How to test a thermostat?

My 2000 323Ci has been occasionally overheating and since I have recently replaced the water pump and thoroughly bled the air out of the cooling system according to e46fanatics standards, I believe that the thermostat on my car may be stuck shut. I was wondering how I would go about testing the thermostat to make sure that it is faulty before replacing it. So if anyone has a method of testing, please tell. Thanks

it usually sticks open....that said, there is probably a way to test. Its a realatively cheap replacement, and not a terrible DIY. I did it in about an hour in 15deg weather. Probably worth replacing.

I believe you can put it in a pot of water and see if it opens when the water reaches that temp.

This is an electric thermostat. So don't boil the connectors.

My bad. Still new to the BMW world so was going off old knowledge.

I thought about boiling mine since by the time I suspected it was stuck shut, the electrical heater circuit had already fried itself. Since the thermostat was on the car, I tried this instead:

1. Bleed cooling system and remove main fan (Auto) or only fan (MT)
2. For bonus points, unlock hidden OBC menu to watch coolant temp on instrument cluster
3. Start car and drive around until temp gauge is in middle
4. When temp gauge climbs beyond middle turn heater to 91F, fan high, vent dial to 3 red dots to prevent overheating since the heater core acts like a mini radiator. Do NOT let gauge go into the red zone.
5. Park car, feel upper and lower radiator hoses plus radiator. WARNING, may be hot enough to burn you
6. Replace thermostat if needed. Rebleed if you replace thermostat.
7. Repeat test to make sure new thermostat opens when car is at operating temp
8. Put fan back on.

If upper hose is hot while lower hose and radiator are cold, your thermostat is probably stuck closed. But since it's supposed to be closed when the car is cold, you have to let the car warm up first before checking the hose temps.

Can you tell us specific conditions under which it overheats? Does it ever happen when you are traveling over 40MPH? Some of these signs may help you to identify the problem without needing to take the thermostat off. In my opinion if you are going to test the stat by removing, might as well just get a new one (depending on how old it is).

Currently it's pretty hot in Ga. My car only overheats when stopped, and when I start driving again it will cool down. It doesn't overheat every time I stop, but sometimes. I generally just blast the heat in my car and the engine cools down.

Originally Posted by wroberts

Currently it's pretty hot in Ga. My car only overheats when stopped, and when I start driving again it will cool down. It doesn't overheat every time I stop, but sometimes. I generally just blast the heat in my car and the engine cools down.

It's not your thermostat. It's your fan clutch getting weak. With the stat stuck closed the temp will rise even if you are moving fast.

Agree with ZHPRegistry. Ignore my thermostat test plan. If you have an AT, it's your fan clutch. If you have MT, it's either temp sensor in lower radiator hose or relay for electric fan.

Originally Posted by wroberts

Currently it's pretty hot in Ga. My car only overheats when stopped, and when I start driving again it will cool down. It doesn't overheat every time I stop, but sometimes. I generally just blast the heat in my car and the engine cools down.

sounds like an air movement problem. are the fans working?

hey guys, i need u'r expert opinions. Are these fan clutches reliable? What causes the fan clutch to burn out? This is my second one it lasted 80k, is this normal?
THX