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Engine mechanical testing using an in-cylinder pressure transducer

Saturday, September 1, 2018 - 07:00
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The next waveform shows several good compression pulses and then a complete loss of cylinder compression (Figure 3). This 5.3 V8 GM truck engine had a cylinder #3 misfire. A compression test was performed with a gauge and the tech said the compression was the same on all cylinders on that bank. This is the same truck discussed in my June article and the problem was a broken valve spring. This problem would not escape the cranking compression test done with a transducer! If the engine is cranked over for at least 10 seconds and the compression peaks are varying more than a few psi then valve sealing issues are likely present.

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Figure 3 — This cranking compression waveform shows the first two pulses at 165psi, the third pulse at 75psi and all remaining pulses at 4psi. A broken valve spring is the culprit. Because a gauge style compression tester uses a hose with a Schrader valve which traps pressure, the gauge showed about the same value as the other cylinders.

Timing is everything

The next item I’d like to mention is timing, both ignition and valve timing. Many technicians today do not have a quick and reliable way to verify timing on most engines. Ignition timing marks are mostly a thing of the past and few techs have a working timing light. If a car comes in as a cranking no-start or lack of power, how does one eliminate a timing problem as the issue and do it quickly?  The answer is to compare the ignition event to an in-cylinder pressure event of the same cylinder using two scope channels and a pressure transducer (Figure 4).

I recently looked at a 2004 Toyota Highlander 3.3 V6 that another shop was struggling with diagnosing a lack of power complaint. When test driven, the vehicle seemed like it was starting off in third gear but the transmission had no codes and was shifting through the gears. Wondering about a possible torque converter problem, I decided to make sure the timing was correct on the engine and found the problem. The timing was severely retarded due to a grooved-out crankshaft timing belt sprocket which also has the crank sensor trigger wheel cast into the sprocket. This technique has uncovered many similar problems and is a simple test to verify a very important relationship that is all too often taken for granted because timing is no longer adjustable.

Figure 4 — This ignition timing relationship waveform was captured on a 3.3 V6 Toyota during a mild brake torque. The scope rotational rulers are used to measure the ignition signal and show the ignition pulse in blue occurring 14 ATDC. This means the timing is retarded about 22 degrees because the scan tool timing PID was reading 8 degrees BTDC.

Some readers may think this is advanced diagnostic testing and would only be used in rare cases but nothing could be further from the truth. A nearby shop towed over a 2010 VW CC they were stuck on. Considerable time had been spent trying to diagnose a cranking no-start on this 2.0, GDI turbo engine with no answers. After verifying a few basics, a cranking in-cylinder waveform was captured and analyzed. The waveform quickly pointed out what I will call an exhaust path restriction (Figure 5). The scope rulers are showing the 4-stroke cycle and there is a high-pressure pulse at the point where the exhaust stroke is ending and the intake stroke is beginning that measures 117 PSI (Figure 6). This is caused by the exhaust cam being advanced nearly 90 degrees due to a jumped timing chain. I will point out that older engines that jumped time usually resulted in retarded camshafts but today’s engine with their complex camshaft drive mechanisms can have cams that jump and end up advanced or retarded. Exhaust cams that are out of time have very little effect on cylinder compression. When an intake cam is out of time there will be a large effect on compression, retarded cams will lower compression and advanced cams will raise compression. When checking any V-style or opposed engine with two banks, if one bank has different compression than the other you should immediately suspect camshaft timing. Advanced intake cams are why an engine can have too much compression, if the measured compression is above specifications suspect an advanced intake cam possibly from incorrect installation, a jumped chain or frozen cam phaser assembly.

Figure 5 — This cranking compression test from a 2.0 VW engine looks odd with what looks like a double compression event. The waveform will be zoomed in for a closer look.
Figure 6 — This is the classic waveform for what should be called an exhaust path restriction. Pressure on the exhaust stroke should be the same as atmospheric pressure which would be the zero point on the waveform scale. This was caused by an advanced exhaust cam and it prevented the engine from starting.
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