I’ve been scratching my head over something with my 1999 M3 S52 for a while now, and I’m hoping someone can shed some light on this. I’m using OBD2 readers, two different ones to be precise, and they consistently show a coolant temperature around 95°C (203°F). From what I’ve gathered on forums, the M3 should ideally run closer to 88°C (190°F) under normal conditions, which aligns with the 88°C thermostat.
Being keen on track days, I’m all about making my car as reliable as possible. So, I proactively overhauled the cooling system, as the original components were quite old and nearing their lifespan.
Here’s what I replaced:
- Complete system flush (repeated 10 times with distilled water for thoroughness)
- New full aluminum radiator
- Upgraded water pump (rated for +20% performance at 6500rpm)
- New auxiliary fan and clutch fan assembly
- Full hose replacement, front to back
- New coolant expansion tank
- New coolant temperature sensor (located under the intake manifold)
- New 88°C thermostat and housing
- Lower temperature auxiliary fan switch (87-89°C instead of 91-93°C)
Now, you might be thinking, “Did you bleed the system properly?” And yes, I believe so! I’ve bled it multiple times to ensure there are no air pockets, and the car’s heater blows hot air, which is a good sign.
Despite all these upgrades aimed at lowering the running temperature, the OBD2 reader still stubbornly reads 95°C.
This led me to do some further investigation with a laser thermometer to compare readings:
- OBD2 reader: 95°C (203°F)
- Laser thermometer on thermostat housing: 88-90°C (190-194°F)
- Laser thermometer on hose before radiator: 88°C (190°F)
- Laser thermometer on hose after radiator: 60°C (140°F)
- Laser thermometer on coolant tank: 86-88°C (187-190°F)
I even cross-referenced the laser thermometer readings with a multimeter (resistance measurement), and the values were consistent.
So, here I am, still puzzled. These temperature readings remain consistent even after hard driving during HPDE events. Even letting the car idle at 2500rpm in the garage for a couple of minutes after a hard drive, with no airflow, the OBD2 reading remains at a flat 95°C.
This raises a couple of questions:
- Could the OBD2 reader be incorrectly calibrated, causing it to consistently over-report the temperature?
- Is it possible that the coolant temperature inside the engine itself is actually higher than what’s being measured at the thermostat housing?
Any insights or suggestions would be greatly appreciated!
