I know i have a bad head gasket, but i didn't know until the car utterly lost power one day driving it until it stalled on me. it all happened in like 10 seconds - i was going up a hill, and realized i was going too slow, tried to speed up again, and it stalled. I don't think the engine overheated. I didn't look at the temp gauge right away, but i looked at it about a minute afterwards and it was normal/low.

My mechanic is telling me that if we take the top end off only to find out that there's real damage to the block or pistons (i'm reaching a little here, not a mechanic at all myself) that would be the worst case because then i'll be paying him for the labor of tearing it down and and still throwing the whole engine away for a new one, whereas if we know it's beyond repair i can just get a new engine.

So I'm wondering if we simply start the engine - and if the engine appears run normally at least for a short while - would that mean that the engine damage is minimal and that i should expect to just have the heads machined and replace the gasket? Or is there another way to try to figure out the extent of the internal damage before doing the work of removing the heads so i can just swap the whole engine i needed?

  • If water has mixed with the engine oil and is milky in color, then engine damage is for sure, it has damaged all the bearings on the crankshaft for sure. Other than that, there is no good way to determine other heat related damage until it is torn down and inspected.
    – Moab
    Sep 17, 2019 at 16:41

2 Answers 2


Unfortunately the temp gauge isn't always the most reliable indicator of whether or not it has overheated - the rapid loss of power into a stall leads me to think a fairly sudden loss of compression on at least one cylinder. Which may be a result of the headgasket failure directly or may indicate other damage. But what caused it to lose power in this way won't have changed.

If you do your proposed test and the engine starts and runs I'd say it's an encouraging sign but not a sure-fire indicator that the engine is ok. Equally it may well (as I suspect) start and run poorly - but this doesn't necessarily mean that it's scrap either. A sufficiently severe failure of the head gasket that it's losing compression could cause it to run badly (if at all) but still not indicate that there is more severe internal damage.

I suppose what I'm trying to get at here is that your mechanic is correct - the only real way to tell one way or the other if there's significant internal damage is to strip the head off and have a look.

You could have your mechanic do an inspection using a borescope but there's still time (and hence labour cost) associated with doing that and you're only going to be able to get a limited amount of information that way.

It frustrating but there's no way around it I'm afraid.

  • thanks for taking the time to reply, i appreciate it
    – tntwyckoff
    Sep 17, 2019 at 15:17

What you need is to have a mechanic perform a cylinder leak-down test. It is inexpensive and does not require the removal of the heads. If there is a leak, it will be possible to determine where it is by where the air is coming out. That should be the first suggestion of a professional mechanic; someone who wants to start tearing the cylinder head off is not competent. Find a different mechanic.

Here is a professional mechanic working through a cylinder leakdown test: https://www.youtube.com/watch?v=YPfL4TOU_eM

  • way cool, sounds promising. cheers.
    – tntwyckoff
    Sep 19, 2019 at 14:06

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