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General E46 Forum
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Old 04-18-2015, 08:35 PM   #1
Ldub
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Cool Assist Diagnosing Temperature Sensor Issue

Hi There,

Advance apologies if I'm posting in an incorrect area and thanks for any assistance.

We have a 2002 BMW 325i with 98,000 miles. A good well loved car. A few months ago I happened to catch that the temperature was in the red hot. I was at a light, no where to pull over. As soon as it turned green I went to pull over but by that time (maybe 1 minute), it was back at the normal halfway point.

Recently this has started happening frequently and driving my crazy since at first I was pulling over every single time, until again I was stuck at a light and saw it do the quick return to normal.

It shows no signs of actually overheating- and will go from red to normal abnormally fast. It will only happen once while driving. Once it's done it's craziness it will stay at the normal halfway point and the coolant light will come on. The coolant light is NEVER on when it's in doing it's moving into the red and out.

The coolant level is right where it should be. The only code is an 02 sensor - unrelated? Reading other posts I understand this is likely a temperature sensor issue... and there are multiple sensors?

Sorry for the untechnical language- I'm not the mechanic in the family. Just the member who gets the ball rolling by posting to find out the plan and pass it along.

Thank you in advance.
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Old 04-18-2015, 08:47 PM   #2
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Its possible there is a problem with your fan or the module which controls it.

It seems that once you come to a stop, there's no air flowing through the radiator, so everything will heat up.

Once you start driving, things cool down again.

I would check those first then move on.
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Old 04-18-2015, 08:57 PM   #3
Ldub
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Wow, thank you for the quick reply.

Sorry I should have specified. It's not isolated to happening when stopped. I've tried to time it to see if there is a common time for it to happen... about 7 -12 minutes into driving- it can happen on the highway... stop and go..
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Old 04-19-2015, 08:21 AM   #4
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Has the car's cooling system been "overhauled"? The cooling system on these cars is made of plastic. The heat cycles and age cause the plastic to get brittle. The consensus here and on other E46 forums is that the system needs to be overhauled at or close to the 100k mile mark.

I bet your cooking system has s major problem at this point. Replacing or repairing an overheated engine will cost you thousand ($2500 min). Either follow Mango's cooling system stickie, or find a good independent ("indy") shop to do it for you. Either way make sure ALL COOLING SYSTEM PARTS ARE EITHER BMW OR OEM! Don't scrimp by using cheap eBay or aftermarket parts. In the end those cheap parts will let you down. Btw, I've owned and worked on my BMWs for more than 40 years.
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Old 04-19-2015, 09:09 AM   #5
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Agree with Marcus for a total system overhaul. The issue to me sounds like the thermostat failing.
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Old 04-19-2015, 12:29 PM   #6
Ldub
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Thank you both. Within the past 2 years we've replaced the radiator, expansion tank, upper and lower hoses and I believe the thermostat.

From performance, and the unusually quick way the gage is moving I don't get the sense it is actually overheating- though since I can think of anything worse to do to my car besides maybe drive it into the ocean, I also don't want to take any chances that it actually is overheating. I've had computer issues in the past that affected the transmission, so once after it shot up into the red, I pulled over, turned it off and turned it back on a few minutes later and it was back to the middle. It will also only happen once in a driving session.

Would you recommend our first step maybe be replacing the thermostat? I will look for the Mango Cooling System Stickie.

Thank you again.

Last edited by Ldub; 04-19-2015 at 12:29 PM.
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Old 04-19-2015, 12:42 PM   #7
BaliDawg
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It sounds like there may be air in the system. If the gauge moves quickly, might be because of an air pocket there. Bleed the system.

Also, check the coolant flow and maybe replace your water pump. More than once I have seen an impeller disintegrate leaving little pumping ability.
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Old 04-19-2015, 01:33 PM   #8
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Quote:
Originally Posted by My325XI View Post
Agree with Marcus...
C'mon, at least spell his name right. It's MarkusMarkus, so bad ass they named him twice.
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Old 04-19-2015, 03:16 PM   #9
Ldub
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Thank you.

We're going to start with bleeding the system and see where that gets us. Will report back- but probably not going to get to it until mid week- and won't be driving it until we give this a shot. Watched videos from E46Mango and WorldMechanic... Sounds promising and a solid DIY. All crossables crossed!
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Old 04-19-2015, 05:37 PM   #10
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Start with the bleeding...and then therm sounds like the best guess, since it's responsible for maintaining the flow of coolant. While you're at it, though...make sure you don't have pulleys hanging up...that your belts are good...but I think it's probably either bleeding or therm...sounds like it's stuck closed...doesn't happen often, but it does...and get an OE therm. Off market ones don't play nice with our cars. I think Wahler is OE, but don't quote me.
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Old 04-19-2015, 05:45 PM   #11
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Use the OBD hidden menu to view your water temperature and see how it is reacting when being driven.

It will give you a much better idea on what may be happening.

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