Odd behavior from temperature gauge

'92 M50 with ~133k on it. While cruising, my temp gauge shows right around the 12 0'clock position. If cool out, slightly under, if hot out, slightly above. Pretty stable.

However, occasionally when I come to a stop after driving a bit, the temp gauge starts moving up QUICKLY. Very scary stuff. The strange thing is, if I put it in neutal and rev it a little, the temp goes back to normal, again quickly. This is a matter of maybe 5-10 seconds. Or, once I am in motion any my revs exceed 2k or so, everything returns to normal.

It has happened before, which scared the hell out of me. I checked the coolant, and its completely full. No signs of leaks anywhere, and frankly the engine is behaving normally. Tonight, I pulled over when it happened, and popped the hood to feel the radiator hose. It was not anywhere close to overheating, simple normal warmth. Again, no coolant, no smells. All seems normal.

This leads me to believe that I have a faulty sensor, so I'll probably wind up replacing the coolant temp sensor for the guage. If I'm reading realoem correctly, there's also one for the ecu right next to it? If so, I'll replace both, since they're fairly inexpensive.

I'm just concerned about other aspects of the cooling system. The only other thing that I can think of is the water pump not delivering enough flow at idle, but I would think the upper rad hose would feel very hot to the touch (the gauge is reaching the red zone) if the gauge was accurate.

Anyone have any insight on this? I may very well just change the thermostat and water pump for peace of mind, but it'd be nice to know I'm not totally crazy here.

unplug and plug in the connectors to the sensors a few times,maybe dirty connections,could also be the fan clutch by the sounds of it

I would second checking the fan clutch. With the engine running (preferably in a garage or a non-windy place), you should feel considrable air flow with your hand hovering over the waterpump (safely of course, no pirate shirts allowed). After changing mine, there was a distinct difference at idle.

Also check the archives for other diagnosis methods, maybe even bruno site.

Good Luck,

Dave M

I thought fan clutch as well, but I do have good airflow while running, and with the motor still, I get significant viscous resistance when turning it by hand.

The fan clutch was replaced a few months ago by the PO.

I'll keep an eye on it though. Thanks for the suggestions.

The gauge is not accurate -- it has a large buffer to prevent unknowing owners from complaining about the car "overheating" while sitting at a stoplight. When it shoots to the red, it's been hot for a bit already.

Does the car have an original plastic impeller water pump? Maybe a broken thermostat? That's what I'd be looking at. Remember, the newer factory water pumps for these cars have composite impellers...the crappy original plastic units are just that -- crappy plastic. Same with the tstat housing (although it's guaranteed to be unrelated in this case). But tstats do break on these cars.....



best, whit

Quote:

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Originally Posted by lowell

The gauge is not accurate -- it has a large buffer to prevent unknowing owners from complaining about the car "overheating" while sitting at a stoplight. When it shoots to the red, it's been hot for a bit already.

Does the car have an original plastic impeller water pump? Maybe a broken thermostat? That's what I'd be looking at. Remember, the newer factory water pumps for these cars have composite impellers...the crappy original plastic units are just that -- crappy plastic. Same with the tstat housing (although it's guaranteed to be unrelated in this case). But tstats do break on these cars.....



best, whit

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I agree, but that's the thing. It's not really getting hot. I've had overheating motors before, and this aint it. No steam, no extra heat, no drama. The t-stat housing is aluminum, dunno if it's original or not. I have no idea if the impeller is plastic or not. The sensors do have a bit of grime on them.

Aw, screw it. I'll just replace the t-stat, water pump, and the sensors. $100 for peace of mind is a good investment in my mental health (what's left of it anyway). :)

Before the plastic impeller pumps shatter (bad), the impeller itself can sometimes break free and spin on the shaft, giving partial cooling and whacky scenarios like yours. Tstat makes more sense, though, given the fact that the car is getting cooler when under way -- the water pump/radiator wouldn't become more efficient were they failing, but I learn something new every day.

Might as well flush the coolant while you're in there if it's been more than two years, and use BMW coolant while you're at it.

Good luck!

best, whit

Your problem sounds exactly like what mine did before I replaced the fan clutch. Your strategy of replacing everything will cure your problem though....

My old fan clutch felt normal, and was difficult to turn by hand, but upon replacing it, the creeping gauge went away.

Quote:

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Originally Posted by dave b

Your problem sounds exactly like what mine did before I replaced the fan clutch. Your strategy of replacing everything will cure your problem though....

My old fan clutch felt normal, and was difficult to turn by hand, but upon replacing it, the creeping gauge went away.

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Interesting. I wonder if the PO put on a crappy aftermarket clutch? Well, shoot. The very odd thing is that it doesn't do it ALL the time. I just got back from a drive, and it worked just fine.

I'll go the route of replacing the t-stat and pump first, and if that doesn't solve, it I'll do the clutch. I can only throw so much money at a problem at a time :)

Thanks for all the suggestions guys.

Well, I got the pump, the thermostat and both sensors replaced. So far it seems to idle without overheating okay. Although, it was an intermittent problem enough to not call it "fixed" until I get some more time driving it.

As a point of reference, the water pump on there was metal, but it has a very different design than the one I installed. It seems to be cast and very heavy, as opposed to the new one which seems to be stamped and welded. I'm not positive the old one wasn't a better design.

The new one is on the right:
http://www.guapozx.com/guapozx/d/3596-5/IMG_1803.JPG