I checked the compression on my 91 1.6 with 144k miles, and am not sure what to make of the numbers.

The dry compression is (front to back) 150, 155, 160, 180. To check it wet, I added just a little oil, and there was no change. After adding a little more, it was ~200 on the first cyl, but I think I may have overfilled. It's also an approximation because I used one of those gauges that you have to press down into the spark plug hole instead of the screw-on type, and apparantly I wasn't holding it down tight enough

Since that's not particularly helpful, I'll try to re-do it using an equal amount (1 teaspoon?) of oil in each.

I also have an explanation for the higher compression on cyl 4. The #4 spark plug looks like it had never been changed, as it was rounded at the top and nearly impossible to remove. I'm pretty sure it was the orignal plug from the factory... So that cylinder wasn't really working all that well until I got it removed around 2k miles ago.

What does this mean? It seems to run fine; I do notice a little vibration, but it doesn't seem all that serious.

I was thinking of going with a mild turbo setup. Is it safe to boost?

I'm very skeptical of those all-in-one cure-all magic bottle treatments, but there appear to be some out there which are able to raise compression via cleaning. Some BG products, auto-rx, and Marvel Mystery Oil seem to come up a lot on various forums. Are any worth trying?

Thanks!

 

From the sound of it you need to use a better compression tester. Seafoam works well and I don't have any experience with others.

 

I had considered that the gauge was off, but I did it twice with the same results. I actually checked the compression, used seafoam through the vacuum line, let it sit for an hour, ran it for a bit, checked compression again, and changed the oil & filter. I didn't add seafoam to the oil or fuel, just vacuum.

Before seafoam, everything was ~5psi higher, but that doesn't seem relevant since it's also possible that the engine was hotter or the battery better charged.

What concerns me is the huge difference between cyl #4 and the others. Should I add some Marvel Mystery Oil for a bit and see what happens?

 

Leak it and listen for air through the intake, exhaust, or dip-stick hole.

 

No. Take Trey's advice and invest in a leakdown tester, which can be bought at Harbor freight for $50 or less, or even rented at some automotive chains I think. You need to try something other than the gauge you are using that will make a good, tight, screw in seal.

 

Ok, will do. I'll report back when I can.

Thanks!

 

Dude just drive your car a bit and redo your test. In order for test results to be valid your testing method needs to be valid. Your oil overfilling makes your results worthless.