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      11-26-2014, 11:52 AM   #1
vachss
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Transient Overheat Problem

Help me diagnose a problem in my '07 3.0si. The temperature gauge has always read rock solid in the middle of the scale when warmed up. The last two days, however, I've had the same alarming behavior:

5 minutes after starting up in the morning (~50 deg ambient temperature) just after the engine reaches operating temperature and I'm driving at speed on the freeway the gauge suddenly jumps to full scale and the overheat alarm goes off. The first time this happened (2 days ago) I immediately shut the engine down, coasted to the side of the road and then tried starting up again - only to find the temp gauge reading completely normal. I then warily continued my drive to work, but saw no other issues - including on the drive home after starting up about 10 hours later.

Just a transient glitch right? Then the same exact thing happened yesterday (at the same exact spot on the freeway). I still shut down immediately, but this time tried turning the engine back on after about 20 seconds. The gauge still read hot, but about 80% of scale - not quite in the red zone. I watched this for another 10-15 seconds, and then it quickly (in about half a second) jumped down to 50% of full scale where it should be. Then I repeated the watchful drive to work and back again without unusual readings or incident.

So what do you guys think? Flaky sensor? I kind of doubt a failing thermostat or water pump since the temperature change was so fast (both jumping into the red and retreating to normal), but I suppose it could be a physical blockage somewhere in the system. I'll do another coolant flush this weekend prophylactically (it's been too long anyway), but if anyone's seen this behavior before and can tell me what else to try I'd appreciate it.

Thanks for your help.
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      11-26-2014, 05:33 PM   #2
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Could be the water pump. Your car (and mine) uses an electric water pump. They are prone to fail at about 70,000 miles. When beginning to fail, they can sporadically stop working. Have your mechanic pull the codes. It will show if the water pump has intermittently stopped working. If so, time for a new pump and thermostat.
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      11-28-2014, 05:14 PM   #3
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You should be aware that these temp gages are "buffered". In other words, the gauge will stay in it's middle position even though the temp may vary by a significant amount. Therefore, you receive very little actual warning of an overheat condition and the gauge will return to a normal reading fairly soon.

Although I do not have an idea of what issue may be causing your problem, my guess is that you are experiencing a genuine overheat issue and should be cautious. Do you own an OBD II port reader to try to verify?
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      11-30-2014, 06:27 PM   #4
vachss
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Curiouser and curiouser... Did a coolant flush today and when venting the system the water pump was totally random when it would turn on.

The technique I'd read about of pressing the accelerator for 10 seconds with the key in the ON position didn't work at all. The only way I could get the pump to turn on was was to run the motor for a couple of minutes (until the temp gauge started to move) and the turn the motor off - then the open reservoir vent cap would spew coolant as required (but only 2 of the 4 times I tried). Is this normal or a symptom of a water pump on its way out, bad thermostat (or just a faulty sensor somewhere)?

Edit: And I just tried the 10 second pedal - vent burp trick again and it worked just as it should... not sure if that's a good thing at this point or sign of a maddeningly intermittent failure.

Last edited by vachss; 11-30-2014 at 07:03 PM..
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      11-30-2014, 10:35 PM   #5
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Dunno how much mileage on your n52 but the electric water pumps are known to go as early as 60k. I replaced mine at 65k and since then, car warms up in 5 mins regardless how cold!
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      12-01-2014, 09:18 PM   #6
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Epilogue: It was the water pump. No big surprise given BMW's history in this area. This one lasted for 137K miles so I shouldn't complain.

Anyway, went to my fave local Indy shop. They pulled the codes and found water pump electrical faults. Since I needed the car back on the road I opted to cough up the money and have him install it (and the thermostat while he was there). Ended up costing me about $400 more than if I'd done it myself (labor + markup on parts). A bit painful, but at least its done and the damn thing's back on the road.
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