bump anyone know if this is true? I've only seen them fail open and have been told by indies that when they fail they fail open. But then again, if they only failed open then how could the car overheat? LOL. Maybe member HealthServices can chime in. Usually when the thermo fails it sticks open from my knowledge.

If someone wants to test their thermostat that is bad they can just stick it in a pot of water, let the water boil to test it - distilled water only of course. (that was a joke, you're only TESTING it)

 

they usually fail open, but can fail closed too...

 

If you are having the same problems that i was having, then i would recommend removing the thermostat and just putting the housing back on. I replaced the oil separator a few months back due to the diaphram splitting inside. symptoms was constant 02 fault codes due to diaphragm causing a massive air leak making the car rough idle and bad mpg. When i got the engine light back on a few days ago i had the P0599 heater circuit plus the 02 codes. I thought i had another air leak pus the thermostat problem, although now ive put the thermostat housing back on and cleared the codes, im not getting any codes whatsoever. So i presume that the bad thermostat was causing the other codes as well.

Like i said i will keep you updated when i fit another thermostat to see if my problem is electrical.

 

Hey guys. I am experiencing about the same problem on my 2004 525i. My wife was driving around when it suddenly overheated. I later found out that the fan was not functional. But when I disconnect the wire on the bottom left hose of the radiator the fan turns on. But still,the care overheats, am starting to suspect the thermostat now.

 

2004 E60 525i is a belt driven water pump. So, the N52 bleed procedure doesn't apply.

Here is the page from the Bentley manual:
IMG_0082.jpg

What made you replace the thermostat in the first place? Perhaps your water pump is spent??

 

Sounds like you have a bad coolant temp switch? They're fairly inexpensive even from the stealership and they're easy to replace. I'd start there. That switch tells the fan when to kick on. With that switch disconnected I believe by default to ecu sees 220 degrees (don't quote me on the temp) and turns the fan on. Another test is to reconnect the coolant sensor then turn the AC on. The fan should kick on. If these tests check out the problem isn't the fan, but more likely a coolant switch, t-stat or water pump. If the car has high miles it's probably best to replace it all anyway and save yourself the headache of getting stuck somewhere and needing a tow.

 

Here's another one for you guys, not to intrude on the thread, just didn't want to create another thread:

1. Over heat indicator goes on, radiator fluid is at max.
2. Fan is working properly, turns on and activates when hot.

Prior to overheat warning: when the heater is turned on with the car in motion, it blows warm/hot air. After a few minutes, suddenly the air becomes cold followed by the over heating warning messages.