Surprisingly good compression!?

[NickR]

92 Tercel, 285,000kms. This car has not been properly maintained. It has had the oil changed only when I can persuade my daughter that it really, really needs to be changed (usually around 10,000kms, or more). Since it uses some oil, and she is not over careful about checking the oil, I'm sure she has run it low on oil more than once, and she confessed to driving several kms with the oil light on, a few years ago. Last summer I checked the compression, and was pleasantly surprised to see it at around 160/165 on all 4 cylinders (stock is 184).

This summer, she hit the oil pan on a rock (as described in my other thread), causing some serious oil leaking. She confessed again to driving "quite a way" (my suspicion is a few dozen kilometers, if not more) with the oil light on (in her defence, she was miles from anywhere, on a dirt road in Northern Ontario). Amazingly, the brave little car got her home. First thing I did was to check the compression, wondering if the car was worth salvaging. The results were not good: 120, 105, 130, 100. I decided to go ahead and fix it, hoping it might get her through one more year. I gave it an engine flush (Canadian Tire bottle of engine flush added to the oil), then changed the oil pan, oil, and filter.

I ran the engine for a few minutes, then checked the compression again: 170, 175, 175, 180. I was very surprised, after all that car had been through, not to mention the mileage!

Three possibilities:
1. The previous times I checked the compression. maybe I didn't do it right. The engine was warm, but maybe I forgot to hold the gas pedal down, or didn't crank long enough, or didn't make sure the compression tester was screwed in tight. (I did remember to remove all the plugs every time).
2. Carbon build-up is giving an artificially high compression reading, by reducing the effective size of the combustion chamber.
3. The engine flush really did work, by getting all the gunk off the piston rings, and freeing them up.

What sort of compression numbers are others getting, on Tercels with similar mileage?

Has anyone else had such beneficial results from an engine oil flush?

Or do you think I must have failed to do it right on the previous times I checked the compression?

[tyler88]

no i have done a compression test at 277xxxkms and i read between 160 and 190 across the engine. IMO my car is in the best shape its ever been in

[Tercel Drifter]

my theory is that because the valve stem seals tend to fail at a relatively "early stage" in the engines life the oil in the cylinder help lube thing up a little and protect the rings, piston and such. IMO

[NickR]

^Interesting theory. It makes sense, because a leaking valve stem seal would ensure there is a little oil in the cylinder at start-up, before the oil pressure is up, when most of the wear is supposed to happen, right?