning of the fuel in the combustion chamber |
|
White/Gray Smoke |
• Indicator coolant is being burned in the cylinder. • Smoke may be accompanied by a sweet smell from the burning coolant. • White smoke is normal during a cold weather from condensation that forms steam in the exhaust. • All engines produce 1 gallon (3.8 L) of water for every 1 gallon (3.8 L) of fuel burned. • Prolonged burning of coolant can damage oxygen sensor(s) and catalytic converter(s), which will then need to be repaired or replaced. |
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*ECT-Electronic Coolant Temperature | ||
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**IAT-Intake Air Temperature | ||
Using Scan Tools to Diagnose an Exhaust System
3-05 Analyze exhaust system problems by using a scan tool.
The technician of tomorrow must be able to use all available tools to diagnose problems with today’s automobiles. Use a scan tool to diagnose exhaust problems in a vehicle to get a direction of where to investigate further without disassembling much of the exhaust system. An efficient diagnosis should be the purpose of every operation a technician performs.
Using the Scan Tool for Exhaust Restriction Diagnosis
Being able to effectively use a scan tool to diagnose exhaust restrictions relies on understanding volumetric efficiency (VE) and on having the knowledge to select the correct proportional-integral-derivatives (PIDs) to test VE. VE is the ability of the engine to breathe correctly and is very useful in the diagnostic process. Most engines that are naturally aspirated will operate between an 85% and 95% fill rates. Forced-induction engines can exceed 100%, depending on boost pressures. These readings are taken at WOT under load and assume that the air inlet system is free of defects and restrictions. The engine efficiency varies due to intake and exhaust manifold designs, variable valve timing use, and the exhaust system’s efficiency at removing exhaust gases. Any increase in the exhaust’s restriction to flow will reduce VE.
Calculated load is the best indicator of an engine’s VE when using PID data on a scan tool. Note: calculated load data can vary significantly based on engine design. Possessing a knowledge base of what is considered “normal” is another reason for testing, collecting, and storing data on known-good vehicles—as opposed to only vehicles with faults.
Two methods can be used to determine airflow into an engine on modern vehicles. The most abundant at this time is the mass air flow (MAF) sensor, which directly measures the airflow into the engine. The other option is speed density, which uses a manifold absolute pressure (MAP) sensor to calculate airflow when combined with other input sensors, typically engine speed and throttle position. Due to their differences in operation, each system provides different diagnostic approaches for determining a plugged or restricted exhaust.
Exhaust Backpressure Testing Using a Pressure Transducer
The common complaint resulting from a restriction in one catalyst of a V-bank engine is a lack of power. Depending on the severity of the restriction, a misfire on one bank may go along with the lack of power. Complicating the diagnosis is the fact that the misfire is not always on the bank with the obstruction. An exhaust restriction upsets the airflow through the engine on both banks. When one bank is restricted, less than 50% of the air that enters the engine passes through the cylinders of the clogged bank. The remaining air that enters the engine flows through the unrestricted bank. The MAF sensor reports the total air mass entering the engine to the PCM for fuel control. The PCM divides the fuel mass delivered to both banks evenly, with each bank receiving 50% of the fuel. The result of the air imbalance is that the bank with the additional air runs considerably lean, whereas the bank with less air operates overrich due to the restriction. Unequal airflow typically results in skewed fuel trim readings from bank to bank. The fuel trims are usually close to normal at idle unless the restriction is severe and skewed at higher rpm or while driving. If the fuel trims continue to move in increasingly opposite directions as load and engine speed increase, suspect an exhaust restriction. The bank with the negative fuel trim number is always the side with the restriction. If the restriction has been present for a long time, the skew split may only be seen in long-term fuel trim (LTFT) while short-term fuel trim (STFT) shows 0 due to the PCM’s correction.
The concern when diagnosing an exhaust restriction is that vacuum testing at idle, or 2,500 rpm, is usually helpful only on a severely restricted exhaust (FIGURE 3-13). Vacuum testing for a partial restriction in only one bank is usually inaccurate and easily missed. Pressure testing in the front oxygen sensor’s holes for comparing bank to bank may increase the chances of finding a partial restriction. A partial restriction often requires increasing the load on the engine by power braking to develop any significant backpressure. When load testing, unless the restriction is severe, rapid pulsations of the needle on the pressure gauge make comparison difficult.
FIGURE 3-13 A waveform capture using an in-cylinder pressure test to find an exhaust restriction. The accuracy of a transducer aids in finding even a partially restricted exhaust. The restriction in this waveform is creating a backpressure of 565 mbar or 8.2 psi. On a V-type engine, perform the test on a cylinder for each bank, and compare the results. A partially plugged exhaust pressure that exceeds 1.5 psi (103.4 mbar) is enough to cause drivability problems on some engines. The restriction did not prevent engine vacuum from reaching 700 mbar (20.7 inHg), as shown in the expansion and intake pockets.
Partial exhaust restrictions are difficult to find by using traditional testing methods, including pressure testing by using a gauge. Pressure transducers increase testing accuracy, providing detailed results of the conditions in the cylinder. In-cylinder pressure transducer testing in the hands of a skilled technician will often find partial restrictions that traditional testing often misses. On a V-bank engine, remove one spark plug from each bank one at a time, and then install the transducer. Record and save both waveforms for comparison. Viewing and comparing the waveforms results in a decidedly accurate and precise comparison, helping to locate even a slightly plugged converter. When testing an in-line engine, comparing banks is not an option. As with all scope testing, saving known-good test results, waveforms, and graphs for vehicles tested will aid in future diagnostics.
Testing Mass Airflow Sensors for Exhaust Restrictions
MAF sensors directly read and report the airflow entering the engine. Consequently, anything that affects the engine’s ability to breathe, including a restricted exhaust, decreases the airflow into the engine. The symptoms found on a vehicle equipped with a MAF sensor will differ significantly from those found on a vehicle equipped with a MAP sensor when using PID data to diagnose a restricted exhaust.
A restricted exhaust in a MAF system will not produce a rich air-fuel mixture, as it would in a MAP vehicle, but both will have a lack of power. The reduction in power is relative to the amount of the restriction present. A concern with this diagnosis may exist in that a bad MAF sensor can mimic a plugged or restricted exhaust. Typically, a MAF sensor suffering from contamination will overreport airflow at idle, so the engine will run rich; on the other hand, underreporting airflow at higher speeds and rpm will result in a lean condition and thus a lack of power. To remove the possibility of a contaminated MAF sensor as the cause, unplug it and test-drive it again. On most vehicles, disconnecting the MAF sensor will restore power if the sensor is at fault. If the vehicle still suffers from reduced power, suspect the exhaust.
Since the exhaust restriction reduces airflow into the engine, the MAF sensor will report this information to the PCM, which adjusts the air-fuel ratio by reducing the amount of fuel for the given amount of air. The reduction of fuel and air reduces the power that the engine can develop—a classic symptom of a MAF vehicle with a restricted exhaust.
TECHNICIAN TIPSome manufacturers (e.g., Ford) use the MAF sensor to calculate barometric pressure at high throttle openings under load. A restricted exhaust will result in a barometric pressure reading that is lower than it should be for the given elevation. The given specification is 159 hertz (Hz) at sea level. For every 1,000-foot (305 m) increase in elevation, expect a drop of 3 to 4 hertz. Therefore, a restricted exhaust could infer an altitude that is significantly higher than where the vehicle is operating. Incorrect barometric pressure can affect fuel delivery, ignition timing, valve timing, and transmission shift points.
Testing Manifold Absolute Pressure Sensors for Exhaust Restrictions
The MAP sensor is regaining acceptance, particularly on vehicles using forced induction. While some manufacturers never wavered from their use, others are coming back to it. As technology continues to develop, the MAP sensor found on today’s vehicles responds faster and with more reliable information than those found on “older” systems.
MAP sensors read pressure in the intake. The pressure present in the intake varies from atmospheric (or above when equipped with forced induction) to less than atmospheric (vacuum). With the throttle plate closed, a difference exists between atmospheric pressure on the outside of the throttle plate and the vacuum that exists in the intake manifold, a result of the restriction from the throttle plate. As soon as the throttle opens, the MAP reading in the intake will begin to match atmospheric pressure. The pressure that enters the engine is directly related to the opening of the throttle plate. The more the throttle plate opens, the more atmospheric pressure enters the engine and the lower the vacuum. The PCM responds to the throttle opening (more air) by adding fuel to match the airflow into the engine. With a restricted exhaust, the MAP reading will equal atmospheric pressure as soon as the throttle opens, resulting in a rich command to the injectors. The inability of the exhaust gases to escape causes a backup of exhaust into the cylinders, then into the intake manifold, reducing vacuum while increasing the pressure. To verify, graph the MAP sensor while accelerating and note the sudden increase in the reading, accompanied by reduced power on acceleration. A normally operating MAP sensor should show a gradual increase in pressure that corresponds to the throttle opening. Additionally, a MAP-equipped engine will also generally run rich as the PCM incorrectly determines that the engine is under load, due to a lack of vacuum (increased pressure) in the intake manifold.
Another method of exhaust restriction testing is to monitor the MAP sensor at high constant engine speeds. If a significant loss of vacuum is noted or if a steady decline of vacuum occurs at a constant highway speed and rpm, suspect an exhaust restriction.
TECHNICIAN TIPIf a WOT throttle snaps with the vehicle stationary, it may not produce the stated calculated load percentages. If the vehicle is not under load, the PCM may alter fuel, spark timing advance, and valve timing, reducing performance. The vehicle may need to be driven for an accurate calculation.
Valve Timing
3-06 Describe how camshaft timing affects engine performance.
A customer complaint of a no-start or lack of power could lead to a problem with valve timing. A broken timing chain or belt causes a no-start, accompanied by a faster-than-normal cranking speed due to a lack of compression. Another potential problem with camshaft timing is a belt or chain that slips or jumps. Problems occurring from a valve timing issue vary depending on how many teeth the belt or chain has jumped. Retarded camshaft timing ordinarily results from a belt or chain that jumps time, reducing engine power and efficiency. Advanced camshaft timing is usually the result of a stuck variable valve timing (VVT) actuator/phaser or incorrect assembly during service. A belt or chain that jumping only a tooth or two results in any number of customer complaints, including reduced engine performance, hard starting, a buck or jerk while driving, reduced fuel economy and an increase in emissions, and a possible camshaft position code (or codes). If a belt or chain skips more than a couple of teeth, expect a no-start that is accompanied by an irregular cranking pattern/sound.
Incorrect camshaft timing can also result in engine damage. The amount of damage depends on the type of engine and the operating rpm when the belt or chain breaks or jumps teeth. A noninterference engine offers some protection against damage by providing clearance between the valves and the top of the piston. However, engine damage can still occur normally, due to bent valves or a possible cylinder head rebuild, particularly if the fault occurs while operating at higher rpm.
Zero-tolerance engines increase the potential for catastrophic engine damage due to a failed or jumped timing chain or belt. The cam-to-crank timing relationship operates in a very tight window, leaving little if any margin for error to occur. The opening valves and the crown of the piston occupy the same space at the top of the cylinder at precisely timed, distinct intervals. A difference of less than a second between the events can cause catastrophic internal engine damage. Follow the manufacturer’s recommend timing belt replacement guidelines, generally ranging from 60,000 to 100,000-mile (96,000 to 160,000 km) intervals, to prevent engine damage. Incorrect timing of the camshaft, crankshaft, and balance shaft or oil pump driveshaft can result in engine damage or performance and vibration problems. Verify that the components have been indexed correctly, by referring to the manufacturer’s service information during service.
TECHNICIAN TIPUse caution when rotating the engine. Some engines do not index the gears to the camshaft(s) or crankshaft. The attaching bolt and a diamond washer use friction to hold the gear/sprocket/pulley in the correct position. Using the bolt to turn the engine in the wrong direction can loosen the bolt, allowing the pulley to move independently of the crankshaft or camshaft, altering the timing.
The difficulty associated with removing the timing cover varies widely depending on the engine design. Typically, timing belt access is easier than removing a timing cover on a timing chain–equipped engine. With the timing cover removed, verify the correct alignment of all the drive gears/sprockets to the corresponding indicator. The alignment indicators vary widely by engine: An indicator may be part of a cover, a mark on the cylinder head or block, or two marks that point toward each other; or special tools may need to be installed to hold and align the crankshaft or camshafts. Refer to service information for the correct alignment instructions. Some engines require the use of special tools to position and hold the camshaft or crankshaft or to check or set valve timing. If the engine is out of time due to a stretched timing chain, inspect the tensioner. During inspection for a stretched chain, the tensioner is usually fully extended and there is still looseness in the chain.
TECHNICIAN TIPA visual test can be used to infer timing chain stretch. With the timing cover removed, inspect the tensioner. On a new or slightly worn chain, the tensioner normally extends about one-half inch (1.3 cm). If the tensioner extends over three-quarters of an inch (1.9 cm), the chain is showing significant wear and will require replacement. Most tensioners have an operating range that maxes out between seven-eighths of an inch to 1 inch (2.2 to 2.5 cm). If the tensioner exceeds its limit, the chain will be excessively loose, with the possibility of noise and jumped camshaft timing. Before condemning the chain for stretch, verify that a broken or missing chain guide is not responsible for the tensioner’s overextension. The tensioner will attempt to overcome the damaged or missing guide to keep the chain properly tensioned.
Manually Verifying Camshaft-to-Crankshaft Timing Chain Alignment
At times, the camshaft-to-crankshaft timing will need to be verified manually, to either verify or rule out the possibility that the crankshaft is out of time with the camshaft. The application determines what procedures are necessary to verify the timing on the vehicle being worked on. Timing chains are meant to be a long-term solution to maintaining the crankshaft–camshaft relationship, but like everything else, it does have a service life, which does allow it to fail and cause a poor running condition. In some situations, a valve cover can be removed so that the timing can be verified with minimal intrusion; in other situations, the engine must be removed and disassembled to verify timing chain alignment. Consult the service information to determine the best possible way to verify the crankshaft-to-camshaft timing.
Using a Lab Scope to Check Camshaft Timing
A less intrusive method of checking cam timing involves using a digital storage oscilloscope (DSO). There are several methods of applying the DSO: An amp clamp, in-cylinder pressure testing, camshaft-to-crankshaft relationship, or even a power balance test can guide the technician toward a camshaft timing fault. Using the lab scope takes practice and may require the help of a known-good waveform, but it is much less intrusive and time-consuming than checking the relationship manually. Some of these methods require advanced tooling and training, but in the end, they are beneficial to the technician and the vehicle’s owner.
Using the Amp Clamp to Check Camshaft Timing
Performing the amp clamp method requires a high- or low-amp clamp placed around the starter cable and a trigger from the coil control signal. Set up the lab scope by following the same procedure as described for the scope test of cranking compression. Attach the amp clamp to one channel of the scope and the trigger to the second channel. Crank the engine and record the pattern. After cranking for three to five seconds, stop the recording. View the waveform, and compare the compression peak to the “turn off” signal of the coil. When a coil is on, it is saturating: building a magnetic field. Turning the coil off collapses the magnetic field, causing the coil to fire. During a cranking compression test, the spark created by the coil turning off should occur at or very near top dead center (TDC). If the trigger signal is off from TDC, a potential problem with timing exists, and thus the engine will need to be disassembled to verify the problem (FIGURE 3-14). Remember that cam timing affects compression. A retarded camshaft will produce lower compression, whereas an advanced intake camshaft on a dual overhead camshaft (DOHC) engine will increase compression (FIGURE 3-15).
FIGURE 3-14 Relative compression waveform using a high-amp clamp to monitor starter amperage. Every other compression is low due to cam timing being out specification—the result of a jumped timing chain. Unlike when AC (alternating current) coupling a low-amp clamp, the amperage reached by each cylinder is accurate. The peak on the good cylinders is just under 110 amps. There is no trigger event on this capture to identify which bank is weak. After identifying a problem, add a trigger to determine which bank is out of time.
FIGURE 3-15 The waveform above displays cranking current (red trace) with the sync from coil #1. The compression levels across the waveform are all equal, indicating that compression is even. The spark plug should fire close to, or slightly advanced (to the left 0-14° BTDC) relative to, the cylinder’s current peak. The concern seen in the waveform is that the trigger (blue) occurs between two compression humps, not at TDC, when the spark plug should fire during cranking. The difference between the actual firing versus when it should occur indicates a possible cam timing concern.
Using Camshaft Sensor–to–Crankshaft Sensor Synchronization
To check the synchronization between the camshaft(s) and crankshaft, attach the scope leads to the output of the camshaft sensor (CMP) and the crankshaft sensor (CKP). With the engine running, record the waveform (FIGURE 3-16). The CKP-to-CMP synchronization waveform varies depending on the manufacturer. Using the cam-to-crank comparison (sync) is more difficult due to the wide variety of patterns for different engines and manufacturers. A known-good reference library is required to compare the readings to find a timing fault. As with any diagnosis without knowing what “good” is, knowing what bad looks like can be extremely difficult. There are several reference books, Internet websites, and even waveform examples included in some scope software to aid the technician in finding a known-good example. Record and store known-good examples found during testing to use for future reference (FIGURE 3-17).
FIGURE 3-16 At first glance, there does not seem to be a problem with either the camshaft or the crankshaft sensor patterns. The blue trace is the camshaft sensor; the green trace is the crankshaft. They appear to be in sync also. Analyzing crankshaft-to-camshaft signals requires a known-good waveform for comparison. This waveform is actually off due to a timing chain that originally stretched and then jumped, altering the cam–crank relationship and thus resulting in low power and increased fuel consumption.
FIGURE 3-17 After replacing the timing chain, tensioner, and guides, a known-good camshaft-to-crankshaft sync waveform for the same engine. While the visual difference appears slight, it was enough to create a drivability concern.
TECHNICIAN TIPExcessive machining of a cylinder head can cause camshaft correlation codes on an overhead cam engine. Removing too much metal from the head’s surface alters the distance between the crankshaft and camshaft. Altering this relationship can cause the PCM to set a code (or codes).
In-Cylinder Pressure Transducer Method
When using compression waveforms to verify camshaft timing, it may take some time and practice to become comfortable with analyzing the results of in-cylinder pressure testing. Some scope manufacturers include software that offers automatic analysis of the captured waveform. Using a sync trigger also aids in the analysis. Capture a waveform and then select one 720-degree engine rotation (FIGURE 3-18). Placing the markers from the software onto the waveform divides the complete engine cycle into 180-degree sections, the four strokes of an engine. Some software includes additional markers dividing the 180-degree sections into 30-degree segments. The overlay aids the technician in determining whether the valve timing is correct (FIGURE 3-19).
FIGURE 3-18 An in-cylinder pressure test waveform showing late ignition timing due to a jumped timing belt. The sync trigger is from the coil-off command, illustrated by the red vertical lines. The ignition event should occur at or near TDC of the compression towers during engine cranking or at idle. The yellow vertical lines display the correct ignition timing location. When an ignition timing or valve event is retarded, the waveform moves to the right. If the event is advanced, the waveform will move left on the screen.
FIGURE 3-19 Using the scope-included software for one firing cycle divides the waveform into the four strokes of the engine. Pictured on the waveform is one complete engine cycle of 720 degrees. The tall, purple divisions represent 180 degrees of crankshaft rotation. The smaller purple lines are 30 degrees of crankshaft rotation. Intake valve opening should take place 20 degrees after top dead center (ATDC). Intake camshaft opening on a VVT engine is 30 degrees ATDC; both intake valve–opening specifications are ±5 degrees. The intake valve should close between 40 and 60 degrees after bottom dead center (ABDC). Typically, exhaust valve opening occurs within 30 to 60 degrees before bottom dead center (BBDC).
Using Relative Compression on a Scan Tool to Find a Jumped or Stretched Timing Chain
Using scan tools for an embedded test, such as relative compression, can also help to identify a jumped or stretched timing chain (FIGURE 3-20). Similar to the cranking compression test, using a low-amp clamp the scan tool measures the crankshaft speed and compares the cylinders against each other. Whenever an entire bank is affected, suspect that a jumped timing chain or a stuck VVT actuator is altering cam timing. The relative compression test should not be the only test used to determine whether a mechanical timing problem is present. A cam-in-block or single overhead camshaft (SOHC) engine that uses only one timing chain can pass the test even though it is out of time. Remember that a relative test compares each cylinder against one another. Therefore, if all the cylinders are equally low, the test will not find a fault.
FIGURE 3-20 Ford integrated diagnostic system (IDS) scan tool relative compression test showing four cylinders with low compression. On a Ford engine, the cylinders are numbered 1 through 4 on the right bank and 5 through 8 on the left bank. Each bank has its own timing chain that runs from the crankshaft to the camshafts. Bank 1 has low compression on all cylinders, indicating that a timing chain that has jumped or that a cam phaser is stuck. Jumped timing chains on these engines frequently occur due to a leaking hydraulic tensioner or broken timing chain guide. The test may not run if cam timing codes are present. To execute the test, clear the codes and attempt to perform the test again.