Engine Coolant Thermostat Tolerance

[wrs 120]

Has anyone ever measured when a coolant thermostat actually opens and closes?

Engine M50B30 (219kW), radiator A1 Radiators 50mm core, fan delete, electric fan set to come on at 93'C with 1' hysteresis, switched by Link ECU.

I'd been chasing the reason for my engine running at 95-98'C all the time even though it has a 88'C thermostat. If the thermostat opened at 88'C while driving with good airflow (especially in winter) the engine temp should sit round 88-90'C. However, it's always been 7'C too high.

Today I took the thermostat out of the engine and did a boil-up test to see where it opened. It did not open at 88'C. In fact it wasn't until it reached 94.5'C for around 15 seconds that it began to open. Interestingly it didn't close again until 88'C. I had a couple of 80'C thermostats on the shelf so did some tests on these too. One opened at 84'C, the other at 85'C. Both closed at 80'C.

So I put the 80'C in the car and did some more testing. The engine now sits between 88-90'C when cruising and peaks at around 92'C under heavy load. At idle the temperature rises to 93'C then the electric fan comes on until the ETC drops to 92'C then the fan switches off. The temperature then drops down further to about 90'C then slowly rises back to 93'C and the cycle repeats.

I have a link ECU and can monitor the exact engine coolant temperature while the engine is running. I've also previously removed the temperature sensor and characterised it so the ECU could be calibrated using a Cal Table to get ECT within ±1'C across the whole measurement range.

 

Is it normal for a thermostat to open several degrees higher than it's rating but close at its rating?

[Allanw 1071]

Thermostats are one thing I tend to buy OE or OEM. It's not worth trusting a cheap one.

Having said that... they are a pretty simple device and precision isn't one of their strong points - they do creep open and closed, so the speed at which you increase the temperature can dramatically affect the measured "open" temp and "closed" temp.

[wrs 120]

I should have added a bit more info about the opening profile:

On the original 88'C TStat it began to open at 94.5'C and opened up about 0.5mm and sat at that gap. As the temperature was very slowly increased up to 97'C it opened up more and was fairly linear with the temperature change. It appears thermostats have a fairly linear region where they go from 0-100% open over several degrees of temperature change. It makes sense that they do this as it allows the cooling flow to match the load with a relatively small temperature change.

The 88'C TStat above was an OE from the local BMW parts dept...

[M3AN 4016]

There will be variances because of the simple mechanical nature of the 'stat as Allan says but this variance is in the "it doesn't matter" range, it's not worth bothering about. If you can keep the temps to below 120*C, nominally at 100*C you have nothing to worry about. Your coolant changes temp way faster that your oil, head or block. You should ideally monitor oil temp, not coolant temp. Only if oil temps become unmanageable do you need to consider the cooling system.