Running In Cylinder Compression Intermittantly Drops 5psi at Idle

1998 Nissan Almera ( Euro spec Sentra ) 1.6L GA16DE engine

I just built my own pressure transducer, and was testing it out on the above vehicle, which has been having some idle quality and minor fuel efficiency issues, and saw the following behavior on both the #1 and #3 cylinders:



The running compression is about 36psi on the two cylinders I tested, but as can be seen above, the compression peak will occasional drop by about 5psi. I'm guessing the same thing is probably happening on the other two cylinders, although I haven't tested them.

I'm pretty sure the peak compression should stay pretty much constant. The only thing I can think of which might affect all the cylinders in the same fashion like this would be a loose timing chain or maybe weak valve springs across the board.

Any ideas for a diagnosis or further testing? The engine runs fine at even slightly higher rpms. I did not capture any waveforms at higher rpms.

Inconsistent compression peaks are an indication of upper-end sealing issues. (i.e. Valve seats, carbon build-up, etc)
I would slow the time base down to 5 seconds and do a cranking compression test. Is is totally random? Or, is there a pattern?

Also, some cursors at every 180 degrees would make valve openings and closings on the running compression waves easier to compare.

I did cranking and running compression tests on cylinder #1 at longer time bases and wasn't able to reproduce my previous results:

Cranking gave me an consistent 134 PSI with the throttle open:



The running test gave me 10.5 psi vacuum ( 21.4 inches of mercury ) and 40.5 psi peak pressure, again seemingly consistent:



So, I'm not sure what was really going on before. I made some modifications to my pressure transducer power supply to eliminate allot of the noise that was in the signal, and I also noticed that this cheap chinese transducer has a response time of about 2ms, so at lower time bases like 20ms / div I get this stepping artifact in the wave forms.

These cheap hantek scopes don't support the 180 cursors like the picos do.