Tired of overheating every few months...

M

MrNiceguy

Newbie
American Zeds
Joined
Jul 10, 2026
Points
1
Location
Kansas, USA
Model of Z
Z3 2.8L 6 Cyl
The patient: 1998 Z3, M52 engine, 6 cylinder, 2.8L

How the problem always starts: If I'm starting with the car completely cold, everything runs perfectly. Starts up, engine heats up to operating temp and stays steady. I've driven an hour or more at highway speed without any problem. However if I shut it off and then attempt to drive it again before it's completely cold, it begins overheating. If I it shut off for an hour or so, then it's fine.

I assumed a stuck thermostat and replaced it. This resolved the problem completely for a couple of months, then it started happening again. Talked with a mechanic family member who thought it likely I just got a dud part, so I replaced it again. Once again, zero problems for a few months, then same symptoms.

The car is old enough that the cooling system has had a fair bit of work. In the last couple years, I've replace the radiator, water pump, and most of the hoses, either in response to leaks or just, "might as well while I've got it apart." I've always run the BMW-specific antifreeze as well, and I know to bleed the system after any work.

I've searched through the forum and read through other threads on overheating, but none of them seem to have the same symptoms, or describe steps I've already taken. Anybody have any suggestions?
 
Use a digital non contact thermometer to diagnose the thermostat by taking temperature readings ate the housing and hose.
Make sure there is no air in the system.
 
Maybe try a blownhead gasket test kit (the one that changes color).
 
Just a thought, you don't say if the only symptom of over heating is the temp gauge going into the red. Is it possible you are getting false readings at the cluster. If this is the case I found the info below that maybe useful.
  • Coolant Temperature Sensor: The Z3 uses a dual-circuit coolant temperature sensor. One circuit feeds the DME (engine computer) and the other feeds the instrument cluster gauge. If the cluster circuit fails or sends a distorted resistance signal, the dash needle can spike to hot while the actual engine temp remains normal.
  • Wiring Issues: Chafed, corroded, or loose wiring between the engine sensor and the instrument cluster can intermittently ground out or alter resistance, causing the gauge to peg.
  • Instrument Cluster Malfunction: Internal circuit board faults, bad solder joints, or failing capacitors inside the speedometer/cluster assembly can cause erratic gauge behavior.
 
I only fit BMW thermostats, and Bosch coolant temp sensors. Thermostats generally fail open though. I've had similar overheating problems and it's always a head gasket - bit by bit, air gets into the system and on slowing down an air pocket finds its way around the water pump impeller.

A block test as suggested above is a good start. A 1998 US spec Z3 with the single vanos engine should have an iron block which is a plus.
 
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