6.2 Cylinder Low Compression Diagnosis

[adf5565]

I performed a compression test on my 6.2 today and got the following results.

1- 395
2-390
3-335
4-410
5-345
6-400
7-390
8-390

Cylinders 3 and 5 are low and below the spec of 380psi. I retested those cylinders twice to confirm as well. My question is, is there any way to narrow down the cause? Whether rings, valve/head issues, head gasket, etc? I already plan on doing some head work this winter but would like to know (if possible) if the piston/rings should be on my list as well.

 

[ezgn]

I performed a compression test on my 6.2 today and got the following results.

1- 395
2-390
3-335
4-410
5-345
6-400
7-390
8-390

Cylinders 3 and 5 are low and below the spec of 380psi. I retested those cylinders twice to confirm as well. My question is, is there any way to narrow down the cause? Whether rings, valve/head issues, head gasket, etc? I already plan on doing some head work this winter but would like to know (if possible) if the piston/rings should be on my list as well.

You can do a leak down test. If you get air leaking out of you radiator you have a blown head gasket. Air out of your exhaust would indicate a exhaust valve leaking. Air out of your manifold would indicate a leaking intake valve. Air out of your oil dip stick or oil fill tube would indicate rings or cylinder problem. On gas engines and I am assuming diesel would be similar you can put a couple of teaspoons of oil in the cylinder and do a compression test, if the compression comes up considerably that would indicate rings.

 

[Barrman]

Pouring a cap full of oil into the cylinder through the glow plug hole and then doing a compression test will boost the psi if it is rings. No boost and it is valves or head gasket.

 

[ezgn]

I performed a compression test on my 6.2 today and got the following results.

1- 395
2-390
3-335
4-410
5-345
6-400
7-390
8-390

Cylinders 3 and 5 are low and below the spec of 380psi. I retested those cylinders twice to confirm as well. My question is, is there any way to narrow down the cause? Whether rings, valve/head issues, head gasket, etc? I already plan on doing some head work this winter but would like to know (if possible) if the piston/rings should be on my list as well.

You should be able to use the hose from your compression gauge. Just unscrew the hose from the gauge and put an air hose fitting on the hose, then use your glow plug fitting and put 70 psi into the cylinder to do a leak down test.

 

[adf5565]

You should be able to use the hose from your compression gauge. Just unscrew the hose from the gauge and put an air hose fitting on the hose, then use your glow plug fitting and put 70 psi into the cylinder to do a leak down test.

Thanks! I was just thinking about doing something like that. If there are leaks into the intake or exhaust, is it typically pretty easy to tell?

 

[ezgn]

Thanks! I was just thinking about doing something like that. If there are leaks into the intake or exhaust, is it typically pretty easy to tell?

You will definitely hear the valves leaking. If the valves are in need of replacement you will hear air leaking through the tail pipe or the manifold. You will need to put each cylinder at top dead center to check the valves. There are a few different ways to determine top dead center. Once you are hooked up to your compressor the gauge on your compressor will also be an indicator. If you don't leak at 70 psi you can increase to 100 psi max.

 

[Barrman]

Does the engine run good? If so, don’t stress too much about those 2 cylinders. Yes, they are below spec but unless it is the head gasket. You probably won’t feel the difference.

 

[87cr250r]

Does it blow bubbles in the coolant when running?

 

[adf5565]

Does the engine run good? If so, don’t stress too much about those 2 cylinders. Yes, they are below spec but unless it is the head gasket. You probably won’t feel the difference.

Yeah it seems to run fine. Just trying to get a feel for motor condition before I go about putting money into it, and the plan is to install a turbo so I want to make sure everything is intact beforehand.

 

[adf5565]

Does it blow bubbles in the coolant when running?

Also a great point. I'll check that this week, it will help with the troubleshooting.

 

[Mullaney]

Yeah it seems to run fine. Just trying to get a feel for motor condition before I go about putting money into it, and the plan is to install a turbo so I want to make sure everything is intact beforehand.

.
If the truck is blowing bubbles into the coolant, the leak is pretty bad.
I'm sure there are other brands - but what about this? (About $10)

 

[ezgn]

Yeah it seems to run fine. Just trying to get a feel for motor condition before I go about putting money into it, and the plan is to install a turbo so I want to make sure everything is intact beforehand.

Here is an article about turbos that may interest you. Scroll down to #138 and #139https://www.steelsoldiers.com/threads/84-m1009-cucv-chevy-blazer-build-up.77114/page-7

 

[adf5565]

.
If the truck is blowing bubbles into the coolant, the leak is pretty bad.
I'm sure there are other brands - but what about this? (About $10)

View attachment 885594

Interesting, I’m guessing if there were head gasket issues this product would show in the oil?

 

[adf5565]

Here is an article about turbos that may interest you. Scroll down to #138 and #139https://www.steelsoldiers.com/threads/84-m1009-cucv-chevy-blazer-build-up.77114/page-7

I’ve read through that thread before but forgot about that part specifically so thanks for the reminder! I am planning on the ARP head studs so using that sealer is an important point

 

[Mullaney]

Interesting, I’m guessing if there were head gasket issues this product would show in the oil?

.
Yes sir! That is what that product or something like it was made for. You might find a problem before the oil looks like mud.

 

[cucvrus]

I had several with bad head gaskets and not leaking into the oil. CUCV head gasket / head/ or change engine survey | SteelSoldiers
For your reading pleasure. Happy Holidays. The bottom ends are strong the heads and head gaskets are the weak part.

 

[adf5565]

Alright I finally got around to performing these tests. I’ll go through my process and y’all can let me know if I did something wrong since my results seem inconclusive.

To start, i ran the motor to see if there were bubbles in the radiator. All seemed well there, no bubbles. Then I proceeded with the pressure test.

I based the cylinder TDC on the fluid damper balancer marks (which are every 90 degrees). Cylinder 5 should be 360+90 degrees as it is the 6th cylinder in firing order. I also confirmed this by rotating the motor 90’ at a time (across 2 full crank revolutions) and giving cylinder 5 a few psi and gauged cylinder location based on where air was easily escaping (intake or exhaust). That also confirmed my original #5 TDC location.

At TDC, I gradually increased the air to ~50psi . Air was heard/felt out of the intake as well as a small amount from the dipstick. Much less air than when the cylinder was in the intake stroke, but still noticeable.

Based on this, I was going to conclude intake valves and cylinder rings were the issue. However I decided to check cylinder 1 (which had no compression issues) to compare. This cylinder, at TDC and similar psi (40-50), also had air escaping from the intake as well as the dipstick.

So now I’m not really sure what to think. Based on the fact the motor runs fine I think I’ll just move forward without rebuilding the bottom end. But still seems very odd.

 

[ezgn]

Alright I finally got around to performing these tests. I’ll go through my process and y’all can let me know if I did something wrong since my results seem inconclusive.

To start, i ran the motor to see if there were bubbles in the radiator. All seemed well there, no bubbles. Then I proceeded with the pressure test.

I based the cylinder TDC on the fluid damper balancer marks (which are every 90 degrees). Cylinder 5 should be 360+90 degrees as it is the 6th cylinder in firing order. I also confirmed this by rotating the motor 90’ at a time (across 2 full crank revolutions) and giving cylinder 5 a few psi and gauged cylinder location based on where air was easily escaping (intake or exhaust). That also confirmed my original #5 TDC location.

At TDC, I gradually increased the air to ~50psi . Air was heard/felt out of the intake as well as a small amount from the dipstick. Much less air than when the cylinder was in the intake stroke, but still noticeable.

Based on this, I was going to conclude intake valves and cylinder rings were the issue. However I decided to check cylinder 1 (which had no compression issues) to compare. This cylinder, at TDC and similar psi (40-50), also had air escaping from the intake as well as the dipstick.

So now I’m not really sure what to think. Based on the fact the motor runs fine I think I’ll just move forward without rebuilding the bottom end. But still seems very odd.

An actual leak down tester has areas that measure the amount of air escaping. If the volume is acceptable it will measure in the green area. You will get air seepage but the amount is what is important.

 

[adf5565]

An actual leak down tester has areas that measure the amount of air escaping. If the volume is acceptable it will measure in the green area. You will get air seepage but the amount is what is important.

Ah okay that makes more sense now.

 

[nyoffroad]

I've never done a leak down test so keep that in mind. TDC=Top Dead Center, what that means is that piston is all the way at the top of its stroke. Assuming that you have the engine turned TDC on the COMPPRESSION STROKE then both valves are closed. But as soon as you put compressed air into the cylinder the piston is going to move down (correct?) When that happens one or the other valve will get cracked open and thus an air leak.
IDK, Maybe I'm missing something or not.
If the lower pressure really bothers the OP then I'd do (or not) a dry then wet compression test and plan on a rebuild. Does it matter if it's a valve or rings? Personally I've never rebuilt a lower without also doing the heads and learned long ago on a gas engine, if it needs head work chances are it will also need block work.