3.0 poor compression

[BK911]

At a recent DE event, my car ran awesome and gave no indication anything was wrong, but I was told my engine was smoking a little bit. So I did a compression check when I got home and found 160 – 165 across the board except #4 was 100. I couldn’t find my leak down fixture, so I set #4 at TDC and pressurized it to 100 psi to listen for a leak. I couldn’t hear anything. Then I did a wet compression check on #4 and got 120. A little bit better, but still not good enough.

Any ideas on what it could be? The engine has about 3k miles on a complete rebuild from a reputable shop. Everything is new except the pistons and cylinders. She seems to have poor compression but good leakdown. I am going to pull the valve covers and look for a broken head stud. Is there anything else I could do? Would you run it as is or tear it apart?

Thanks!
BK

[justinp71]

Maybe the cam timing is off and letting the air leak out during compression test ....

Or they replaced the piston with one that has the wrong height or wrist pin height, I don't have any experience with this, just a thought...

[304065]

Find the leakdown tester, then find the leak.

The diagnostic capability of a leakdown is really amazing. I did multiple tests on my '66 engine and discovered 45% leakdown in a few of the cylinders. . . when I tore it down, the top rings were broken on FIVE of the rings. The leakdown tester does not lie.

You should try to have the engine at operating temperature when you do the test, but be extremely careful removing the plugs from the engine if it's hot. And I don't mean for your personal safety, although that is a consideration: you don't want to accidentally remove the helicoil in the head along with the plug!

Don't use 100 PSI for the test, either. When I did my test, I used about 35 psi, it makes the test more accurate and there is less risk of accidentally turning the engine over with a ratchet attached to the crank.

Once you have a good, repeatable test for all six cylinders, then you can think about tearing it down.

Any oil leaks?

[BK911]

Thanks guys. I will spend more time looking for the leak down fixture and I will pull the valve covers off tonight and look for broken head studs.