Remember the old days when the diagnostic value of the conventional vacuum gauge was the rule of the thumb? Not to undermine the importance of these well tried and proven testing results, there are some new technologies that have been out for several years now that need to be embraced with the advent of the scan tool and the modern variable valve timing systems. We will cover what is available on the scan tool and more importantly we will cover the diagnostic value of the electronic pressure transducer.
Fig. 2: WOT snap test viewing MAP and TPS, using scan tool graphing mode.
First let’s use what is available on the scan tool. When dealing with the speed density electronic fuel injection (EFI) systems or the mass air flow (MAF) EFI systems that also incorporate a MAP sensor, we can graph out the manifold absolute pressure (MAP) sensor with our scan tool by looking at a stable MAP values at idle. See Fig.1 We can also do a WOT (wide open throttle) snap test by viewing the MAP and throttle position sensor (TPS) when using the graphing mode of the scan tool. See Fig. 2.
An ignition misfire will not cause major positive pulses from the MAP sensor during a power brake condition. A mechanical valve train issue will cause major positive pulses of the MAP sensor. A restricted converter will cause the MAP values to slowly indicate recovered MAP values when the throttle is slammed shut during a WOT snap test.
As the TPS indicates a closed position, notice the MAP values indicate a good vacuum value greater than the MAP values that were indicated at idle before the WOT snap on this good example. A clogged converter or an engine with a retarded valve timing problem will not show these good results. Note that this WOT snap test will only work on cable operated throttles. The electronic throttle control (ETC) systems will not go to WOT in your bay when doing the WOT snap test.
It may be necessary to power brake the vehicle at around 1,200 rpm and note the lack of vacuum caused by a restricted converter of retarded valve timing issue. A misfire caused by a mechanical problem such as leaking intake valve will most likely create positive shifts from the MAP during a light power brake test. We simply refer to the MAP sensor as our electronic vacuum gauge.
Most manufacturers have now equipped their engines with variable valve timing. The reason here is two-fold.
First of all, we can eliminate the EGR system by retarding the exhaust cam timing thus leaving the exhaust valve partially open during the intake stroke. This essentially allows some EGR gases to be pulled into the cylinder during the intake stroke.
This is a popular strategy on the domestic engines. The Asian manufacturers tend to advance the intake cam timing off idle. This advances the opening of the intake valve when the piston is on the exhaust stroke. The net result is the same as to allow some exhaust gases into the combustion chamber. You will see some domestic engines where only the exhaust cam timing is varied and the intake cam timing is fixed such as the 4.2L GM Vortec engine and other domestic engines.
Secondly, as the valve timing is varied off idle this increases the engine’s ability to breathe. The Asian manufacturers such as Toyota vary only the intake cam timing whereas the exhaust cam timing is fixed on some engines.
The first thing that needs to be checked is the oil level and oil condition. Sludge build up may impede oil pressure from reaching the variable valve timing (VVT) control solenoid or the cam actuator. Low oil pressure is also a major concern especially on high mileage engines. It is sometimes necessary to conduct a manual oil pressure test. This seems to be common on the Ford Triton engines before model year 2010. Some technicians have fixed this problem by replacing the original oil pump with an aftermarket high pressure oil pump.
“Premium” engines will vary both cams. The domestic systems will usually retard the exhaust cam timing and advance the intake cam timing off idle, whereas the Asian engines will advance the exhaust cam timing and retard the intake cam timing off idle. The net result is the same in that both designs create valve overlap. This creates a cylinder scavenging effect that actually increases the engine’s ability to breath.
Fig. 3: A GM engine captured at idle. Notice the 0% duty cycle command to VVT control solenoids.
Always keep in mind that we can only vary the cams’ timing off idle. The proper amount of oil pressure is also needed to force the cams to their park position at idle. Notice Fig. 3 from a GM vehicle captured at idle. Notice the 0% duty cycle command to VVT control solenoids. This will only occur at idle. Most GM systems will not vary the cam timing off -idle in your bay. The PCM needs to see a VSS signal to vary the valve timing so a test drive might be on order with the scan tool in the record mode.
Fig. 4: The variable valve timing (VVT) system is a once-per-trip monitor, so you have very robust test results from the Mode 6 menu of your scan tool.
The variable valve timing system is a once per trip monitor, so you have very robust test results from the Mode 6 menu of your scan tool. See Fig. 4. GM and Ford systems will also give us a new parameter to allow us to detect when a camshaft is out of phase. GM uses the term cam variable where Ford uses the term cam error. Normally + or – 5 degrees is a good spec. If the timing chain or timing belt has jumped 1 notch, the valve timing will be retarded 13.4 degrees. These parameters are very reliable from the scan tool.
Fig. 5: GM V6 engine when viewing both Bank 1 and 2 intake and exhaust cams captured during a test drive. Notice the 49% to 50% duty cycle command to the VVT control solenoids.
Fig. 6: The most important parameter to monitor on the GM systems is the cam variance values.
Fig. 7: A capture from a problem engine. The desired position of the cam is to move 20 degrees. Notice that the variable shows that the cam is 17 degrees out.
One thing to always remember when viewing this parameter is that the rpm must be steady. When blipping the throttle you will see these values as unstable. Notice Fig. 5 from a GM V-6 engine when viewing both bank 1 and bank 2 intake and exhaust cams captured during a test drive. Notice the 49% to 50% duty cycle command to the VVT control solenoids. The intake cams were advanced 8 degrees while the exhaust cams were retarded 14 degrees. The most important parameter to monitor on the GM systems is the cam variance values as seen in Fig. 6. We now have bi-directional controls on the scan tool to vary the cam timing in your bay. Notice Fig.7. This was captured from a problem vehicle.
Fig. 8: Ford system with VVT on the exhaust cam only. Notice the 41% duty cycle command to the VCT solenoid causing the exhaust cam to retard to 24 degrees. The cam error PID indicates 0 degrees. The rpm must be stable when monitoring this PID.
The desired position of the cam is to move 20 degrees. Notice that the variable shows the cam is 17 degrees out. Now let’s look at a Ford system with variable cam timing on the exhaust cam only in Fig. 8. Ford uses the term cam error, indicating how far the cam is out of phase. Notice the 41% duty cycle command to the VCT solenoid causing the exhaust cam to retard to 24 degrees. Notice the cam error PID indicates 0 degrees. The rule here again is that the RPM must be stable when monitoring this PID.
Some systems such used by Asian manufacturers will show the cam position as desired and actual. You simply have to do the math. For example, if the desired is 0 degrees from the intake cam and the actual is 8 degrees, then the cam is 8 degrees out of position.
Fig. 9: A Chrysler scan from a problem engine with slow response codes for both intake and exhaust cams. Notice the test drive scan tool capture. The PCM wanted the exhaust cam set point at -120 degrees, and the actual matched at -120 degrees. The value indicates degrees of crank rotation.
The Chrysler systems indicate the cam positions as actual and set point. Some aftermarket scan tools may use different terminology for these parameters. The set point is the PCMs desire for cam position and the actual is the position of the cam. Let’s look at some Chrysler scan data from a problem vehicle with slow response codes for both the intake and exhaust cams. Notice the test drive scan tool capture in Fig. 9. The PCM wanted the exhaust cam set point at -120 degrees and the actual matched at -120 degrees.
Fig. 10: Data from the same Chrysler engine during a power brake condition. Notice the 21 degree difference between the actual exhaust cam position and the PCMs set point.
This value is represented in degrees of crank rotation. Notice the 63% duty cycle command to the VVT control solenoid. Now note the changes in RPM when the actual set point varies, which is normal. Now look at some data from the same vehicle during a power brake condition in Fig. 10. Notice the 21 degree difference between the actual exhaust cam position and the PCMs set point. There is actually a TSB on this car to change the oil and the intake and exhaust cam control solenoids as well as a PCM reflash.
Fig. 11: A relatively new test involves the use of an electronic pressure transducer and a DSO (digital storage oscilloscope).
Moving on to some relatively new testing procedures which involves the use of an electronic pressure transducer coupled with a DSO. See Fig. 11. In Fig. 12, this shows the Bosch pressure transducer and the conventional compression gauge. The conventional compression gauge was only designed to conduct a WOT cranking compression test.
Fig. 12: Example of a Bosch pressure transducer and conventional compression gauge. This is only designed to conduct a WOT cranking compression test. The Schrader valve must be removed from the gauge in order to monitor all four strokes when monitoring a compression waveform with a pressure transducer.
There are many manufacturers such as Pico and Snap-on who also make a pressure transducer. In a conventional compression gauge hose there is a Schrader valve that must be removed in order to monitor all four strokes of the engine when monitoring a compression waveform with a pressure transducer.
You’ve heard the old adage that a picture is worth a thousand words. Using this equipment allows us to view cranking, idle, cruise and WOT snap compression values. In addition, we can view exhaust valve opening points and intake valve opening points in degrees of crankshaft rotation in the case where a sloppy timing chain may be the lack of power complaint.
Fig. 13: WOT cranking compression test on a high mileage 3.8L Chrysler. Notice the waveform indicates 125 psi.
Fig. 14: Idle compression value at idle of 45 psi on the same 3.8L Chrysler engine. Idle compression values will normally be 40 to 60% of cranking compression value.
Let’s take look at a WOT cranking compression test in Fig. 13 on a relatively high mileage 3.8L Chrysler engine. Notice that the waveform indicates 125 PSI. Now moving on, let’s look at an idle compression value at idle in Fig. 14. Notice the value of 45 psi. Notice the uniform and consistent values of 45 PSI. Leaking valve issues can show up here with compression values dropping off.
Fig. 15: Cruise compression values of the same 3.8L Chrysler. Cruise compression values are normally captured at around 1,500 rpm.
Since the throttle plates are barely off-idle, the engine is simply not pulling in a lot of air. The idle compression values will normally be 40 to 60% of the cranking compression value. Now let’s move on to the cruise compression value. Note Fig. 15 indicating 25 psi. The cruise compression values are normally captured at around 1,500 rpm.
Fig. 16: WOT snap value on the same 3.8L Chrysler. The WOT snap values equaled that was captured during WOT cranking.
The reason for the normal drop in compression value here is that the rpm is higher and the throttle plates are barely open. This is also where you may see compression values dropping off, caused by leaking valves. Now let’s look at a WOT snap value in Fig. 16. Notice that the WOT snap values equaled what was captured during WOT cranking conditions.
Fig. 17: Compression waveform. This can help indicate a timing belt or chain problem.
Now let’s look at all of the critical points of a compression waveform in Fig. 17. EVO stands for exhaust valve opening, while IVO stands for intake valve opening. With some minor calculations we can determine if there is a timing chain or timing belt issue. View Fig. 18 from a 3.8L Chrysler engine. Notice that the time between the two TDC cursors measure 176 milliseconds.
Fig. 18: Note that the time between the two TDC cursors of 176 milliseconds. Dividing this time by the 4 strokes shows that a stroke happens every 44 milliseconds.
Fig. 19: The EVO point on most engines with fixed valve timing is usually about 40 to 45 degrees BBDC. This high mileage engine shows about 6 degrees of slop.
Knowing that all 4 strokes would have occurred, we simply divide the 176 milliseconds by 4 which equals every 44 milliseconds a stroke occurs. Now let’s take a look at Fig. 19. We have moved the right cursor from the TDC mark to the left at 44 milliseconds. This identifies BDC at the 180 degree mark. Note the EVO point occurred before the BDC mark. If you count the number of minor divisions between the TDC mark and the 180 degree BDC mark you should come up with 20. 180 degrees divided by 20 minor divisions equals 9 degrees of crank rotation per each minor division. Now notice the point of EVO.
There are 4.5 minor divisions between the EVO point and the 180 degree cursor. 4.5 times 9 means that he exhaust valve opened 39 degrees BBDC. The exhaust valve opening spec. on this engine is 45 degrees BBDC. The EVO point on most engines with fixed valve timing is usually 40 to 45 degrees BBDC. The timing chain has about 6 degrees slop on this high mileage engine.
Fig. 20: On a fixed valve timing engine, the 180 degree cursor splits the exhaust ramp in the middle. Again, this helps to determine if the timing belt or chain is worn or damaged.
Notice Fig. 20 where the 180 degree cursor pretty much splits the exhaust ramp in the middle which is what you want to see. This only applies to engines with fixed valve timing. This may be a good test to determine if the engine has a sloppy timing chain or timing belt. Labor-wise, it sure beats the removal of the timing case cover to check for alignment marks.
Now let’s look at an engine equipped with variable valve timing on both the intake and exhaust cams. As we stated earlier, the scan data for variable valve timing systems is very robust and can be relied on. But let’s look at some waveforms from a GM V-6 engine that varies both the intake and exhaust cam off-idle only. We are going to look at these waveforms at idle where both cams are in their park positions. Again, the domestic engines will retard the exhaust cam timing off idle and advance the intake cam timing off-idle.
Fig. 21: 2012 Chevy Equinox 3.0L V-6 compression waveforms captured at idle. This example shows 52 milliseconds per stroke.
Most GM systems will not vary the cam timing without a VSS signal. We talked earlier about the ability to control the VCT control solenoid with a scan tool in your bay. Obviously commanding a VCT solenoid at idle will create an RPM fluctuation from too much valve overlap. Let’s take a look at the compression waveforms from a GM V-6 engine in Fig. 21. Captured at idle the distance between the two TDC cursors measure 209 milliseconds. Divide this by the 4 strokes and we come up with 52 milliseconds per stroke. Now let’s move the second cursor to the 52 millisecond mark. See Fig. 22.
Fig. 22: The same 2012 Equinox 3.0L at idle. When driving this vehicle, the PCM will retard the exhaust cam timing.
This identifies the 180 degree mark at BDC. Notice that there are 10 minor divisions between the TDC cursor and the 180 degree cursor. 180 degrees divided by 10 minor divisions tells us that the crank turned 18 degrees per minor division. Now notice that the point of EVO occurred 3 minor divisions before the 180 degrees cursor. 3 minor divisions times 18 degrees per minor divisions tells us the exhaust cam is in its advanced peak position. Again, when driving this car, the PCM will retard the exhaust cam timing. Now let’s look at the point of IVO (intake valve opening).
Fig. 23: On the same 3.0L GM engine, we move the second cursor over to the 104 millisecond mark. This shows that the intake cam is in its retarded park position.
We simply multiply the 209 milliseconds times .50 to establish the 360 mark. 104 milliseconds times .50 gives us a value of 104 milliseconds. Now we move the second cursor over to the 104 millisecond mark. See Fig. 23. If you recall, the crank turned 18 degrees per each minor division. Note that the point of IV0 occurred 3 minor divisions after the point of the 360 degree mark, telling us that the intake cam is in it’s retarded park position. When the PCM sees a VSS signal it will advance the intake cam timing.
This again improves the engine’s ability to breath. The Federal EPA mandates that the PCM must flag a cam variance code if the cam does not return to its park position. A reminder here is that it takes good oil pressure to vary the cam timing as well as to force the cams back to their park position at idle. We all know that oil pressure values decrease as RPM decreases.
Fig. 24: Example of a 3.4L GM V6 with a number 4 cylinder misfire, with the firing event on the exhaust stroke, caused by a DIYer’s mistake.
Now let’s look at some problem vehicles. Fig. 24 is from a 3.4L GM V-6 engine with a number 4 cylinder misfire. Note that the firing event occurred on the exhaust stroke. This was caused by a DIY customer. With spark advance we always need the firing event to occur before TDC.
Fig. 25: Example of a good engine exhaust back pressure test. At the point of EVO (exhaust valve opening), the exhaust pocket is smooth and normal.
Fig. 26: Example of the exhaust pocket at idle. Note the slight rise in pressure after the EVO point with exhaust back pressure on bank 1, cylinder 1.
Fig. 27: This example shows a throttle snap. Note the abrupt rise in exhaust pressure caused by a restricted converter.
Now let’s focus on the EVO point and the exhaust pocket from a good engine. Notice at the point of EVO, the exhaust pocket is smooth and normal in Fig.25. This is actually the point of barometric pressure values. Notice the exhaust pocket in Fig. 26 captured at idle. Do you see a slight rise in pressure after the EVO point? Now let’s do a throttle snap and monitor the exhaust pocket in Fig. 27.
Notice the abrupt rise in exhaust pressure caused by a restricted converter.
Fig. 28: Cylinder leak-down testers may not be as reliable on modern engines. A better option is to apply EVAP smoke machine and monitor the flow gauge.
The misfire monitors on today’s modern day engines are very tight and sensitive. We may have a vehicle in with a single cylinder misfire whereas new plugs, coils and injectors have not addressed the misfire. Let’s say a cranking compression test indicated a good value of 145 PSI. The next logical step in most cases is to do a cylinder leak down test with a conventional cylinder leak down unit. Note Fig. 28. These newer units apply 20 to 40 PSI to the cylinder at TDC.
We have found these units somewhat unreliable on modern engines that experience valve leakage. A better option is apply smoke to the cylinder by using your EVAP smoke machine and monitor the flow gauge. If the flow gauge is up from the bottom you have a leak. Smoke waffling from the intake tells us that an intake valve is leaking.
On gasoline direct injection systems, carbon buildup on the valve face and seat is a common problem causing intake valve leakage and misfire codes. This is caused by vaporized oil from the PCV system. Going back to the cruise compression test you are likely going to see the compression values start dropping off. A good chemical cleaning of the intake system off-idle usually fixes this problem.
Well, that’s all I have for you on this round. I hope that I shared some good info with you and thanks for your commitment to this very resilient industry.