head gasket and low compression

[c4racer]

Need some advice. Our P5 overheated on the highway this week and radiator blew out. I replaced it only to learn the car would not start. So I did a compression test and came back with:

100-50-50-75

Normal spec is 170 for the 2L engine according to my service manual.

So clearly the headgasket is fried. But is this low of compression enough to prevent the motor from running?

Does anybody have any experience with this?

I need to pull the head at a minimum, maybe the motor - but just wanted to make sure there isn't also something else going on here.

 

[CrazyCaker]

It's not just the gasket that you fried... if you over heated badly enough to blow the rad, I think it's pretty safe to guess you have warpped the head.

 

[pasadena_commut]

So I did a compression test and came back with:

100-50-50-75

Normal spec is 170 for the 2L engine according to my service manual.

So clearly the headgasket is fried.

Please don't take this the wrong way, but since a similar compression test came up in this forum about a week ago...

Did you rotate the crankshaft before testing each cylinder so that all valves on that cylinder were closed?

If so, and those numbers are real, then the car might also have warped the head, cracked the block, or suffered a couple of other equally heinous injuries. Unless you do a lot of engine work this might be a good time to consult with a mechanic.

 

[flash75]

"Did you rotate the crankshaft before testing each cylinder so that all valves on that cylinder were closed?"
Closing valves for a compression test? No. The engine spins during a compression test, valves open and close as the as the engine spins. Maybe you are thinking of a leak down test.

"So clearly the headgasket is fried. But is this low of compression enough to prevent the motor from running?"
Yes that is low enough to keep an engine from running.

Clifton

 

[c4racer]

"Did you rotate the crankshaft before testing each cylinder so that all valves on that cylinder were closed?"
Closing valves for a compression test? No. The engine spins during a compression test, valves open and close as the as the engine spins. Maybe you are thinking of a leak down test.

"So clearly the headgasket is fried. But is this low of compression enough to prevent the motor from running?"
Yes that is low enough to keep an engine from running.

Clifton

Right - I did a compression test, not a leakdown test. I didn't see the point of doing a leakdown although I do have a gauge and I do understand how to use it properly. But I digress..

I guess I am coming more from a V8 perspective where a blown head gasket on one bank is not normally enough to keep the engine from running - but I guess at least 3 of the 4 cylinders are leaking very badly, which is 75% of the motor, so that is a lot different than a V8 missing 2-3 cylinders where it will still run although badly.

And I fully expect the head to be warped - it comes down can it be cleaned up with a surfacing. And my other concern is did the rings also get damaged in the process. The motor was not run for very long, but newer motors can have low tension rings and they may not seal well after this overheating. and the tough part about that - there is no way to know until I fix the head, install it and then do a leak-down test.

And yes - I am a mechanic and have rebuilt engines before, so none of this scares me. My experience is mostly limited to pushrod V8's, however. So there will be some learning curve.

If I find a decent used engine it would also be a lot easier to do a swap than digging into this one, so I may go that route too.

 

[c4racer]

luckily there is no water in the oil, so that gives me hope the bottom end is salvagable.

But if I find a complete used engine with reasonable miles, I may just swap that in instead if the economics of it make sense.

 

[jlnoury]

I really think that maybe that there is 2 possibilitys: 1) the fact that there is no cyl that has 0 psi means that the engine was overheating enouft to burn the head gasket but not overheating enouft to make some big big damage. 2) The engine was high-overheating and there is really a big engine problem. For experiance, an engine overheating that kills the head gasket at 90% of chances to give this compression test (150-0-140-160) or like (150-140-0-0). Questions for fun: dose the engine starts fast?
Find a sparkplug filted air hose (to put some 90 psi compressed air in the hole that the spark plug has in). fill the radiator of water and turn slowly the crankshaft to see if there is littles bubbles that come out of water in the radiator. if there is some bubbles, head gasket is damage. if there isin't no bubbles, head gasket is good. there is an other probleme in the engine.

 

[pasadena_commut]

Right - I did a compression test, not a leakdown test.

... and I had a brain fart and thought "leakdown" when I read "compression".

Sorry.

Compression test tells you there is a leak somewhere, but as jlnoury said, pressurize the cylinders and it can tell you what type of leak it is. Hopefully it will be into the radiator or out the side between the head and the block and you are only looking at a head gasket. If there are valve problems too, even on a cylinder with just 50 psi on a compression test, you will still be able to hear or feel air going out the exhaust or the intake. Ring problems it goes into the crankcase (and the compression test goes up, leakdown rate goes down, if you squirt some oil into the cylinder). Cracked block, maybe you hear a whistle or hiss down on the side of the motor. Easy enough to do that you might as well get this information before you do anything else.