Fresh Rebuild Coolant in Exhaust/Exhaust in Coolant - Cracked Head?

[thisismedriving]

I recently finished my 2.9 stroker build and everything was going great. Unfortunately, 200 miles into the break-in period (and probably 10+ heat cycles) and the car starts misfiring and blowing steam out the exhaust. The temp gauge also creeps above halfway (but never makes it to the red) and the cooling system is clearly pressurized by exhaust gas. (Elring HG with Elring head bolts, all torqued to spec in the prescribed sequence)

I pulled the head and the gasket wasn't obviously failed anywhere, so my next step was to have the head pressure tested for cracks and flatness (this was a used head with unknown history and my original machinist had commented that the block had clearly been overheated due to signs of piston rings losing tension in the cylinder walls, but decked the head and claimed it should run forever). The (new) machine shop I brought it to said it checked out fine and that nothing was obviously wrong with it (although they aren't BMW "specialists" (do those exist anywhere anymore?), they build racing engines and seem competent). There are no cracks visible to the naked eye and I visually inspected the coolant passages and there wasn't any obviously horrible corrosion around the edges, but that's hardly definitive.

The block was fully checked out and given a clean bill of health at my original machine shop before I assembled the motor, plus everything I've read (and every mechanic and machinist I've spoken to) says a cracked block/cylinder would be both rare and obvious (especially to a machinist boring it out to 85mm) and I'd hate to pull the bottom end out of the car and apart for nothing, so...

I reassembled everything with a fresh Corteco gasket (in case the Elring was a problem) and ARP head studs (to eliminate the TTY bolt variable) and burped the cooling system. Car got up to temp and idled fine (and cool) for ~15-20 minutes before I took it out for a test drive. I took it to a gas station about 5 miles away to fill it up and all vitals were good (good oil pressure, temp gauge was staying right below the middle). When I left the gas station, I drove a couple of more miles before flooring it from ~45 to ~70 mph (hitting maybe 6000 rpm) and the tailpipe immediately started bellowing white steam (worse than ever). The temperature gauge started climbing around this point and it was misfiring badly on at least one cylinder, so I babied it home and now I'm at a loss.

I'm back to thinking there's a crack in the head that only becomes obvious when the car is fully up to temperature. I'm thinking my next step is to have another (known good) head off a different motor (the one that was originally in my car, this time) machined and swap my cam and valvetrain in and try again, but I also don't want to waste my time chasing the wrong gremlins if that wasn't the original problem. Unfortunately, paying for more diagnostic time on the (presumed) bad head doesn't seem like a smart bet, either.

Does anyone here have any input on whether I'm on the right track or anything else I should be looking for? I'm already living on borrowed time to drive this car much more this season, but I'll be ruminating about this all winter until I diagnose and address the problem. TIA!

[e30davie]

That whole story is a bit of a bummer.

If it were me, I think a coolant pressure test whilst it is still assembled will tell all, but you'll need a borescope to see which cylinder it is going into (and be careful not to fill the cylinder with water - and dont start it if you do). I am not that familiar with where exactly m20s typically crack between to the water galley but if its in the ports you might be able to see it better with the manifolds off. (if you can get the intake off whilst retaining the coolant, i don't think you can actually....)

[digger]

can you check the spark plugs to see if one or more look different?

[82eye]

go for a quick run but don't overheat. then check your plugs. which one is the cleanest? concentrate on that cylinder. if one is pristine you have a good clue.
even brand new plugs on a new engine will get a little white build up on them if they are firing well after running a short time. the clean plug will have been kept steam cleaned in the failed cyl.

[thisismedriving]

can you check the spark plugs to see if one or more look different?

The first time this happened, I pulled the plugs and 6 was obviously fouled and wet with (blue) coolant. The piston crown and combustion chamber were notably cleaner than the other cylinders, too (steam cleaned). Cylinder 3 seemed to have similar symptoms, but less pronounced.

It's obvious to me which cylinder(s) is/are affected. The question to me is why. If it's definitely a cracked head, then the next step is obvious. If it's not, then I need to figure out where to look next.

[82eye]

not sure what is left aside from a pressure test and remove the head for inspection. had heads on cars crack both to the inside and outside. only found internal cracks after removal. outside ones are obvious.

[thisismedriving]

not sure what is left aside from a pressure test and remove the head for inspection. had heads on cars crack both to the inside and outside. only found internal cracks after removal. outside ones are obvious.

Yes, I had removed the head, taken it to a machine shop, had it pressure tested, and they found nothing. Looking for ideas for how else I can diagnose this.

Is it possible a pressure test at room temp wouldn't conclusively find a crack that reveals itself at operating temp?

[82eye]

Yes, I had removed the head, taken it to a machine shop, had it pressure tested, and they found nothing. Looking for ideas for how else I can diagnose this.

Is it possible a pressure test at room temp wouldn't conclusively find a crack that reveals itself at operating temp?

pressure test should find it no matter. if it's not cracked something is either warped or not mating properly. it could be either the head or block. block itself could also be cracked but that again is generally easy to find.

[thisismedriving]

if it's not cracked something is either warped or not mating properly. it could be either the head or block. block itself could also be cracked but that again is generally easy to find.

Yes, as stated. The head was milled and confirmed flat by two machine shops, though. There are no obvious cracks in the block or coolant in the oil, which you would expect if the block itself failed.

[82eye]

Yes, as stated. The head was milled and confirmed flat by two machine shops, though. There are no obvious cracks in the block or coolant in the oil, which you would expect if the block itself failed.

are you actually losing coolant? if you are it is going somewhere. if it's bad enough to be making white smoke it's significant. there's a fail somewhere, and it results in the head coming off in every circumstance.
you started with a known good flat block and head, they may no longer be that way at present.

with luck it's just a head gasket. probably poor from factory. it may not have overheated to the point of warping a head.

[thisismedriving]

are you actually losing coolant?

Yes, obviously it's going out the tailpipe...

if you are it is going somewhere.

No shit.

there's a fail somewhere

NO SHIT?

it results in the head coming off in every circumstance.

No shit. The head HAS BEEN OFF and checked for cracks and flatness by a machine shop. That's why it's so frustrating that the same failure happened again after putting everything all back together.

you started with a known good flat block and head, they may no longer be that way at present.

The head was checked for flatness both before AND after failure. The engine was not overheated at any point since I assembled it, especially before the first failure, so there's no reason anything should have warped in the meantime.

with luck it's just a head gasket. probably poor from factory.

Thanks for your insight. If a manufacturing defect were the root cause, this wouldn't have happened twice in a row with two different brands of gasket.


Thanks and no (okay some) offense, but I think I'd prefer for you to step back and let someone else weigh in if you're not going to read anything I've written. You haven't actually given any pertinent advice and we're not really getting anywhere.

[reelizmpro]

The head could still be cracked somewhere. They would need to pressure test it while at operating temp to know for sure. There is a dye they can use to pinpoint leaks. Perhaps find another shop that specializes in cylinder heads and have them test it? They might have better equipment or more insight on aluminum heads. You could also try another head in the meantime if you can find one locally.

[thisismedriving]

The head could still be cracked somewhere. They would need to pressure test it while at operating temp to know for sure. There is a dye they can use to pinpoint leaks. Perhaps find another shop that specializes in cylinder heads and have them test it? They might have better equipment or more insight on aluminum heads. You could also try another head in the meantime if you can find one locally.

I'm suspecting exactly what you've suggested re: dye and testing at operating temperature. I just don't want to throw a bunch of money at this head which (in my mind, at least) is probably bad just to confirm my suspicion when I could put that money towards prepping a known good head.

I have another head (off the good running 2.5 that came out of my car before I swapped in the stroker), so I'm thinking that even if I have the machine work done and it doesn't solve my issue, I'll at least have one head (or maybe two) that I know is good. It's kind of a lot of work to swap again without knowing for sure, though, which is why I was hoping I could find a smoking gun in the meantime.

[e30davie]

Are you sure theres not something really dumb we're missing here. ive done a top end rebuild on an m20 and i cant think of anything too useful....isn't there water in the throttle body on some cars? that cant get sucked into the intake manifold somehow if that fails?

I mean if you've got a 2nd head there, you'd think your luck cant be bad twice. id go down that route, maybe with a different shop...?

[thisismedriving]

Are you sure theres not something really dumb we're missing here.

Nope, that's why I'm asking the community.

isn't there water in the throttle body on some cars?

Yep.

that cant get sucked into the intake manifold somehow if that fails?

Nope.

I mean if you've got a 2nd head there, you'd think your luck cant be bad twice.

That logic only works if a bad head caused the problem in the first place.

id go down that route, maybe with a different shop...?

Yeah, that's the provisional plan, unless someone else has other ideas I can try in the meantime.