M42 Overheated...No start (with video)

[Vivek]

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.

So will that be 5 jumps up on the compression gauge?

[kwill22]

Usually about 2-3 to get to its peak and 2 more after for peace of mind and to make sure its completely done. Once it hits peak it should barely move after that

Any news?

[Dj Buttchug]

what killwill said ^

[Vivek]

Okay update time! New compression test. Car was totally cold and it was being jumped (connected the whole time) and getting 13.8 volts.
Cyl 1: 125psi
Cyl 2: 25psi
Cyl 3: 90psi
Cyl 4: 75psi

Poured some (kinda a lot of) oil down the spark plug holes and the results were:
Cyl 1: 250psi
Cyl 2: 160psi
Cyl 3: 225psi
Cyl 4: 250psi

So I put the plugs back in, reconnected my fuel pump fuses, and it started! Drove it around the block a bit to burn off the oil and it seems to be running okay. It was still overheating (didn't let it go past halfway this time but it got there really fast) and the coolant in the reservoir was boiling but it was cold on the other side of the thermostat housing, so it looks like my thermostat is stuck closed.

[Dj Buttchug]

interesting. Theres no way the car would run if the first test compression numbers were correct. Are you sure your psi gauge is accurate?

Anyways. good to here it started up and drove. Figure out whats up with the coolant system and get it bled properly. Then all should be well?

[DesertBMW]

So I put the plugs back in, reconnected my fuel pump fuses, and it started! Drove it around the block a bit to burn off the oil and it seems to be running okay

I am happy that you listen my advice and got your car back on the road.

what are you talking about??

.

I am talking about something that you knew nothing about so I am happy that you learned something today.

interesting. Theres no way the car would run if the first test compression numbers were correct.

It is magic. My advice put him back on the road, and that is what I am talking about.

It was still overheating (didn't let it go past halfway this time but it got there really fast) and the coolant in the reservoir was boiling but it was cold on the other side of the thermostat housing, so it looks like my thermostat is stuck closed.

It is cracked head, the compression is blowing into cooling system. I was able to fix mine with $10 block seal. It will flow into the crack and seal it permanently.

Autozone sells it. K & W Permanent Metallic Block Seal Head Gasket Repair - 401016 (16 Oz), follow the instructions on the back, it worked for me.

Then after it seals the crack and car runs great. Then use Bar's Leaks Liquid Copper™ Block Seal - 1109 (18 Oz) this one keeps the crack shut.

first one you got to flush out, the second one you keep running in the cooling system.

[bmwman91]

Damn, you learn something new every day. How exactly do the rings get pinched / between what?

[Vivek]

interesting. Theres no way the car would run if the first test compression numbers were correct. Are you sure your psi gauge is accurate?

Anyways. good to here it started up and drove. Figure out whats up with the coolant system and get it bled properly. Then all should be well?

I hope so. I'm gonna try to fix my tstat tomorrow. It kinda seems too good to be true though. If it was reading 25psi on cylinder 2 i have my doubts that a bit of oil would permanently fix that. It was still reading well below the others (160 vs 225-250) but I suppose for all I know it could have always been that way. What do you think could be wrong? Worn rings or maybe a crack in the head by that cylinder?

So DesertBMW thinks it's a cracked head, anyone else have an opinion? Would that cause the overheating and bubbling/boiling in the cooling system? I suppose I could try that block sealer...wouldn't have much to lose.

[Dj Buttchug]

I am talking about something that you knew nothing about so I am happy that you learned something today

cool story. You can get off your high horse now


Also OP. If head is cracked fix it right. Theres no such thing as a mechanic in a can.

[bmwman91]

Yeah, cracked head for sure, or at the least a warped head that is allowing combustion chamber gases to be pushed past the HG and into the cooling system. The bubbling in your coolant tank isn't it boiling...it's combustion gases being pushed in. Most auto stores sell a chemical test kit that will tell you if the bubbles in your coolant are from exhaust gases or not, but based on the M42's history, it's most likely a crack between the exhaust valve guides of cylinder #2.

Pay attention during hot starts after it sits for 5-10 minutes...you'll probably see white smoke for 10-15 seconds.

[DesertBMW]

I suppose I could try that block sealer...wouldn't have much to lose.

With the KW you must drain all of the water from the block, the block drain is located on passenger side rear block, 19mm bolt. Remove spark plugs let it all dry for at least 24hrs, if you live in colder weather, allow 2 days.

[wazzu70]

Just replace the head with a known good one. Its not like M42 heads are really expensive!

[roguetoaster]

Just replace the head with a known good one. Its not like M42 heads are really expensive!

Sometimes they can be hard to find, and shipping is often a 25% price increase over base cost. But I do agree with your sentiment, crack sealer is not the right solution in this case.

Replace the stem seals when you get it resurfaced (along with the timing cover) if you get a new to you cylinder head!

[LoneWolf]

Sometimes they can be hard to find, and shipping is often a 25% price increase over base cost. But I do agree with your sentiment, crack sealer is not the right solution in this case.

Replace the stem seals when you get it resurfaced (along with the timing cover) if you get a new to you cylinder head!

This.

M42 stuff isn't as common and cheap as it once was. Still cheaper than most other engines I suppose.

[Vivek]

In all likelihood, if the heads cracked, I may just swap in a new engine. A new head would be 1-200, if I can find one. Then probably a hundred at a machine shop to get it resurfaced, and easily 200 for the headgasket and other seals and gaskets for it. At that point I'd be into it more than the price of a new m42. And I can source one from a friend locally who's swapping his so I know the history and mileage on the engine, and I've driven the car and it's always felt stronger than my m42.