Hello, I have a 91 M5 that has a overheat problem. I have had the thermostat and fan clutch replaced and the temp is within range however the coolant is still getting super hot and is spilling out. I cannot find anything leak. I am hoping someone might be able to help me out here.
It seems to me that your temp gauge is bad. How can you have boiling coolant and have temps "within range"? I just had the opposite happen in the 530 that I recently bought. The previous owner had replaced the radiator, water pump, thermostat, etc. and the engine was still "overheating" according to the gauge but there was no boiling or coolant loss. I got the car for $1,200 and after checking temps with an infrared thermometer, I replaced the sender ($19 from Ebay). Temp is steady at almsot 12 o'clock after that. Anyway, back to your problem. Since you haven't mentioned the radiator, have it checked by a shop as it might be plugged restricting air flow. Also, bleed your coolant system multiple times as these cars tend to be prone to having air bubbles stuck in the system. Make sure that your heater is on so that the valve opens and coolant flows through the heater core also. Worst case scenario, check your oil to see if you have the "milky" look which would mean new head gasket time and maybe some head work (milling). Good Luck!
Have the system pressure tested. Another common failure item is the coolant reservoir cap. If it deos not seal, it can cause the system to boil over for sure. That is a cheap item to replace. Also make sure you bleed the cooling system several times to be sure. They are difficult to get all the air out of. Make sure heat is on full blast on defrost when you bleed it and that you have good heat.
First thing I would do is when you get the boiling over CAREFULLY touch the upper radiator hose and the lower radiator hose to see if they're the same temperature. If the bottom one is cold then you have a circulation problem which could be caused by an air pocket after changing the thermostat. If you find the hoses very hard or look like they're swollen then you might have a head gasket leaking compression into the cooling system. If you had the problem before changing the thermostat then I would change the water pump since this is a common failure point on these cars (I have two new spares just waiting for mine to go). Pressure test first before changing the water pump because you might have a leak somewhere. Make sure the system is full of coolant before pressure testing. If you see the coolant level drop after testing but no visible sign of an external leak then we might have a head gasket problem. Notice any steam like vapor coming out of the exhaust pipe? Oh, and change the radiator cap too since they're cheap and can sometimes not hold pressure when they get older. The cooling system requires monitoring closely on these cars. Paul
