About to dive into a head gasket change

[Guapo]

But I wanted to run the symptoms by the gurus here before wading in neck deep.

1992 525 M50 (10/91 prod) auto sedan. ~140k

I've had an intermittent problems with temps rising when at rest. I suspected the fan cluth, but it is very firm when hot, and seems to be blowing plenty of air. Also, it was changed out by the PO maybe 2 yrs ago.

About 6 months ago, I changed the thermostat, t-stat housing, water pump, and drive belts. I had a hell of a time getting the coolant system bled, but I finally had it fairly reliable. I did suspect a slight leak in the head gasket, as it seemed to build up quite a bit of pressure over the course of a semi-long drive.

This past Monday, I was driving to work and had one of the accessory belt pulleys fail. The bearing completely seperated, and of course I lost both belts. The car got into the red before I was able to get it pulled over, but did not max out.

Yesterday, I installed all new pulleys, tensioners, and belts. Car is running fine, but now it wants to overheat at stoplights again. I've tried bleeding the system as before, and I there is a LOT of pressure buildup in a very short amount of time (10 minutes tops), no matter how many times I bleed it. The upper rad hose gets rock hard.

My thinking is that overheating the car on Monday caused the leak in the head gasket to get worse, causing more combustion pressure to enter the cooling system. I still do not have any fluid crossover at all, but I ran compression to see if there was any discrepancy. From front to back, my compression reads:

130 165 160 168 168 168

I get no smoke of any color from the tailpipes, no oil in the coolant, and no coolant in the oil. Still, the compression numbers and the very high pressures say head gasket to me.

Unless I'm simply overlooking something and anyone has any other suggestions, I need to read up on the Bentley on the procedure. Also, I need to look into getting the special tools required for cam alignment.

Anyone have any thoughts or suggestions?

[Mendozart]

I wonder if maybe you're not bleeding correctly. The m50 bleeds pretty easy, but the first time I did it I wasn't doing it right. With the bleed screw out start engine and fill the reservoir all the way to the top till it starts crossing over. Keep adding coolant as needed and wait till just coolant is flowing from bleed screw with no bubbles showing. Hope this helps.

[Guapo]

I wonder if maybe you're not bleeding correctly. The m50 bleeds pretty easy, but the first time I did it I wasn't doing it right. With the bleed screw out start engine and fill the reservoir all the way to the top till it starts crossing over. Keep adding coolant as needed and wait till just coolant is flowing from bleed screw with no bubbles showing. Hope this helps.

Yes, I've done that numerous times.

[632 Regal]

if the aux fan isnt kicking in when it gets hot then thats your problem not the head gasket.

the system builds pressure from expansion dude... every ones does.

[cortez]

my dads 735i kept overheating and boiling out the pressure cap. turns out the pipe connection was blocked up on the expansion tank (the small pipe that runs from the top of the radiator to the exp tank). I cleared it out with a fretsaw blade and now its running perfect. totally reliable. hope this helps

[Guapo]

if the aux fan isnt kicking in when it gets hot then thats your problem not the head gasket.

the system builds pressure from expansion dude... every ones does.

I forgot to mention that in trying to diagnose the issue, I shorted across the contacts on the plug for the aux fan sensor. This turns the fan on high, and it still doesn't make a difference.

I realize that the system builds pressure, but this seems excessive. Does the M50's cooling system build more pressure than other cars? Also, when it has warmed up and built the high pressure, I crack open the bleeder screw and nothing but air comes out for a good 10-20 seconds. The air is coming from somewhere.

[Guapo]

my dads 735i kept overheating and boiling out the pressure cap. turns out the pipe connection was blocked up on the expansion tank (the small pipe that runs from the top of the radiator to the exp tank). I cleared it out with a fretsaw blade and now its running perfect. totally reliable. hope this helps

If I squeeze the upper hose when there is coolant in the hose, coolant comes through the small port under the neck, so I'm assuming it is clear.

[genphreak]

If I squeeze the upper hose when there is coolant in the hose, coolant comes through the small port under the neck, so I'm assuming it is clear.

It sounds like you have the problem well diagnosed, the compression is a worry.

However steam escaping from the bleed screw each time is either gaseous coolant or fuel mixture or exhaust gas.

Since you want to be 100% that the air coming out is not just steam buildup which occurs when an air block reduces flow enough to boil the coolant in the head or when the water pump is not pumping properly- have you pulled the pumpo again to make sure it is not faulty? What type did you put on?

Or maybe you have a physical blockage in your head which is causing a localised hot spot- ie a small boil, which creates steam and causes an air-lock. I would suggest getting someone experienced with these engines to check your bleed procedure.- or use the forum members- What is your exact method? Have you drilled a 1/8" hole in yout thremostat plate as many others do to make bleeding easier (beware I am not really sure if this is applicable to an M20/50) as well as M30- this works a charm for me.

Regardless if you follow the BMW bleed procedure, it should be fine- if you are doing this then you will know if you have a problem. Next step might be a coolant analysis. The procedure is in the TIS; perhaps search for it on this forum, someone like Bill R might well have posted the relevent section before to help someone else.

Perhaps pull the plugs and post pics of each of them- someone might even unravel this for you that way OR if unrelated perhaps help diagnose your compression issue...

Oh, and what is the service history like- has it always run on the right coolant and had it changed regularly?

[gmannino]

I agree with sending the car to a experienced BMW mechanic to verify that the cooling system is properly bleed. Also check the radiator cap, and DO NOT allow your car to run if it STARTS to overheat, the last thing you want is a cracked cylinder head.

[Ross]

I agree with genphreak that you seem to have a handle on it.
Checking the water pump as he suggests is a good idea, the impellers break or become loose on the shaft so there is no circulation causing symptoms like you describe.
The impeller problem will make it heat up right off, idling or driving doesn't matter.
Sounds like the gasket though. What did the plug look like?
Good luck,
Ross