A problem has crept up on my R53 as of yesterday. It seems to be overheating, at least according to the temp gauge. Last night, were were leaving a drive-thru and my wife noticed the temp gauge was peaked and the red light was on. I drove another couple hundred feet, and all of the sudden the gauge went from peaked to 1/2 in about 4 seconds. Today when I got to work I noticed the gauge was peaked again. My initial thinking is a stuck thermostat. Any thoughts on what else it could be? How hard is a thermostat to replace?

 

checked your fluid levels right?
...can do it from low coolant..air gets in..makes a bubble.....gets loit usually gets low throgh 2 common leaks..the thermost gasket or the .refill/overflow tank seam that leaks
The fan is working right?! you did hear it turn on right?
The final think to look for or rather listen for is sound...any higher pitched noises?
the water pump runs off a pto that is on the bottom of the SC..oil runs low...gears grind up...just before toal failure, car runs hot unless at speed. not very common, but it happens.

 

I know the high speed fan is working, it sometimes runs after I turn the car off lately. I'm not sure about the low-speed fan, though I'm pretty sure I can hear it running. Suggestions on how to check the low-speed? Just start the car, open the hood and look? I assume the low-speed is the quieter of the two?

 

if the high speed is running car should never overheat....
The run on fan is a sign of low coolant....
the infamous low speed fan failure is common...many threads..heck i even swapped out may fan...most likely 70% of mini's have it failed...and owners don't know/care.
mostly just let your car idle...no a/c.
a good scan guage can tell you exact temps...
but as the car warms up...low speed..pretty quiet will come on....and as the temp goes up more, the hi speed will come on...it is loud...and may cause an idle change momentarily.

 

The little evidence I have indicates it only gets hot when I'm not moving.

 

early 2005 cars..pre 2005 production, have the rad fan on the same fuse as the the ps fan...not an issue here since you heard the HI speed fan.
Check the coolant...if the tank is empty, refill, and bleed.
look for the leak,,usually the tank seam, or the thermostat gasket.

 

Like Zippy already asked... Have you checked the coolant level in the expansion tank?

 

It sounds like a bad thermostat. Basically whats happening is its not opening up allowing the coolant from the radiator to flow into the engine. So the coolant in the engine is getting hotter and hotter. Then finally when the thermostat opens the cold coolant in the radiator flows into the engine thus cooling it down which explains why the temp went from hot to normal so fast. anybody have any other ideas?

 

I agree with Autobot. The next time it does the overheat/cool down cycle, try driving aggressively (legally of course) for awhile to see if you can make it overheat. Then let it sit idling for 5 minutes or so. If all is normal, that would indicate a bad t-stat. If it heats up when sitting then investigate the fan. Lots of good info on repairing both the t-stat and the fan in the Modifications/How To section.

 

I've confirmed that it's leaking coolant somewhere. The tank was basically dry when I checked it yesterday, I filled it up and it was really low again today. I've inspected the expansion tank, and it seems to be OK. That leads me to believe that it's the thermostat housing. Any other ideas?

 

Thermostat housing gasket. Doing your preventative maintenance fluid checks on a periodic basis is the first step in identifying a potential problem. In your case, running the expansion tank basically empty allows air to get in to the cooling system and causes the temp gauge to behave exactly as you describe. In doing the fluid checks, one should notice the fluid level dropping and ask themselves, where is that fluid going. Jumping to conclusions about failing parts rather than following a methodical process of identifying the problem often simply costs the owner additional money.

The thermostat housing gasket issue is quite common and fairly easy to replace. Look for blue/green staining, if you still have factory coolant, below the housing to verify that there is indeed a leak at the housing gasket.

 

You should fill up your car with coolant. Be sure you bleed it correctly. If there are air pockets inside your engine then it will still overheat since water isnt flowing thru evenly. There is a screw at the front of the car connected to a hose that you would unscrew and let the air out. Then let the engine run more and bleed it again. IF you have a pump to put pressure in the resevoir tank to force the water thru then that would be easier. get all the air out and then see if it still is leaking anywhere after.

ooh and check your oil. Make sure its not milky. Just incase its a bad head gasket

 

Is it usually the full thermo housing that fails or just the gasket that comes with the thermostat itself? In other words, should I plan to buy housing and thermostat or just thermostat and gasket?

 

The gasket is intergeated into the mini thermostat ...
Most of the pro's say the housing is fine..clean and reuse...many folks here have replaced both...
So take your pick!!

 

Well, I got to the thermostat and it doesn't appear to be leaking from there. It appears to be leaking from somewhere on the front of the engine. I can't quite tell, but there is dried nasty coolant all over the front/bottom of the oil pan and the trail leads up towards the top of the front of the engine. Doesn't appear to be coming from the upper radiator hoses, either. Any ideas on what else it could be before I start removing the front of the car to get to the front of the engine?

 

Could be blowing back from the radiator...
They tend to fail where the metal tubes are crimped onto the plastic ends...but road debris, etc could cause failure.
With airmovent, it is tough to follow a leak....

 

Turns out it was the water pump.