Difference in vacuum pressure observed before and after exhaust stroke

 

2014 Kia Forte LX 1.8L engine.  No codes, no check engine light or MIL, 100769 miles.  When looking at in cylinder pressure waveform during cranking.  I observed a greater vacuum pressure (about 2.2 PSI) before the exhaust stroke compared to after the exhaust stroke.  I have seen something about how this could mean a mechanical problem like burnt valves, incorrect timing etc.  Since the engine doesn't seem to be running poorly I'm wondering if 2.2 PSI is within what would be normal for this engine?  I performed a cylinder leak down test and all four cylinders showed 97-98 psi when 100 PSI was put into them.  So if all cylinders are only losing 2-3% I don't think that would be considered an unacceptable amount.  Should I be concerned about this difference in vacuum or am I just imagining a problem where one does not exist?

It is normal for the expansion pocket vacuum to be deeper than the intake stroke when cranking. When cranking, the piston is moving slow enough to completely fill the cylinder with air. Very little vacuum is created. Your waveform has a very distinct separation between the exhaust and intake strokes. That is a bit unusual for cranking waveforms.  Many times when cranking, you will not see a difference between the exhaust stroke and the intake stroke, but you will see an expansion/exhaust pocket.

This is because there is so much aie in the cylinder and the piston is traveling so slowly that, during compression, some of the air volume is lost, forced past the rings into the crank case, at the end of the the compression stroke. Because there is now less air volume in the cylinder when the piston returns to bottom of the expansion stroke, a deeper vacuum is created. This is called the expansion/exhaust pocket.





 

I think I may have figured out the reason for the distinct separation between the exhaust and intake strokes. I only removed one spark plug at a time for the cylinder that I was testing rather than removing all of the plugs before conducting the cranking test. I might test this again with all of the spark plugs removed to see what kind of result I get.