strange overheating

[bsell]

Not knowing what engine you are talking about doesn't help but here goes...when is the last time the water pump was replaced? BMW used plastic impellers from the factory and they tend to snap off from the shaft when they get old. Which means you have no water flowing in the engine, hence the overheat and not screaming hot hoses.

If you have a known good pump, then your thermostat may be the culprit.

Brian

[robert g]

I was driving to work today and the outside temp was about 43 and I noticed that after driving 5 to 6 miles the temp guage went to red and the message on the dash said overheating or something.
So I got to work opened the hood and looked to see if I had any fluid in the radiator. It was full. I also noticed that the top hose was hot but not boiling and the bottom hose was warm. If you have ever opened up the cooling system and you have gotten air in there and you do not bleed it the top hose just boils. It was not doing that.
Later tonight I was driving home and about 5 to 6 miles I noticed the temp going up so I turned the heat on. The temp only got about 3/4 up and than went back down to half. Almost home I turned off the heat and let the car get hot again. 3/4 up I got home popped the hood and checked the hoses touched the hood and the same thing again. Engine bay cold, hood cold and the hoses warm and not boiling over.
I must say I have not been driving the car for about 1 week and half. Took the car and this happens. It seems when the car sits and than I drive it something always goes wrong. I guess I will have to drive it everyday or every other day. What do you guys thing temp. guage sending unit?

Thanks

[Ross]

X2
Sometimes the impeller cracks around the shaft but remains loosly in place, sometime it pumps, sometimes it doesn't.
Don't trifle with overheating issues on these engines.

[DaveVoorhis]

Thermostat might be sticking shut.

[robert g]

I forgot the most important information... The car is a 1994. And yes the waterpump is new. What else can I say... I havent driven it today... I will later.....

[Paul in NZ]

disconnect and reconnect both temp sensor connections a few times...that will ensure the connections are now good so at least half of the instument side of things is known to be good.You dont say what sort of speeds you are doing...does the high temp coincide with low speeds...might be the viscous fan.

[robert g]

I drove the car for about same distance temp started to go up so I turn on heat and it went down. Drove it for another mile in half and temp started to go up pulled over and both hoses the same temp. engine bay normal temo and outside temp was 34. I hate to start taking things apart with out trying to go about it a sensable way. Any ideas. thanks again.

[Chad535]

Are you sure there is no air trapped in there ? My 535 overheated a bit sometime back and eventually vented some of the coolant, when i added coolant later i overlooked the bleeding bit and ended up having strange overheats for a couple of weeks till i read some posts here and bled it properly.

[Robin-535im]

If your cabin heater gets nice and hot (really hot like it should) - you know the coolant is moving around the engine and exchanging heat, so I wouldn't suspect the water pump or thermostat just yet. Plus if the coolant level is fine, it's probably not a leak or pressure loss anywhere.

Aged coolant doesn't work as well, and your fan clutch could sure cause the overheating if it's not pulling the air through hard enough. I would flush the system and replace the fluid, and clean off the radiator to see if that helps. If the fan clutch is too quiet or real old you could replace that too, and you can test the lockup with boiling water I think.

[robert g]

new:belts
radiator
fan clutch
fluid new

I am driving normal speeds like 45 on street to about 55 and on the highway about 85 mph. Why when I turn the heat on it goes down. I have had air trapped in the past and the top hose boiles alot and the heat in the cabin cold. This is not the case. It takes about an hour for the car to cool off when there is air trapped. Both hoses hot but not boiling. If it was any of the guages or temp sender why would it go down when the heater is turned on it would stay hot the guage? I do smell some coolant in the cabin of the car. But I have been smelling that for a long time. The coolant is also full. The thermostat is not new but seems to be opening up.
I did bleed the system when I replaced the coolant level sensor about one month and half and probably driven the car one tank full. It has been sitting for about 9 days or so. Every time the car sits for a while something goes wrong. Something minor at least... I have 3 cars now and one car doesnt get driven. I need to start driving the 525 since it is the only one that gets good gas mileage.
You can tell when your car is over heating. Engine bay is very hot the fluid is boiling and none of that is happening. I dont have any visual leaks. I live in small town and no one has the adapter for pressure testing. Is there a different way I can check it.
I have not checked the guages or sending unit yet. I will do that monday. Both my kids ate something and got sick all day sunday. What a mess. I only got to drive it to the store to check and see if it was still doing the samething. Maybe something was stuck and got unstuck.... Not really but wishful thinking.... Thanks guys. The turning on the heat and temp goes down that has me puzzled. If there was air trapped nothing makes it go down. Park it and wait. Last time I got lot of air and it took a long time to get all the air out. That was in the begining of this year.

[whiskychaser]

The turning on the heat and temp goes down that has me puzzled.
If you mean you put the fan on and it stops the engine getting too hot, that makes perfect sense. Just proves the engine cooling system isnt working normally. I'd be thinking stat as has been suggested. If it cools as soon as you turn the heat on (no fan) I'd be thinking air lock. Did you have the heater on full when you topped up last time?

[Robin-535im]

Pressure is the other necessary component for proper cooling, if there is a pressure leak the temp will rise.

Without seeing any evidence of a leak (and it would be good to check all the fittings if you haven't yet) it could be the cap on the reservoir.

I had a jeep that kept overheating, I replaced EVERYTHING in the system except the cap on the reservoir b/c when I got the new tank (the overheating made the expansion tank crack) it didn't come with a new cap. After many hundreds of dollars, it turned out to be the pressurized cap wasn't working (letting pressure out at too low a pressure). Worst part was that it was a $12 part, after spending many hundreds chasing the problem!

[Chad535]

The filler cap was a source of misery for me too in the past

[robert g]

The cap is new since I replaced it when I replaced the radiator in October. Yes I did cap off the radiator with fluid when I bleed it. Both top and bottom hoses are pressurized when car is running at normal temp. I guess I can bleed the system again and see if that helps. When the car is running hot for example and the guage is 3/4 and you turn on the heat the guage takes it's time going down. What this is doing the guage takes but 1 or 2 minutes to go down. I havent timed it put it just doesnt seem like the temp stays up there to long. All the parts I have replaced in the cooling system have been due to old age or it was time to replace. I might have a bad radiator cap. Who knows... I will bleed again and see what else I can find. Can I check engine temp with a thermometor. Not for humans of course.... I dont hear it coughing... Ha Ha. How about one of those digital temp checkers. Thanks again for the help and quick response. Just because it is new doesn't mean it will not fail.....