M42 Overheated...No start (with video)

[Vivek]

Backstory:
I was driving home, playing with a new radar detector, and the coolant pipe on the block snapped, dumping out all my coolant. Then temp gauge was all the way red but the car ran fine. I have no clue how long it was in the red.
I towed it home, fixed the part, filled up coolant, and went to drive it. It started and ran fine, but still overheated. I made sure to not let it get way too hot (didn't go far) and drove home, assuming I needed to bleed it. I was busy and it sat for a couple of days at this point, and when I went to start it again to bleed it, the engine barely cranked and wouldn't start. It still has trouble cranking and won't start, even when jumping it.

Compression (as I remember, i lost all the data on the phone I had recorded it on):
Cyl 1: 165
Cyl 2: ~60-75
Now after these two, I had to run so I couldn't finish the test. The low compression on cyl 2 worried me.
Tested it the next day and cylinders 3 and 4 were low also (50-75), but at the same time I tested those, I tested cylinder 1 again and it only got to around 50.

Stomp test is throwing a AFM code. I swapped the AFM with a known working one and there was no change. It's getting fuel and spark.

Video (note the pssh sound at ~:06 is something I hadn't heard before. Also this video is with the AFM disconnected, but it won't start either way):

http://youtu.be/yg6_SP4dJYE

My best guess/fear is that I fucked my piston rings, which are now scraping against the block and causing it to have trouble cranking, low compression, and not start. Any help is appreciated. Thanks!

[bmwman91]

How's your oil look? Overheating the M42 often leads to a cracked head. Your compression numbers are low, but then again the engine was stone cold when you did it.

Are you getting fuel and spark? It's odd that it does not even seem to be trying to fire at all. Sorry to hear about your trouble.

[DesertBMW]

yeah the pistons expanded too much rubbed then when the engine cooled down the pistons shrank pinching the piston rings, and now you have no compression. Cranking it will just wet the plugs and foul them.

Do this:

1: remove plugs.
2: put some little amount of thick oil in each cylinder,
3: make sure plugs are dry, put them back in
4: remove fuel pump fuse
5: charge battery, put good charger on it
6: with wide open throttle crank engine, let it crank for 30 seconds then while still cranking put in fuel pump fuse. Once engine fires keep it up in revs.

At this point cylinder head probably developed a crack, blue devil block sealer additive can help for few days.

[Vivek]

How's your oil look? Overheating the M42 often leads to a cracked head. Your compression numbers are low, but then again the engine was stone cold when you did it.

Are you getting fuel and spark? It's odd that it does not even seem to be trying to fire at all. Sorry to hear about your trouble.

It seems to have burned a bit off but there's no signs of water in the oil. It's getting fuel and spark

yeah the pistons expanded too much rubbed then when the engine cooled down the pistons shrank pinching the piston rings, and now you have no compression. Cranking it will just wet the plugs and foul them.

Do this:

1: remove plugs.
2: put some little amount of thick oil in each cylinder,
3: make sure plugs are dry, put them back in
4: remove fuel pump fuse
5: charge battery, put good charger on it
6: with wide open throttle crank engine, let it crank for 30 seconds then while still cranking put in fuel pump fuse. Once engine fires keep it up in revs.

At this point cylinder head probably developed a crack, blue devil block sealer additive can help for few days.

So even if I do that, it looks like the engine's kinda fucked, right? With a cracked head, and damage to the rings, pistons, and block I might as well just swap in a running one.

Cranking it will just wet the plugs and foul them.

You're right, it seems that all I'm doing is getting gas on the plugs.

[wazzu70]

Did you take all the plugs out when cranking to do the compression test?

If you have low numbers this way, put some oil down the spark plug hole and try again. If this helps, your compression....your rings are dead.

If you take a spark plug out and ground it to the valve cover while cranking, does it spark?

[Vivek]

Did you take all the plugs out when cranking to do the compression test?

No, pulled them one at a time. Does this matter? It was the first time I've done a compression test

If you have low numbers this way, put some oil down the spark plug hole and try again. If this helps, your compression....your rings are dead.

I could try that this weekend

If you take a spark plug out and ground it to the valve cover while cranking, does it spark?

Yes it does

[Dj Buttchug]

yeah the pistons expanded too much rubbed then when the engine cooled down the pistons shrank pinching the piston rings, and now you have no compression. Cranking it will just wet the plugs and foul them.

Do this:

1: remove plugs.
2: put some little amount of thick oil in each cylinder,
3: make sure plugs are dry, put them back in
4: remove fuel pump fuse
5: charge battery, put good charger on it
6: with wide open throttle crank engine, let it crank for 30 seconds then while still cranking put in fuel pump fuse. Once engine fires keep it up in revs.

At this point cylinder head probably developed a crack, blue devil block sealer additive can help for few days.

what are you talking about??

op, pull all plugs and do a good compression check. 5 cranks per cylinder and report back
You pull all the plugs so you are not fighting compression on other cylinders.

[DesertBMW]

op, pull all plugs and do a good compression check. 5 cranks per cylinder and report back
.

Why? We already know the there is no compression.

So even if I do that, it looks like the engine's kinda fucked, right?

I bought a 1991 318is with the same problem you have, and I was able to revive it with the steps I gave you.

[kwill22]

never heard of that.. and he checked the compression with the plugs in removing them one by one. it can give a false reading.

i think you probably cracked the head.

[bmwman91]

Yeah, you need to pull all the plugs for the comp test. Leaving them in will lower the numbers, especially since you are usually supposed to do it with a fully warmed engine.

I've never heard of what DesertBMW was saying, but who knows. Never seen it myself, but I have also not overheated an engine.

In the video, it sounded like the battery was dying. If it was fully charged, then something is pretty fucked because the starter was struggling. Really, a cracked head or busted HG is what I would expect form overheating. Ruined rings and stuff are also possible, but you'd need to have it be overheating for a while (which may have been the case it sounds like).

Honestly, at this point, I think I'd just suck it up & pull the head to look around. Chances are that it'll need a weld & decking no matter what, if not replacement with a good used one (and a decking).

[brads2002]

I can see what desert is saying. If the rings are pinched out his method might pop them back in to the piston.

[Dj Buttchug]

im sorry but theres no way a ring can pinch in the piston ring grove from a slight overheat. Desertbmw, i dont know where youve seen this other than maybe a sized engine but I too HAVE NEVER seen this

op. if the engine was still running great before you noticed the temp needle in the red chances are the bottom end is not damaged from heat. it takes alot to waste a bottom end from heat

Now you could have cracked the head... or you popped a hg.


Where are our compression numbers?

[bmwman91]

I can see what desert is saying. If the rings are pinched out his method might pop them back in to the piston.

I guess I'm not seeing how the rings are going to get pinched out. The ring grooves will grow wider as the piston undergoes thermal expansion. Now, the piston will expand more than the block since aluminum has a higher coefficient of thermal expansion than iron, and I could see you MAYBE wearing the shit out of the rings, but I doubt that you can really get the engine hot enough to expand things THAT much without other major issues first.

Or is the idea that the piston expands a bunch and the ring ends get popped apart and don't mesh back together when things cool?

im sorry but theres no way a ring can pinch in the piston ring grove from a slight overheat. Desertbmw, i dont know where youve seen this other than maybe a sized engine but I too HAVE NEVER seen this

op. if the engine was still running great before you noticed the temp needle in the red chances are the bottom end is not damaged from heat. it takes alot to waste a bottom end from heat

Now you could have cracked the head... or you popped a hg.


Where are our compression numbers?

My thoughts exactly.

The Conclave of M42 knowledge demands numbers so that it may render a verdict and possibly talk shit until such a time that the Conclave tires of the talking of the trash. Submit!

[Vivek]

Okay I'll have new compression numbers doing it the right way in the next few hours. I'll also squirt some oil in the cylinders and see how that affects it. Any specific way to do that? I'd think that if I just poured a few drops of 20-50 in the spark plug hole it'd just sit on top of the piston.

[Dj Buttchug]

do a test first without oil. I want to see what the compression is before you wet test the rings which usually/ almost always yields in higher numbers.
also: the oil in the combustion chamber will not sit on the piston top during cranking. compression spreads the oil to the cylinder wall/rings.

dont forget either. 5 cranking cycles. not 8. not 10. just 5.