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Did a compression check. Thoughts?

B

beyondloadedSE

Hard-core CEG'er
Joined
Sep 29, 2000
Messages
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Location
Louisville, Ky
As some of you have seen or heard, my engine burns oil...well something is burning oil. I thought it was oil dripping down into the spark plug holes from bad valve cover gaskets, but ive since fixed that and the problem still persists. I generally have to add a qt of oil every 300 miles or so. Its burned oil since day one when I fired it up. In my opinion, it has slightly gotten worse though.

I did a compression check this weekend and here were the results:

Cylinder 1: 120 psi
Cylinder 2: 120 psi
Cylinder 3: 120 psi
Cylinder 4: 125 psi
Cylinder 5: 100 psi
Cylinder 6: 120 psi

Im not really sure what to think about the cylinder with 100 psi. Not extremely terrible, but again, its lower than the 10% difference that everyone says it should be with in. I should have done a compression check the day the engine fired up, so I could have known if this is a new problem. At this rate, im hoping the turbo seals on the turbo are bad and thats what been causing the oil consumption.

To me, my hp/tq numbers do seem to be down, unless the non svt cams are just robbing me of power. SZ 05, put down 364 whp, 410 wtq with a pull to 4,700 rpms. Stazi read the boost at 16 psi. Could have been higher..who knows. He was the only witness. This year, I make 354 whp, 350 wtq @ 14 psi with a clean pull. Not really sure what to think.

Anyone have any ideas what might cause a cylinder to lose about 20-25 psi of compression? Cracked ringland? Would this cause that much oil consumption? Any thoughts are welcome.
 
Anything below 170 is something to start worrying about. Looks like maybe you got the wrong piston rings or maybe the machine shop didn't measure the pistons before boring and simply "bored to spec" and the pistons where "on the small side" of the spec :shrug:
 
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Keyser said:
Jim was your throttle body open when you did the compression test? It can make a substantial difference.
Click to expand...
How so ... most gauges build "peak pressure" over multiple compression strokes when both valves are closed. How I ran my tests:

1) Remove spark plug and screw in compression tester
2) Pull connector from coil pack
3) Pull fuel pump relay/fuse
4) Crank engine for 10 seconds
5) Take reading

My tester has a peak hold function and when I'm done, I simply jot down the reading, press the button to releave the pressure in the tester and then move onto the next cylinder.
 
If you suck all the air in the intake manifold out, and the blade is closed, will it not lose momentum during cranking? On a 2JZ engine it can make a 20psi difference.

Edit: I believe Terry Haines made the same reference in the old forums as well.
 
beyondloadedSE said:
As some of you have seen or heard, my engine burns oil...well something is burning oil. I thought it was oil dripping down into the spark plug holes from bad valve cover gaskets, but ive since fixed that and the problem still persists. I generally have to add a qt of oil every 300 miles or so. Its burned oil since day one when I fired it up. In my opinion, it has slightly gotten worse though.

I did a compression check this weekend and here were the results:

Cylinder 1: 120 psi
Cylinder 2: 120 psi
Cylinder 3: 120 psi
Cylinder 4: 125 psi
Cylinder 5: 100 psi
Cylinder 6: 120 psi



Im not really sure what to think about the cylinder with 100 psi. Not extremely terrible, but again, its lower than the 10% difference that everyone says it should be with in. I should have done a compression check the day the engine fired up, so I could have known if this is a new problem. At this rate, im hoping the turbo seals on the turbo are bad and thats what been causing the oil consumption.

To me, my hp/tq numbers do seem to be down, unless the non svt cams are just robbing me of power. SZ 05, put down 364 whp, 410 wtq with a pull to 4,700 rpms. Stazi read the boost at 16 psi. Could have been higher..who knows. He was the only witness. This year, I make 354 whp, 350 wtq @ 14 psi with a clean pull. Not really sure what to think.

Anyone have any ideas what might cause a cylinder to lose about 20-25 psi of compression? Cracked ringland? Would this cause that much oil consumption? Any thoughts are welcome.
Click to expand...

A cracked ring land would definitely cause about that much loss and burn lots of oil. That is what mine did and my compression was probably 30psi lower in those cylinders.

Even with 8.5:1 compression I'd expect it to be higher anyway. I'd think 150ish maybe?
Well anyway, it can still make decent compression and burn oil.
I think at 10:1 on my engine my compression was all in the 170-180psi range.

The first stroke would hit probably 120 the second bring it up to 150 and the third or fourth would hit towards 180. This is done as Jim said with the throttle open.

If you didn't test yours that way then maybe you should try it again that way and compare.
 
fastcougar said:
How I ran my tests:

1) Remove spark plug and screw in compression tester
2) Pull connector from coil pack
3) Pull fuel pump relay/fuse
4) Crank engine for 10 seconds
5) Take reading

My tester has a peak hold function and when I'm done, I simply jot down the reading, press the button to releave the pressure in the tester and then move onto the next cylinder.
Click to expand...

This is essentially the same thing we did with Jim's compression test. I don't know if he pulled the fuel pump relay/fuse, but I don't see that making any difference. Sure enough, we tested that one cylinder twice, and it wouldn't go any higher than 100psi. All others were in the 120 range.

Mark
 
I hate compression tests anyway. Do a leak down a find out for sure.

I believe the cam will make a significant difference in compression numbers, but for reference, my factory spec on my 8.5:1 engine is 125-165 cold.

Which brings up another point. Was the engine hot Jim?
 
Keyser said:
I hate compression tests anyway. Do a leak down a find out for sure.

I believe the cam will make a significant difference in compression numbers, but for reference, my factory spec on my 8.5:1 engine is 125-165 cold.

Which brings up another point. Was the engine hot Jim?
Click to expand...

Yes, the engine was hot...hot enough to burn my pinky finger on the rear valve cover while trying to pull off the spark plug wire. ;)

I guess I'll have to give it another go with the TB open this time.

I suppose I should do a leak down test instead too.
 
Ok I will tell you that the compression is right at 120 psi when it is hot. I did mine a moonth ago and we both have 8:5.1 CR so you are good on the other cylinders. When I lost my old engine the one cylinder was down too. It would build compression but took alot of strokes to do so. I had a broken ring land too. Best way to figure it out is to use a leak down tester if you have one I do have one I can loan you is you need it.

But if you checked your intercooler for oil and it is clean then I doubt it may be the turbo. Plus whent he the engine is colder it would seem to burn more oil due to oil pressure higher geting past the seals.

One quick way is to drop a spoon full of oil on top the ring if you don't mind smoke afterwards. It will tell you the rings are good or bad. But I hope it is not that for your sake.

Oh the hp does seem low since I did that run of mine on 15 psi dropping to 13 psi so I wouls say at 14 psi you should have reached 380whp at least too. The torque is lower indicating something changed since then too.
 
easy way to check if it's a ring land or rings themselves, put a drop, and i mean a DROP, of oil in the cylinder and do the compression check again, if that raises the compression readings then u know what it is. Just an idea worth trying.

Robert
 
It would be nice if it was just the rings only and not the ringlands. Id hate to see my Ross pistons be thrown down the toilet because one piston is bad.

Btw, if it were the rings, could I just buy new rings and install them or would I have to get the block rehoned AGAIN.
 
beyondloadedSE said:
It would be nice if it was just the rings only and not the ringlands. Id hate to see my Ross pistons be thrown down the toilet because one piston is bad.

Btw, if it were the rings, could I just buy new rings and install them or would I have to get the block rehoned AGAIN.
Click to expand...

Don't start sweating the treatment till you know the diagnosis. Leak it down, if you need some input on how to do the test hit me up on PM.
 
Chris, I probably wont be able to do a compression check until next weekend. I just transferred colleges so I dont live at home anymore. I'll probably go home next weekend where I can borrow my friends compression tester and leak down tester. Ill keep you posted.
 
Okay, ran another compression test today, this time with the throttle body open. The results were almost identical to the first test in every cylinder. Could a bad valve cause 1 quart of oil to burn roughly every 300 miles and still cause a loss of compression of ~30 psi. Im still thinking its a bad ring or ringland at this point.
 
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