The next important item in the known-good synced relative compression waveform is the location of the ignition pulse in comparison to the current waveform. A typical engine cranks with ignition timing very close to TDC, sometimes slightly ahead or possibly slightly retarded if the engine uses a catalyst heating strategy. Most technicians do not have a go-to test to verify proper ignition timing if a vehicle is a cranking no-start, but a synced relative compression test will help verify good ignition timing on late-model vehicles with no timing marks. As seen in the known-good synced waveform (Figure 1), the ignition trigger pulse lines up with a current peak, which we will consider cylinder #1, indicating good spark timing. Compare that to Figure 4, where the timing is obviously incorrect. The only caveat here is if the engine has a distributor and the distributor is installed incorrectly, this ignition pulse could occur in sync to a current pulse, but the pulse is not cylinder #1. An in-cylinder compression waveform would uncover this problem and will be covered in the next article. Keep in mind that if cylinder #1 is not accessible due to intake manifold design you can always use the sister cylinder to #1 as your ignition sync input, which will be on the other bank of the engine. Also consider that when performing this test, you will need to prevent the engine from starting. Do not disable the ignition system if you are triggering off a spark event. Remove power to the fuel pump or injectors when performing a synced relative compression test.
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When it’s wrong
When viewing a relative compression test on an engine with low compression in one cylinder, the problem will be fairly obvious; there will be a low or missing peak depending on how low the compression is. Typically, the cylinder following a low compression cylinder will have a slightly higher peak due to the starter speeding up when the low compression cylinder pushes past TDC and then slows down when the next cylinder with good compression comes up to TDC. The next two waveforms (Figures 5 and 6) were captured from a 2005 GMC Yukon with a rough-running, low-power complaint that was diagnosed as a bad catalytic converter at a GM dealer. As the relative compression test displays, the first 4-stroke cycle showed good compression, but then the #3 cylinder lost all compression as the engine continued to rotate. The first cycle proves the cylinder can seal and a gauge-style compression test was done at the dealer and the tech saw good compression on the gauge, but the gauge captures pressure due to the Schrader valve in a compression test hose. As the engine rotates, the cylinder seal is lost. This can only be a mechanical problem, most likely caused by a valve sealing issue due to the complete lack of compression pressure seen on the relative compression waveform. A broken valve spring was suspected and confirmed when the valve cover was removed, the exhaust valve spring was broken (Figure 7).
|Figure 7 - Broken exhaust valve spring on 2005 GMC Yukon. This was found in less than five minutes of testing with a relative compression test after several days of testing and parts swapping by other technicians.|
Sometimes it may not be a low or missing current event that is seen while performing a relative compression test; sometimes it may be the opposite. The next waveform capture (Figure 8) is from a 2008 Pontiac G8 with a 6.0 V8 with GM’s AFM (Active Fuel Management) system, which can cancel four cylinders during light-load operation. The engine is logging a cylinder #4 misfire code and this is an AFM cylinder. A synced relative compression test was performed. The waveform shows almost twice the level of current needed to push cylinder #7 to TDC on the compression stroke. Can one cylinder have higher compression than the rest without a high domed piston or longer connecting rod? Not possible or likely, unless an engine builder is playing games. This is not a carbon issue either as the current is double the other cylinders.
|Figure 8 - Relative compression waveform from a 2009 Pontiac G8. Note the high current seen on cylinder #7. This indicates there is increased effort needed by the starter to push cylinder #7 to TDC on its compression stroke.|
The problem is easily explained: there are two cylinders on compression at the same time, #7 and its sister cylinder #4. The exhaust lifter has collapsed on cylinder #4, so when it should be on its exhaust stroke with the valve open, the valve is closed and both cylinders are compressing air, hence the misfire code for cylinder #4 and the high current event on the sister cylinder #7. It should be clear by now the importance and capabilities of this simple yet effective mechanical test.
“Seeing” cranking vacuum
The second test mentioned was cranking vacuum. To perform this test, a pressure transducer will be required and there are two types. Absolute pressure transducers like the Pico WPS500 (shown in Figure 9) can measure both actual vacuum levels and display what are called vacuum events or pulls created by each cylinder in the engine. There are also differential pressure transducers like the Sen-X Technologies First Look sensor. This transducer displays the change in vacuum or pressure seen by the sensor and not the actual level, but it is very sensitive and well suited to this particular test. Figure 10 shows both sensors connected to the same port and you can make your own decision if you want one or both. There are other companies making high-quality vacuum transducers; I am only mentioning units I have personal experience with. Whatever brand and style transducer you use is not important, only that you actually own them and perform the test.
|Figure 9 - Connecting pressure/vacuum transducers to a GM V8 engine to perform a cranking vacuum test. Connecting to a centrally located vacuum port and keeping your hoses short will produce the best test results.|