Understanding Compression Test Results

[dirty32]

I am looking at a 1g Eclipse GSX and had the owner get a compression test done before making an offer.

Test results:
105/125/125/155

I have read that the standard compression for a very healthy 1g should be about 155-165 while the service limit is around 125. This compression test was done at a shop locally and the engine has roughly 110k miles on it.

As far as I know there is no way to determine what the culprit is without a leakdown test.

If anyone can help me understand these numbers and what I would have to do to narrow down the potential problems it would be a huge help. Thank you in advance.

 

[absolute_DSM]

First off, hats off to you for making him get a compression test. Ive seen lots of guys get cars and then post "why is my new engine acting funny?" without even looking over the car very well.


Well, reasons for low compression. Rings, valve/valve seats, head gasket/head or block warped, incorrect valve timing...

 

[dirty32]

I have a general understanding of the causes of low compression numbers and unfortunatly I did not have the shop do a second run of tests with oil to check if the rings were the cause of the low numbers. It seems unusual that #4 has good compression while the rest are very very low.

If the problem could be narrowed down to simply a head gasket then it would be an easy choice to buy it compaired to having to rebuild the bottom end or head.

When the head needs to have valve adjustments made is it possible for the valves be farther out of spec on one cylinder compaired to another?

Sorry for stringing so many questions into one post, just trying to learn.

 

[dirty32]

Any help would be awesome.

 

[drewvr4]

take out the compression test take a capful and no more of power steering fluid, drop it in there....replace the tester and try it again, if compression goes up you know your ring isn't sealing.

 

[98mitsuGSX-SD]

Do you know which kind of compression check they did? Was it a dry or wet compression test? If they did a compression on a dry motor your numbers aren't bad.

 

[timloomis]

That looks like a possible head gasket since one cylinder is good, and the others are low. Im sure it can happen, but one cylinder shouldn't be ok and the rest bad. But like what was said, cap full of oil and do it again just to be sure.

 

[Littlebeeper]

Do you know which kind of compression check they did? Was it a dry or wet compression test? If they did a compression on a dry motor your numbers aren't bad.

thats not true, those numbers are bad no matter wet or dry. if it was dry use the capful and retest to check the rings. Mine were 155 all the way across dry after sitting all night long. hopefully its head related or head gasket. the rings are a little time consuming to replace.

 

[pboglio]

I am looking at a 1g Eclipse GSX and had the owner get a compression test done before making an offer.

Test results:
105/125/125/155

I have read that the standard compression for a very healthy 1g should be about 155-165 while the service limit is around 125. This compression test was done at a shop locally and the engine has roughly 110k miles on it.

As far as I know there is no way to determine what the culprit is without a leakdown test.

If anyone can help me understand these numbers and what I would have to do to narrow down the potential problems it would be a huge help. Thank you in advance.

I usually do EXACTLY 5 cranks of the engine before checking the compression on each cylinder. Cranking over 2-3 times is enough to bring up a weak cylinder reading. I also do the readings on a warmed up motor, makes a difference. Adding oil will also tell you if its the rings or not if the compression readings change. Otherwise the problem is likely somewhere in the valve/valve seal/valve seat.

A cylinder leakdown test is going to confirm what your already seeing. If on a leakdown test you hear air escaping out the valve cover, its most likely the piston rings. If your hissing in the intake, its probably the intake valve(s). Hissing out the exhaust then it's the exhaust valves. Remove the rad cap, hissing in the radiator would be a bad head gasket.

Have the owner do a treatment of "seafoam" through the intake system. Make sure to have him run it thru the pcv vacuum port fitting AND a fitting somewhere near the fuel pressure regulator at the end of the intake manifold. Run 2 cans thru to make sure. Then redo the compression and/or leak down test.

Previously on my 97 GSX (112,000 miles) setup a compression test resulted in 170, 170, 161, 170 psi. Afterwards the "seafoam" treatment it came out to 172, 172, 172, 172 psi. Initial leakdown tests were 96%, 97%, 85%, 98%. Afterwards leakdown came out to 99%, 98%, 96%, 99%. The amount of carbon deposits were enough to cause a very weak cylinder reading. If this could bring that weak cylinder up above the spec limit, I'd buy it. Cheap and easy to do. Good luck.

 

[ekanddsm]

I usually do EXACTLY 5 cranks of the engine before checking the compression on each cylinder. Cranking over 2-3 times is enough to bring up a weak cylinder reading. I also do the readings on a warmed up motor, makes a difference. Adding oil will also tell you if its the rings or not if the compression readings change. Otherwise the problem is likely somewhere in the valve/valve seal/valve seat.

A cylinder leakdown test is going to confirm what your already seeing. If on a leakdown test you hear air escaping out the valve cover, its most likely the piston rings. If your hissing in the intake, its probably the intake valve(s). Hissing out the exhaust then it's the exhaust valves. Remove the rad cap, hissing in the radiator would be a bad head gasket.

Have the owner do a treatment of "seafoam" through the intake system. Make sure to have him run it thru the pcv vacuum port fitting AND a fitting somewhere near the fuel pressure regulator at the end of the intake manifold. Run 2 cans thru to make sure. Then redo the compression and/or leak down test.

Previously on my 97 GSX (112,000 miles) setup a compression test resulted in 170, 170, 161, 170 psi. Afterwards the "seafoam" treatment it came out to 172, 172, 172, 172 psi. Initial leakdown tests were 96%, 97%, 85%, 98%. Afterwards leakdown came out to 99%, 98%, 96%, 99%. The amount of carbon deposits were enough to cause a very weak cylinder reading. If this could bring that weak cylinder up above the spec limit, I'd buy it. Cheap and easy to do. Good luck.

Very True, It could be as simple as stuck pistons rings due to carbon buildup.

Or the lifters could be going south on the weak cylinders because oil pressure is quite low on cranking

If you dont trust seafoam just used some water(purified) and suck it in through a vacuum line, it's kinda like a steam cleaner for you piston. Just keep the engine around 2500 rpms and suck a little water in through a vacuum line as close to the throttle body as possible.

Go luck, I bought a 1g tsi without the #1 piston attached to the rod, lesson learned!

 

[meliketoball]

All I would tell you is that, if I were you, I wouldn't buy that car. Just move on and a better deal will come along sooner or later.

 

[dirty32]

Thank you for the information. The test was done dry I think but am not sure because he is from a different city and I just had him take it to a local shop and send me the numbers they wrote. I have done two engine rebuilds in the past and was really hoping this was simply a head gasket since that is far easier and cheaper.

Even though I could get the car for the 1500 dollar neighborhood I think it might end up being a better idea to hold out on it.

I will talk to the owner and see if I can meet up with him with my compression tester and do the tests myself with wet and dry and 5 cranks of the motor.

As far as a leak down tester, can those be had at any autoparts store?

Thanks again for everyones help, I am excited to get into a 1g sooner or later.

 

[ktmrider]

The "wet" test is good for diagnosing low "dry" test numbers. Putting oil ( small amount as suggested ) helps determine if the rings are not sealing well ( numbers will go up wet ).
If the numbers stay the same you have head problems.
Most all parts stores carry the compression and leakdown tools, even Craftsman at Sears.

 

[dirty32]

I have found a friend who has a leakdown tester and will test the car sometime later in this week.

Something important I totally forgot was that the car blows smoke on hard acceleration. It is definatly burning some oil either from the turbo or rings I would guess.

I will post results on the leakdown test when I get them.

 

[Burnett03]

The "wet" test is good for diagnosing low "dry" test numbers. Putting oil ( small amount as suggested ) helps determine if the rings are not sealing well ( numbers will go up wet ).
If the numbers stay the same you have head problems.
Most all parts stores carry the compression and leakdown tools, even Craftsman at Sears.

I don't agree with using the "wet" method on these cars. Basically our pistons don't have a dish on top. So when you pour the oil in, it will most likely sit on top of the piston, and NOT run down around it sealing the rings. So this is not a good way to test for worn rings on our engines. On a honda that has a big dish piston, yes it is good because the oil will run off the dish and down the sides sealing the rings. But on our cars ofcourse it will raise compression because it is just sitting on top of the piston.

 

[dirty32]

I talked to the owner of the GSX and he sold the car while I was out of town so no leak down test to find out if it was an easy fix.

Anyone in the Pacific Northwest with a 1g talon/eclipse/laser turbo AWD that is looking to sell let me know. I am in the 3k on down price range and am looking for something that is solid mechanically. Thank you to everyone on here for your help and sorry it didnt work out for me with this car.