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Thread: 850i Overheating Issue

  1. #1
    Join Date
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    850i Overheating Issue

    I'm having an overheating problem with my 850i (12/90 build date automatic). After driving for about 5 minutes, the temp gauge went up to around the 3/4 mark very quickly (<1 minute), and I pulled the car over to let it cool down.

    The fan clutch and electric fan did not turn on. The upper radiator hose was cold, but the 3-way vent hose was hot and slowly bubbling at the connection on the driver's side of the radiator. There don't seem to be any leaks otherwise. Before this, the temp needle stayed at 12:00 in all situations, with/without A/C on. I have not replaced any parts on the cooling system. All fuses are good too.

    I moved the car to a parking space about 1/4 mile away and will be towing it back to my garage to check things out this weekend. Fearing the worst (headgasket). I'm thinking it could be a blockage in a hose or the radiator, or a bad water pump? Has anyone had a similar experience? Searched through some threads but did not see the same situation.

  2. #2
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    Sounds like a bad thermostat to me. Hot coolant in the engine not circulating to the radiator. Temp sensor for the electric fan is on the passenger side of the radiator. Temp sensor for the gauge is on the engine itself.

  3. #3
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    That makes sense. I'll replace the thermostat first. Going to also replace the fan clutch and fan. I need to go through the PO's service history to see if the water pump has ever been changed. Do M70 water pumps have composite impellers?

  4. #4
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    Even with the thermostat and pump not doing anything the top hose should still be hot by conduction - and the reason it isn't is because there isn't any coolant in it. The fact that you actually saw coolant bubbling means you have a leak - and that's where to start before changing bits and bobs.

    More info here:

    https://www.meeknet.co.uk/e31/BMW_M6...ing_System.htm
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  5. #5
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    Thanks Timm. I have new hoses on order as well- might as well do it all while I'm there. Could the small leak I saw between the water pump vent hose and the radiator cause an air buildup in the coolant? I'll have more to report this weekend when I get a chance to work on the car.

  6. #6
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    If you have little coolant in the expansion tank you can be sure that the little leak you saw was enough to lose enough coolant to stop it circulating. See how much coolant it takes to refill the system. In a pressurised system (which the E31 has) there only needs to be a tiny leak to cause problems!
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


    My BMW Repair YouTube Channel
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  7. #7
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    I took a look at the car this weekend. Filled up the coolant with the nose raised slightly for bleeding. I couldn't believe how much coolant it took- probably around 2 gallons (~7.5 liter). Lots of air bubbles came out. The car idled for a while and the temp needle stayed rock steady just to the left of center. I went for a fairly long drive and the needle stayed dead center or just left of center. Seems to be running just fine, but out of caution I'd like to do a compression test.

    Embarrassed to admit that I never checked the coolant level on the car for the past year I've had it- a big oversight on my part. I imagine there is a slight leak that allowed the level to drop enough to create an air blockage. As stated in my first post, I did notice a slight bubbling at the connection from the water pump vent to the radiator when the car got hot. I don't this leak any more (or any evidence of other leaks), but I'll be replacing the hoses.

    Is it possible to connect a pressure tester to the system to determine other leak points?

  8. #8
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    Sorry about the misdirection on the thermostat. I assumed you had already checked the coolant level. I'm sensitive to thermostats because I had one fail in my 850i. Several others have reported similar failures. It would be a good idea to change yours while you are in there. Easy job.

    Quote Originally Posted by David9999999 View Post
    Is it possible to connect a pressure tester to the system to determine other leak points?
    I have a Schwaben Coolant Refill/Air Purge Tool (https://www.ecstuning.com/b-schwaben...l/003466sch01/). Best $75 I ever spent. Connect it to the radiator cap and draw a vacuum in the entire coolant system. Evacuates any coolant lurking in the system. Flip a few valves and it sucks the new coolant into every nook and cranny of the system. My 850i holds 3.43 US Gallons. I put 4 gallons of coolant in a 5 ga bucket and it sucked all but about 1/2 gallon into the system. started the engine and bled very few bubbles of air.

    Ive also used the system on my Jeep Grand Cherokee and it worked a treat. Same results. I'm currently working on my Ferrari 308/GT4 which has a huge, complex cooling system. Front radiator, mid-engine. I did an engine-out to replace sodium-filled exhaust valves (long story). It wouldn't draw 5 lbs of vacuum. Tightened up some of the 26 hose clamps in the system and drew (but wouldn't hold) 15 lbs. More hose clamps to check. If it draws 27-30 lbs of vacuum it's good to go. That system holds almost 5 gallons of coolant!

  9. #9
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    The most PITA air stuck in coolant problems I had were with Jeep SRT WK2.
    Complete nightmare, but guess what finally worked out? A simple hand: I just grabbed the hose and pumped it with my hand intensively. Once it got dancing around the coolamt was able to release the bubbles, so I could add more. It actually took a looooooong while, like half an hour with starting the engine and etc, then pumping hose more and etc, but I was able to pour a gallon more coolant than before.

  10. #10
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    Quote Originally Posted by NevadaMike View Post
    I have a Schwaben Coolant Refill/Air Purge Tool (https://www.ecstuning.com/b-schwaben...l/003466sch01/). Best $75 I ever spent. Connect it to the radiator cap and draw a vacuum in the entire coolant system. Evacuates any coolant lurking in the system. Flip a few valves and it sucks the new coolant into every nook and cranny of the system. My 850i holds 3.43 US Gallons. I put 4 gallons of coolant in a 5 ga bucket and it sucked all but about 1/2 gallon into the system. started the engine and bled very few bubbles of air.
    That looks like a nice tool. Does applying pressure to the system allow you to determine leak points?

  11. #11
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    Quote Originally Posted by David9999999 View Post
    That looks like a nice tool. Does applying pressure to the system allow you to determine leak points?
    The tool doesn't apply pressure, it draws a vacuum. I haven't tried to configure it as a pressure tester.

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