Compression Test Questions
 

Hi guy's new user here.
I have an oil leak in my car which was slow enough that I could put a quart of oil every 2 or 3 weeks and be fine with it. Today the leak got worse as I was driving to school and when I got off the highway I heard the engine knocking. I parked at school and had a friend drive me to walmart to get some oil. I ended up replacing 2.5 quarts of oil.

I did a compression test to check if there was damage to the cylinders and got
Dry: 1 - 150 psi, 2 - 149 psi, 3 - 150 psi, 4 150 - psi.
Wet: 1 - 178 psi, 2 - 175 psi, 3 - 178 psi, 4 - 160 psi.

However I forgot to press down the throttle while cranking the engine :vash:, any idea if this would cause a significant error in the readings? I could go and rent the gauge again but I'd rather not.

Thanks in advance

For your test purposes you should be fine. The numbers look normal and the test was done without the throttle open across the board.

Can you tell the if the noise was from the top of the motor (HLAs) or lower like a bearing?

No I couldn't, I'm hard of hearing.

the wet test suggests your rings are failing.

The 2.5 quarts of oil suggest your bearings are failing.

Oil leaks can bite you in the ass if you don't keep after them religiously. It's too late now, but it sounds like you should have pulled the dipstick every morning as a preflight check.

Car was fine when I started it and wasn't knocking, school is ~60 miles away so the leak could've worsen on the way there, I try to keep on top of maintenance on this car.

Woke up today to find a big puddle of oil in the driveway, so the bearings should be fine as it's a leak.

How big of an issue issue do you guys feel I have on the rings? Is it something I could run with for a few months/years? I track the car so a rebuild/swap is planned to hold fi in the near future.

Its not that bad, thats only ~1.5 quarts in the car. Some spec miatas race with only 2 quarts in the thing. But the knocking sound suggests that 1.5 quarts was not enough. Its still totally possible that the soft bearings did their job and the short time you ran low on oil only took a bunch of life out of the engine rather than killing it completely.

looking at the numbers again i was probably wrong about it. You're even across, and about the same increase across with oil added. fix your leak.

I said your bearings were bad because you had the oil low enough to cause a knock. Knock = damaged bearings

Oh ok, I tought you meant the bearings where burning up the oil.

Thanks guys I appreciate the input.

He did not mention what year the car is, but if he has hydraulic lifters and started suckin a little air with the oil pick up as the oil level was low it may be possible that the lifters started ticking before the bearring had serious damage, it does not take much air in them to make them knock.
Does the car have hydraulic lifters? If it does, did the car kept knocking after running a few minutes once you filled up the oil to the correct level? If it stoped knocking i would fix the leak, skip a few track days and put that money away on an engine pigy bank.

Car is a 97 with the original 1.8, I'm relatively new to the miata scene so I"m not to sure if the engine comes with the hydraulic lifters. The knocking went away a few seconds after running with the right amount of oil. The same knocking comes on certain mornings but goes away after a few seconds as well.

Your knocking just sounds like lifter tick.

The knock, is it a sharp metallic sound, or a deep, thud or THOCK type?

The first would typically be the lifter tick others refer to, and the latter would usually be excessively worn bearing(s) banging on the crank.

Totally normal.