Engine Burned Oil | Trying to Diagnose with Compression Meter

Question

I changed the oil in my car (1997 Volvo S90 with approx 200k miles on it) about 2k miles ago, and recently took a couple long car trips (~400 miles each, up and down mountain roads, etc). When I returned from the last trip I had to do some work and thought I would change the oil as long as I was under the car. The car is supposed to have 5.8 quarts, but I only had 4 quarts in my jug. I was concerned so did a compression test with the following results:

Cylinder# | Dry pressure (PSI) | Wet pressure (PSI)

1 | 205 | 235

2 | 190 | 215

3 | 100 | 165

4 | 152 | 215

5 | 175 | 215

6 | 205 | 235

I am a bit concerned about cylinders #3, 4, and 5. Is there something I can do to narrow down if the low compression is due to a head gasket, or ring damage?

As stated, the car is a 1997 Volvo S90.

Answer 1

It's not that likely to be the head gasket, if it is you'd have oil leaking into the coolant or vice versa, plus lots of white smoke coming out of your exhaust. More likely it's worn rings or your valves aren't seating properly, and I can see from your wet compression tests that it's worn rings or cylinder walls.

The reason I say it's rings is that the wet compression values are much higher than the dry ones, when you put fluid in your cylinder it creates a wet seal which boosts your compression. If your rings are good then you'd see little difference in compression as the rings are already doing their job and the most likely culprit would be the valves.

The manufacturer should have published the expected compression and tolerances, however the guideline I've seen is that a 10% difference in wet to dry compression means you have worn rings or a problem in the cylinder itself. By this calculation all of your rings look to be worn, not just 3, 4 and 5.

If you're having to do the rings you'll want to check the valves as well, chances are there will be work do to there too.