Since I purchased my M3 earlier this year I have been chasing an overheating problem which only occurs on the track. At first the car overheated after about 2 sessions but now it only lasts about 10 min before the temp gauge (stock bmw) spikes.

I bought the car with a new radiator, water pump, fan clutch, and hoses. After the first overheating incident at Watkins Glen a shop looked at the car. They found a faulty pressure cap and no evidence (CO2) that the head gasket was faulty. At the next event the car overheated even quicker. I then brought it to another shop who found no evidence of a head gasket leak (leak down, pressure, and C02 tests). As a last ditch effort I installed a low temp thermostat, ali thermostat housing, and a stewart water pump. Despite this the car overheated within 10 mins at the track. Once I refill and bleed the system the car is perfectly fine on the street.

I am at a loss. From reading other treads I suspect the head gasket but both shops seem sure that it is not the problem. Any input would be greatly appreciated.

Cliffnotes: A 95 M3 (S52) with all new cooling system including stewart water pump and low temp thermostat overheats only at the track. Shops various tests and not found a problem.

Sorry to say it is the head gasket. This is a common problem with E36s....they only show overheating problems on the track and not on the street. It is always a head gasket. I would take the head off and check it for trueness (deck it if needed) and install a new head gasket and that should cure all your problems.

I agree, most likely you've got a small leak in the head gasket or even a cracked head. I've had mixed results with the chemical tests for combustion gases in the coolant. If the leak is small there may not be enough combustion byproducts for the test to detect.

One can usually find a small leak by pressurizing each cylinder with 150-180psi air for several minutes while monitoring cooling system pressure with a sensitive gage. A negative result on a cold engine should be repeated on a hot engine.

A qualitative check would be to throughly bleed the system, then run the car hard enough to get the temp moving towards the red. Allow the car to cool and re-bleed. If you then get air out of the cooling system you have a leak.

In the recent past a friend had the same set of circumstances, but not a blown head gasket, all 12 exhaust valves were slightly bent which did not allow the valves to open far enough to eliminate all spent gases and consequently led to the hot gases remaining in the cylinders and causing an overheating issue only at the track. I hope this helps if in fact the head gasket does not prove to be the issue.

I can't see bent valves as resulting in normal compression test or leak down results. So that seems unlikely here.