Author Topic: Weird Overheating Issue (Read 2669 times)

ClayW · « **on:** February 06, 2013, 11:20:58 AM »

Other than the thermostat, water pump and radiator, what might cause the engine temp to rise?

Background: I've been running an M42 in my 1600 for the last two years (maybe 5000 miles). When I installed it, I bought a new radiator and shortly thereafter, a new thermostat and water pump. I've had intermittent issues with overheating ever since. I replaced the thermostat again recently with the lower temperature unit (75�C), but it seems to be acting like it's not opening fully. I have an electric fan that keeps things cool (no mechanical fan), but it usually runs longer than I think it should. According to my gauge, the engine is running around 92�C.

Is that a favorable temperature? Shouldn't it be lower with the 75�C thermostat? Anything else I should be checking?

bmwman91 · « **Reply #1 on:** February 06, 2013, 12:43:13 PM »

Have you bled the cooling system? The M42 is actually pretty easy to bleed if you install things properly in a stock E30 setup, bit it might be challenging to get all of the air out in a 1600 depending on the height of the expansion tank relative to the head. The bleed screw on the radiator (you using a stock M42 radiator?) should be at least as high as the lowest point on the valve cover, preferably closer to the height of the oil fill cap.

Stick with an 88C thermostat. About a decade ago I ran a 75C one because it was "better" and the car never ran very well since the cooler engine had a lot more piston ring blow-by.

Make sure that the little bypass groove in the thermostat housing is not plugged. Many times it gets completely filled with corrosion. If you use RTV to seal the housing on, I can guarantee that it is squeezing into the bypass and blocking it, no matter how sparing you are with it. I recommend the stock gasket with some red Permatex non-curing spray-on sealant.

ClayW · « **Reply #2 on:** February 10, 2013, 09:08:19 AM »

I have a stock M42 radiator. The bleeder screw is actually higher than the oil cap. I'll try bleeding again and see if that fixes it.

Even though I don't have any oil/water mix going on, is it possible that I've blown a head gasket or have a cracked head?

bmwman91 · « **Reply #3 on:** February 10, 2013, 12:47:07 PM »

If you bleed it, drive it for a while and find that there is lots of air in the system again, then the chances are high that you have a leaky HG or cracked head.

doitover · « **Reply #4 on:** February 11, 2013, 01:12:34 PM »

A lot of people have trouble bleeding these correctly. I've read that pointing it nose up while bleeding helps. Other mention squeezing the top radiator hose. Since I don'e use the factory shroud, I made an upper hose that makes a more direct and lower shot between the pump and radiator. While I was always eventually able to get the stock setup to bleed, it is easier now.

ClayW · « **Reply #5 on:** February 28, 2013, 10:25:36 PM »

I'm about to find out for sure what's wrong with my M42. I just bought a known good M42 with fresh headwork. Once it's in, I'll do a little autopsy on the current engine.