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DIY: Do It Yourself
Post here to share or improve your wrench turning skills! All BMW E46 DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

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Old 02-01-2008, 01:18 PM   #1
Rodney24
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Thermostat stuck open?

I think my thermostat is stuck open. When i am sitting idle my car warms up just fine but when i start to drive the temp drops. This is my first BMW and i have never really worked on them before so my troubleshooting could be off. If anyone can think of something else please let me know. I really dont wanna order one, swap it out and have the same problem. Oh yeah, its a 2003 325i.
Thanks in advance for any advice.
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Old 02-01-2008, 06:02 PM   #2
Paul180
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That sounds like a thermostat problem. It is stuck open a little bit, when you are driving the radiator is more efficient at transferring energy and therefore, the temperature of your engine starts dropping.

I had the same problem and after a new thermostat it worked like new. It's an easy DIY, and I'll be posting pictures and directions for doing it here maybe later this weekend.
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Old 02-01-2008, 06:04 PM   #3
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same thing happened to me, thermostat is a whole housing kit on the e46, couple clamps and some new coolant and you're all set.
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Old 02-01-2008, 06:07 PM   #4
chenja 330CI
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OP, your same exact problem happened to me last week. It is the thermostat, it's about $60-80 for the part, and it takes about 1 to 1.5 hours to finish the installation. If you've never done it before, it's about $200 total at a good shop. Quick easy fix, good luck.
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Old 02-01-2008, 06:08 PM   #5
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When mine stuck open, the climate control (when on Auto) would immediately set the fan to blow even though the engine wasn't warm enough to heat the air. It sets an engine code, if you want to verify your diagnosis.
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Old 02-01-2008, 06:51 PM   #6
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and I'll be posting pictures and directions for doing it here maybe later this weekend.

that will be awesome!
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Old 02-03-2008, 06:21 AM   #7
Rodney24
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I got a new thermostat from a BMW shop yesterday and installed it last night. This is the first time i ever worked on a BMW and it did not go smoothly. The thermostat its self was easy, but i have never worked on a car that you had to take the fan off before removing the fan guard. That is what gave me most of the problems, and i broke all the plastic pins that hold everything together. it would have been alot easier if A: the fan guard didn't go completely around the fan and B: they used nuts and bolts verses the plastics pins. All in all i am embarrassed to say it took me about 5 hours to finish. and i still have to go find some more plastic clips.
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Old 02-03-2008, 11:12 AM   #8
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My thermo is stuck closed once... and the guage was reading RED HOT heh but it's fine now, that the weather isn't so cold. SO I don't think is needs to be replaced but for sure a good idea to do it soon.
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Old 02-03-2008, 12:08 PM   #9
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I got a new thermostat from a BMW shop yesterday and installed it last night. This is the first time i ever worked on a BMW and it did not go smoothly. The thermostat its self was easy, but i have never worked on a car that you had to take the fan off before removing the fan guard. That is what gave me most of the problems, and i broke all the plastic pins that hold everything together. it would have been alot easier if A: the fan guard didn't go completely around the fan and B: they used nuts and bolts verses the plastics pins. All in all i am embarrassed to say it took me about 5 hours to finish. and i still have to go find some more plastic clips.
Ditto. The plastic pin that holds the fan shroud was brittle from age and broke off when I tried to remove it. I had to break off the corner of the shroud in order to remove the rest of the pin. The cost and weight savings for plastic versus a simple metal bolt is fine in a POS econobox, but BMW ought to be ashamed for using such parts in a luxury marque of this cost.
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Old 02-03-2008, 12:13 PM   #10
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My thermo is stuck closed once... and the guage was reading RED HOT heh but it's fine now, that the weather isn't so cold. SO I don't think is needs to be replaced but for sure a good idea to do it soon.
This would seem to be risky. You might be getting some ambient cooling from the winter weather, but the temperature gauge is known to be "damped" and it's only reading the temperature at one spot. Without coolant circulating, the engine's got to be running hot, even more so in internal areas that might not be reflected in the dashboard gauge.

The consequences from overheating run from increased wear to a warped head to catastrophic failure of the entire engine. I wouldn't mess around with a thermostat stuck closed.
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Old 02-03-2008, 03:12 PM   #11
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WTF!!!!!! I changed the thermostat and it worked fine for about 30 min. Then all the sudden the gauge started to rise and with in 15 secs it showed overheating. I dont understand it. There was also a light that came on from my gauge cluster that said my rear passenger light was malfunctioning, but it works fine and so does the break light. As for the overheating problem i dont know cause the thermostat is new and when i drive i little fast it opens up and doesnt let my engine overheat but when i sit at a stop light or something the gauge hits the red zone real fast. Could it be my computer? I dropped it off at a local BMW repair shop here in Germany. the guy seems to be pretty honest. Hopefully he will look at it in the morning and let me know. If it is the computer does anyone know about how much it could cost me? or if anybody thinks it could be something else please let me know. I love the car, but chevy's are hell of alot easier to work on.
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Old 02-03-2008, 03:35 PM   #12
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yes you can have overheating problem after replacing thermostat.
you have to take the pressure out.
just be patient and let the air come out

Anyway, if the temp goes down while driving, does it affect the engine?????
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Old 02-03-2008, 04:11 PM   #13
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yeah the engine runs fine. Thats why i thought that it was maybe the ecu or something because it didnt act like a car that was overheating. And that crazy back light malfunction warning on the gauge cluster. but all the lights work fine
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Old 02-03-2008, 04:39 PM   #14
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WTF!!!!!! I changed the thermostat and it worked fine for about 30 min. Then all the sudden the gauge started to rise and with in 15 secs it showed overheating. I dont understand it.
This frequently happens after DIYers change the thermostat and it's often due to trapped air in the cooling system. Did you bleed the cooling system when you were done?
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Old 02-03-2008, 06:27 PM   #15
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WTF!!!!!! I changed the thermostat and it worked fine for about 30 min. Then all the sudden the gauge started to rise and with in 15 secs it showed overheating. I dont understand it. There was also a light that came on from my gauge cluster that said my rear passenger light was malfunctioning, but it works fine and so does the break light. As for the overheating problem i dont know cause the thermostat is new and when i drive i little fast it opens up and doesnt let my engine overheat but when i sit at a stop light or something the gauge hits the red zone real fast. Could it be my computer? I dropped it off at a local BMW repair shop here in Germany. the guy seems to be pretty honest. Hopefully he will look at it in the morning and let me know.
I agree that air trapped in the system is the most likely culprit. After that, I would check if you remembered to plug the fan back in. A lack of airflow through the radiator when the vehicle isn't moving would explain overheating only when still.
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Old 02-03-2008, 06:44 PM   #16
Rodney24
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yeah i didnt bleed the system. Didnt think you really had to. Oh well, when it started overheating i was close to the shop where i bought the thermostat from so i just dropped it off there. They were closed at the time, but i will give him a call in the morning and have him bleed it. I am tired of messing with it. I have had trouble with doing the simplest stuff on cars lately. This thermostat kicking my a$$ and i helped a buddy change his started we ended up having to remove the motor mounts and jack the engine to get to the bolts. I hope i dont start having these problems at work.
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Old 02-14-2008, 08:51 PM   #17
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mine is stuck open too. Is it bad too engine???? cuz it lowers the temp when going on highway??
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Old 02-15-2008, 04:27 PM   #18
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For those of you wondering how to do the thermostat DIY I wrote a little something up. If your car's temperature is dropping while on the highway and taking too long to heat up, then your thermostat is stuck slightly open.

http://www.bmw330ci.net/maintenance/thermostat.php

Enjoy! - Paul
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Old 02-15-2008, 09:03 PM   #19
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so tell me , should I replace it asap????????????????
is it bad for engine?
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Old 02-15-2008, 09:06 PM   #20
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how about the other parts I should replace with?
timing belt? pulleys? water pump?
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