Compression test results, all 170 except one was 140

[dsmGSTtuner]

I did a compression test on my engine because I thought I had a bent valve. The results were #1-170, #2-140, #3-170, #4-170.

The car was at open throttle. The engine wasn't warmed up either, it was actually cold. Also the engine was rebuilt and hasn't been broken in either.

Can anyone enlighten me on why that #2 cylinder would be lower and would it be safe to run? Thanks ahead of time and it feels good to finally be a Proven Member

 

[96' Eclipse GS]

Drop a couple of drops of oil in the cylinder/cylinders that you believe have problems, and repeat compression test again, if you get different numbers, then your rings are bad. If the cylinder that has lower compression is the one you believe has bent valves, then that is your problem possibly....

 

[NHerron]

How many miles/hours etc has the engine actually been run?

Stock pistons? Do the oil in cylinder like Omar said and that would seal the rings to take piston/rings out of the equation. If compression goes up substantially, then you can blame the rings/cylinder. If not, then perform a leakdown test on that cylinder to see what is causing it. Headgasket, intake, exhaust valves, etc

 

[dsmGSTtuner]

well the engine hasnt even ran long then 5 mins. i was hearing this noise and i thought it was either the lifters pumping up (bleeding) or a bent valve. if i have a bent valve it could still read 140 psi? Im about to go do the oil in the cylinder theory. How much oil should i put in it? llike 5 or 6 drops?

oh yea, stock pistons i believe, i bought the engine rebuilt.

 

[NHerron]

Stock pistons then yes put in a tablespoon (or a capful from a 5 quart jug)

Did you bleed the HLA's (lifters) before assembly?

 

[dsmGSTtuner]

i dont think they were, i bought it rebuilt

 

[NHerron]

Hmm I would bet the head is responsible for the low compression as you said, maybe a slightly bent valve. What kind of rebuild did they say? Was it recently rebuilt then ran for a time then you bought it? Or rebuilt straight up then you bought it?

Okay so the whole engine is rebuilt with timing belt, just dropped the engine in, right?

 

[1983gsp]

Rings might be in alignment. Just a thought. Google motoman method.

 

[dsmGSTtuner]

Yea the whole engine was rebuilt completely. Just a straight drop in. I don't think it was really ran at all. I got it in my car and cranked it right up.

I did the "oil in the piston" idea, I only put about 1/4 a cap full in it and it went up to 150 psi. Should I put more oil in there and try it again?

This may be a dumb question but how much is too much oil to put in the cylinder? It will just burn off right?

 

[NHerron]

Well with only putting a 1/4 cap full in and the compression goes up by 10psi it sounds like rings/cylinder, as Robert said maybe the ring end gaps are in alignment. They should be 180 degrees across from each other

That's not much oil to cause a problem but yes too much can give you a false reading since it takes up volume, which naturally brings up compression. But it won't hurt anything and yes it will all burn off.

 

[dsmGSTtuner]

okay cool. Here is another dumb question, is it safe to run? also how can i tell if the rings are in alignment?

well i mean is it safe to run the car like in that condition. Also, do i need to pull the head or something to get to the pistons?

 

[NHerron]

Do you have any buddies to help you that would be good with this kinda thing? I would find out what's wrong and confirm with a leak down test

You can't check ring end gap alignment without pulling the piston out and honestly, I don't know if that would even cause such a drop in psi, I've always had my gaps offset correctly.

Try this, it's such a better way to see what's going on, it's called a leak down test. Here is a really good link

http://www.dsmtuners.com/forums/articles-engine-fuel/338152-compression-leak-down-testing.html

 

[Albrecht09]

If the motor was just rebuilt, then didn't you check out the head while it was off? What happened that caused it to need rebuilt? I assume the timing belt broke or something like that... Any time I'm rebuilding a motor that jumped timing, I always take the head to my machine shop just to have it hot tanked and have the bearing journals, cams, and valves checked to make sure they're within spec.

When you did the test, on each stroke of the engine the pressure increased evenly without skipping or anything? And did it hold that pressure or slowly start to drop after you stopped cranking the motor? Also was this a wet or dry compression test?

 

[keltalon]

Leak Down Test is next it will for sure let you know what is wrong with that cylinder

 

[BogusSVO]

You have 5 mins of run time.

NOTHING has worked into where it needs to be.

The HLA's need to pump up/bleed down, the rings need to seat.

If you have no leaks, and the rest of the engine install is done. Go drive the car, follow the Motoman break in, Get 20-30 miles on the car, then run the compression test again

Even if the 2 ring gaps were in alignment, they will rotate around the piston/bore a bit till they take a seat.

Being off 30 in one cylinder is not all that bad on engine that has not been run in yet.

If you had a slightly bent valve you would have a skip/pop either in the intake or exhaust, depending on what valve was tinked.

There could be a HLA that is not bleed down enough yet, and is slightly holding a valve open, running the engine, and have the cam compressing the valve spring will provide the pressure on the HLA to work it into place

 

[dsmGSTtuner]

Thanks BogusSVO and HNerron! BogusSVO, what you said is almost exactly what my brother said. I don't think the rings have set so that's why I'm getting 140 in that cylinder. But I did a dry test and it didn't drop any after I stopped cranking it. If it did it was really slow.

I'm just gong to put my CV axles on and let her ride for Motoman style and then check it again! Then I'm goin to run another compression test.