2G Why did I lose compression?

[lapointebrett]

About a year ago my head gasket gave out on the dyno due to the block being pitted and this was from the previous builders error. I rebuilt the engine with all forged parts and had everything machined to be within spec. The tune was almost finished when the head gasket gave out so I decided to break the newly built engine in on that tune.

I recently hit about 900 miles using the Motoman break in method and decided it was time to have the tune finished. Yesterday my car was on the dyno for about 45 minutes, we had done multiple pulls and even made more power than before I rebuilt the engine, the tuner had shut the car off a few times in between pulls to save changes to the ECU, then towards the end of the tune when we were almost done he shut the car off like normal and went to start it back up and of course the car wouldn't start again for some reason.

We checked everything and it all seemed to be working properly so we were very confused as to why it wouldn't start back up, we changed spark plugs just to make sure it wasn't that and still no start so we decided to do a leak down test since it seemed like that was the only option we had left. Cylinder one had 15% leak down and cylinder 2 and 4 had 20%. The big issue was cylinder 3 as it had 80% leak down.

I plan to take the head off and possibly take the pistons out to inspect when I have time next week, but I figured I would ask other DSM'ers to see if anyone has ever had this happen or if they know what happened. This was my first engine build and I'm just confused on what could have happened since I have already put 900 miles on the engine and multiple dyno pulls.

If anyone has any knowledge on the matter it would be greatly appreciated, this was devastating news after I've put so much time into the rebuild and money as well for it to fail basically right at the finish line. All help is appreciated.

 

[pauleyman]

Check mechanical timing and see if it jumped.

 

[99dsmer4g63]

Where was the leakage from?

All while leak down test is running

-Pop off the pcv hose and listen for air in the intake.
-Listen to the exhaust for air
-Take off the oil cap and listen for air

With 80% it should be pretty easy

This where tell you where the problem is.

-Daniel

 

[lapointebrett]

Where was the leakage from?

All while leak down test is running

-Pop off the pcv hose and listen for air in the intake.
-Listen to the exhaust for air
-Take off the oil cap and listen for air

With 80% it should be pretty easy

This where tell you where the problem is.

-Daniel

The air was coming from the oil cap during the test not the exhaust or intake, and I'm pretty sure if timing jumped it would have made a noise if it caused a valve to kiss the piston plus the marks still lined up when putting it to top dead center for the leak down test

 

[99dsmer4g63]

I would suspect something in the bottom end to be the culprit with hearing this.


-Daniel

 

[lapointebrett]

I would suspect something in the bottom end to be the culprit with hearing this.


-Daniel

It definitely seems like it's the rings or something is wrong with the piston but it's just crazy because there was never a noise like anything happened and it didn't die out we just turned the engine off like normal, I've never seen an engine failure be such a mystery

The part that's crazy to me is how it ran 900 miles with no issues but I'll just have to find out when I open it up next week