Engine vibrations at idle, relative compression test

On my 1996 Jaguar XJ6 with an inline six XJ16 engine there are some vibrations at idle. I notice it when I am inside the car and it is also noticeable by looking at the engine. The rpm is completely stable. By Visual inspection the motor mounts seems to be fine. As part of the diagnostic procedure I have performed a relative compression test with a current clamp. On the waveform I notice that 3 and 3 bumps are offset a little bit compared to eachother and maybe differs a little bit in height. Does this waveform seem normal or could it tell something which could explain the vibration? The secondary ignition waveform and injector voltage waveform looks ok in my opinion.

A noise free capture but I think it would be easier to interpret if you do a much longer capture to let the cranking speed stabilise. On my Owon labscope I use a longer screen an then then zoom in , you don't need to capture great detail as long as you caputure the peaks and troughs. Essentially you "look at the forest before you look at the trees" looking for a repeating pattern, usually a low followed by one or two highs. On the voltage channel this would be a high followed by one or two lows.

On the voltage channel try switching to and cranking the volts/div up.

I agree with Andy, a longer capture might give you a better picture. I've noticed that some engines seem to have... peculiar qualities to them that can make their relative compression waveforms misleading. There was a Dodge V8 at the shop recently that produced a very 'raked' series of compression humps that suggested a problem where none existed.

Do you have a compression gauge? If you wanted to test the accuracy of the waveform, you could use an ignition sync to see which three cylinders are high, and which are low. Pick one from each and do a static compression check, and see if they correlate with the relative compression.

Then I will perform the test with longer cranking and post the results. The voltage measurement were mostly performed since I wanted to know the highest voltage drop during cranking, but for the next attempt I will change the volts/div. I have a compression gauge so that could be a possibility. I was also thinking of maybe buying a pressure sensor and a compression gauge kit and replace the dial with the sensor to see the waveform on the scope.. First I will try longer cranking times and observe the waveforms.

I have performed a new test With 2s/div instead of 500 ms. I included a third channel with voltage drop on the battery set at AC coupling on the scope which is the waveform at the top in the oscillogram (I also had a channel with DC coupling when I performed the measurement which is not displayed). I think that the waveforms look ok. Any comments? I also performed a cranking compression test with all spark plugs removed which showed around 7 psi as the highest difference between cylinders.

Nice capture! I think I can almost pick out the three taller peaks that you noticed earlier? I've started using the AC coupling across the battery when battery cable access is a problem - always works well. Just as effective as using an amp clamp, IMO.

Anyway, I think this waveform combined with your compression testing clears the engine of a static compression problem.

I think I can almost pick out the three taller peaks that you noticed earlier?

I see some tendences but not as clearly as the first one. And it is not clearly 3 low and 3 high peaks.

Anyway, I think this waveform combined with your compression testing clears the engine of a static compression problem.

Yes I agree with you I am also going to perform a dynamic compression test once the sensors etc. I have ordered arrives and I get time to build it.

I have also ordered a fuel pressure test kit and are going to replace the manometer with a pressure transducer and try to get an indication of injector performance by looking at the pressure drop waveform in the fuel rail after the injectors fires.

I've started using the AC coupling across the battery when battery cable access is a problem - always works well. Just as effective as using an amp clamp, IMO.

Yes that`s a good idea. And I guess it could be done also with a break out box for even easier access? At least I have read that someone has done that. I inverted the current waveform with the math channel on the scope and overlayed it on the voltage waveform and I would say they matched as good as perfectly.