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Old 05-13-2015, 09:05 PM   #1
josephG
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Overheating mystery

Hi:

I am looking for some advice on an issue I've been having on my 2004 325i automatic. To give a little history, last summer, on a particularly hot day (90 degrees +), my car started overheating. Trying to save some money, i took it to a local shop. After looking at it, they claimed it was the water pump and so I had that replaced. Several hundred dollars poorer, the overheating problem didn't go away. This time it happened again when it was really hot outside and when I was stuck in heavy stop and go traffic. After doing some research, I figured it might have been that the coolant wasn't bled properly and there might be an air bubble. So I took it back to the shop and asked them to bleed it properly. When I got the car back, the mechanic claimed there was indeed an air bubble and they took care of it. Thinking back now, I'm pretty sure he was BSing. Very shortly after, sure enough, car overheated again on a hot day during heavy stop and go traffic. What's interesting is that it would overheat and go to the 3/4th mark and then go back down again within a minute or so.

Due to a hectic schedule, I never got to take the car back to the shop again that summer. Fast-forward to this spring...I'm now living in a different city. Didn't have a problem all winter long, but first 90+ degree day, car overheated again. So I took it to a local shop that specializes in BMWs (highly reviewed shop) and gave them the entire history. The shop owner said it most likely is the electric fan or the sensor itself that's bad. He had both of them tested out and they checked out just fine. He didn't even charge me for his efforts. However he did claim that it is most likely the ECU that is bad and I would have to go through BMW to get that sorted out. I did call up a local dealership and posed the hypothetical of how much it would take to have the ECU replaced. Gonna be around $2000. I'm kinda at a dilemma now. Although I trust the local BMW shop owner, 2Gs is a lot. Researching more about the ECUs, I'm finding out that is rarely ever the problem. I'm wondering what else the issue could be. I just don't want to be in situation where I spend $2000 for something that never needed to replaced.
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Old 05-13-2015, 09:06 PM   #2
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Could be lots of things but I'm guessing the electric fan is not kicking in. Could be as simple as the coolant temp sensor on the lower hose - but the fan is supposed to default on when that goes for the very reason of preventing overheating.

If you haven't done any work lately on the cooling system and this problem just "appeared" I suspect your electric fan.

Check to see if the electric fan spins if you turn the ac on. If it does not (which is what I suspect)- it's a component of the electric fan unit. Simplest to replace the entire unit although you can solder in a new fan relay if you're so inclined and you test the motor ok. When you remove it inspect the connections (3 wires) at the relay and the fan motor (2 wires) - sometimes these get corroded off and can be repaired.

One last thing - - read the threads on here about the aux fan relay - this component is supposed to make the fan go faster when the coolant temp goes up. WHat I have found though is it acts like the FSR and some speeds simply do not work. So although the fan might spin at startup and sometimes even when the ac is on - it won;t get to max speed in stop n go traffic as it needs to.

Last edited by BMW-North; 05-13-2015 at 09:09 PM.
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Old 05-13-2015, 09:36 PM   #3
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I suspect the mechanical fan.
Do check the electric one as suggested, but on the automatics the mechanical fan alone should be able to maintain temperature.

Here is a high tech test procedure:
Let the engine run until it reaches operating temperature. Roll up some newspaper and stick it to the mechanical fan. If it stops easily, it's not engaging.

It's not easy to tell if it's bad, because it will still spin, just not that fast.
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Old 05-13-2015, 09:41 PM   #4
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Quote:
Originally Posted by lszlszx View Post
I suspect the mechanical fan.
Do check the electric one as suggested, but on the automatics the mechanical fan alone should be able to maintain temperature.

Here is a high tech test procedure:
Let the engine run until it reaches operating temperature. Roll up some newspaper and stick it to the mechanical fan. If it stops easily, it's not engaging.

It's not easy to tell if it's bad, because it will still spin, just not that fast.

I don't agree that the mechanical fan can do it alone. perhaps at a decent speed but in stop n go the cooling system is dependent upon the electric fan.

It would be wise for him to check both though
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Old 05-13-2015, 09:47 PM   #5
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I have checked it on my car. I ran the engine at operating temperature and observed the auxiliary fan, it never came on. (A/C off)
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Old 05-13-2015, 10:11 PM   #6
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The local BMW shop owner also suspected that it was the electric fan, but after testing it, he said it was perfectly fine and wasn't the culprit. I can always get a second opinion, but no reason to not trust him. His shop specializes in BMWs and they are highly reviewed. He did say that even though it is very rare, the ECUs do go bad and looks like that is the case with my car. But wouldn't my car show other symptoms then, if that were the case? Besides the car overheating on hot days in stop and go traffic, there haven't been as many major issues.

I can't stand the thought of going to the dealership and them doing this whole dance of guessing what the problem is and wringing me out of thousands of dollars in the process.
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Old 05-13-2015, 10:30 PM   #7
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Not my fan for the radiator the other fan. It moves but when i put my hand in there it stops.. Thats not suppose to happen right?
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Old 05-13-2015, 11:08 PM   #8
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Quote:
Originally Posted by josephG View Post
The local BMW shop owner also suspected that it was the electric fan, but after testing it, he said it was perfectly fine and wasn't the culprit. I can always get a second opinion, but no reason to not trust him. His shop specializes in BMWs and they are highly reviewed. He did say that even though it is very rare, the ECUs do go bad and looks like that is the case with my car. But wouldn't my car show other symptoms then, if that were the case? Besides the car overheating on hot days in stop and go traffic, there haven't been as many major issues.

I can't stand the thought of going to the dealership and them doing this whole dance of guessing what the problem is and wringing me out of thousands of dollars in the process.
For a BMW shop this should be really easy to diagnose, obviously the first one you took your car to didn't diagnose it properly.

Quote:
Originally Posted by jess94 View Post
Not my fan for the radiator the other fan. It moves but when i put my hand in there it stops.. Thats not suppose to happen right?
Jess, start your own thread on that, it gets confusing to respond to two different people.
If the interior fan starts and stops randomly, it's either a connection problem, or a faulty FSR. (resistor)
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Old 05-13-2015, 11:14 PM   #9
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Quote:
Originally Posted by josephG View Post
The local BMW shop owner also suspected that it was the electric fan, but after testing it, he said it was perfectly fine and wasn't the culprit. I can always get a second opinion, but no reason to not trust him. His shop specializes in BMWs and they are highly reviewed. He did say that even though it is very rare, the ECUs do go bad and looks like that is the case with my car. But wouldn't my car show other symptoms then, if that were the case? Besides the car overheating on hot days in stop and go traffic, there haven't been as many major issues.

I can't stand the thought of going to the dealership and them doing this whole dance of guessing what the problem is and wringing me out of thousands of dollars in the process.
Time to find a different BMW repair shop. Them saying "it is likely the ECU" shows they didn't diagnose the problem down to the failed part(s). Which ECU does he mean, the DME?

Since you've had multiple overheats you should first confirm the head gasket isn't blown. If those checks are okay, then have a real BMW mechanic diagnose the cooling system problem.

You're getting dicked around by amateurs here.
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Old 05-14-2015, 09:01 AM   #10
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Thanks for all the responses. The guy was indeed talking about the DME. I'm definitely taking it to another shop to get a 2nd opinion.
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Old 05-14-2015, 09:23 AM   #11
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Therms go bad too. Although they usually fail open, they don't always. It might not be opening enough...it's a usual maintenance item...they don't last forever.

And, you could simply have a leak that is slight, which you're not noticing...not enough to trigger low coolant, but enough to cause that deadly air bubble. If air gets into the WP, it will cavitate, and it might be at low speeds, it doesn't have enough WP power to get past the cavitation. Just a theory.

There are low odds things too, but it's never been the DME before. Honestly, some shops say that to shut you up while they take their time and figure it out. Then you're happy when they say they fixed it...for only $1200...and you have a whole new cooling system!

That's another way to go, of course...rebuild it new. See Mango's cooling thread. For $300-500 or thereabouts, you can buy every component. BTW, does your heat work in the car okay? If you ever start to overheat when it's 90 degrees with AC on, you know to turn off the AC and turn on the heater full blast, I hope...then pull over quickly and turn off the car...unless you know you're going to be moving again soon.

Oh, just make sure belts and pulleys are good too...and, of course, make sure your main fan is working as it should. The clutch on that seems to fail not infrequently.

Sorry...I type quickly...and it could be many things. Read cooling threads until you're feeling cold inside. Then you'll know what to do!
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Old 05-14-2015, 02:56 PM   #12
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Sounds like your fan clutch may need to be replaced.
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Old 07-20-2015, 06:17 PM   #13
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Have an update on this. I took it to another shop to get a 2nd opinion about the DME. They ended up replacing the fan clutch (apparently the previous one was "garbage" and flushing out the coolant. Didn't work...car was still overheating on hot days in stop and go traffic.

Service Engine soon light came on again, and this time the code showed a faulty electric fan. Had that replaced and was assured that would definitely fix the over-heating problems. And of course, that still did not fix the problem. So now it is back to the DME module being the culprit. Mechanic said there are 2 options

a) Get a brand new DME from BMW, which will set me back a few grand.

b) Disconnect the fan from the DME. He mentioned this would possibly trigger the service engine soon light, but the fan will stay on when the engine got hot enough until I turn the car off.

To date, these are all the things that have been "fixed", but never actually solved the problem

water pump
fan
fan clutch
coolant

They said everything else checks out, so that only leaves the DME. If it is indeed the DME, I definitely don't want to fork over a couple grand for a new DME on a car that has over 150K miles and is probably worth around 5-6K.

Is there anything else that could be causing this. ANd if it does need a new DME, is there any way to have a used one re-programmed. Apparently this site says you can

http://www.ecudoctors.com/bmw-ecu-dm...d-rebuilt.html
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Old 07-20-2015, 06:33 PM   #14
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Quote:
Originally Posted by lszlszx View Post

Here is a high tech test procedure:
Let the engine run until it reaches operating temperature. Roll up some newspaper and stick it to the mechanical fan. If it stops easily, it's not engaging.
DON'T DO THIS, or risk breaking a plastic fan blade!!!!

Instead turn the engine off and rotate the fan by hand and feel the resistance of the viscous friction as compare to when engine is cool.

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Last edited by Sapote; 07-20-2015 at 06:50 PM.
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Old 07-20-2015, 06:50 PM   #15
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"What's interesting is that it would overheat and go to the 3/4th mark and then go back down again within a minute or so. "

This is very interesting piece of data that no one had touched on. This proved that the electric fan kicked in and cooled it down fast, and so the elec fan was fine. The only issue is why the fan kicked in too late, only after the needle got to 3/4 mark?

1) is it really overheated engine, or just a bad display caused by bad temp sensor or gauge? Set the dashboard to engine monitor mode to display the temperature in number to see how high it went before the fan kicked in to drop it down.
2) do the temp gauge and the fan use the same sensor or two different sensors, anyone? If two sensors then it could explain why.

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Old 07-20-2015, 08:41 PM   #16
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Yeah, it usually starts overheating when the car is idle or going extremely slow on really hot days and it goes back down within a minute or so. I usually turn off the AC and crank up the heat as soon as I see the needle move past the mid marker. The fan does kick in..but maybe at the incorrect speed?
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Old 07-20-2015, 09:00 PM   #17
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Was the thermostat ever replaced? Less than $100 part. definitely would beat a $2K ECU.
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Old 07-20-2015, 09:06 PM   #18
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"I usually turn off the AC and crank up the heat as soon as I see the needle move past the mid marker."

Why you didn't declare this before in the same post? If you didn't do this, do you think the temperature would drop down?

Bad data input leads to bad answer.
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Old 07-20-2015, 09:23 PM   #19
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My325XI: I believe the thermostat was indeed replaced at one point. This problem has been ongoing for the past 2 summers now. Every mechanic I take it to usually claims they know what the issue is, and every single time, the problem persists, except my wallet is a lot lighter.

Sapote: I learned about the heat trick from this forum. Prior to that, at one point, I did simply just put the car in park and the needle went back down to the middle in a couple of minutes. As I mentioned, this only happens when the car is idling or going extremely slow in traffic on very hot days. Most times, as soon as the car starts moving (5-10 miles), the needle goes back down again.

On a side note, I've had the 'service engine soon' light come on numerous times for the O2 sensor, in fact, way too many times. Just throwing it out there..not sure if it is related.
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Old 07-20-2015, 09:29 PM   #20
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Quote:
Originally Posted by josephG View Post
Have an update on this. I took it to another shop to get a 2nd opinion about the DME. They ended up replacing the fan clutch (apparently the previous one was "garbage" and flushing out the coolant. Didn't work...car was still overheating on hot days in stop and go traffic.
Has anyone confirmed the t-stat is functioning properly?

Quote:
Originally Posted by josephG View Post
Service Engine soon light came on again, and this time the code showed a faulty electric fan. Had that replaced and was assured that would definitely fix the over-heating problems.
What was replaced? The fan blade, fan motor, fan relay module or all of it (New fan assembly - which is what I'm hoping you'll say)?

Quote:
Originally Posted by josephG View Post
b) Disconnect the fan from the DME. He mentioned this would possibly trigger the service engine soon light, but the fan will stay on when the engine got hot enough until I turn the car off.
This is perhaps the most interesting observation - this mechanic is saying that the electric fan could keep your engine cool if only it was not being slowed down or stopped by the actions of the DME?

In order to do this you would need to install a switch to turn the fan on and off from the cabin as it is powered constantly even when the ignition is OFF - otherwise the fan would run your battery dead. You would also need to disconnect the PWM wire and by-pass the fan module and wire directly to the fan motor - otherwise it is still the DME that is telling the fan module to run full speed - kinda like when it detects the lower rad temp sensor is faulty - the dme then instructs the fan module to run full speed.


Quote:
Originally Posted by josephG View Post
They said everything else checks out, so that only leaves the DME. If it is indeed the DME, I definitely don't want to fork over a couple grand for a new DME on a car that has over 150K miles and is probably worth around 5-6K.

Is there anything else that could be causing this.
Well - if you have a brand new complete fan assembly that is good (be aware I've witnessed two recent brand new units arrive DOA out of the box) and you believe the DME to be ok - what's in between? Only one thing - the PWM signal wire.
The electric fan gets a signal through the third wire from the DME to tell the fan module what voltage to send to the fan motor. If this wire was damaged or corroded somewhere from the fan housing to the DME then possibly the fan module is not getting the right signal for fan speed? - I have not seen this but it is possible.

You should conduct the test attached - it will tell you if your DME is communicating to the fan module correctly. I'd do this before I'd buy a new DME.


Edit 1 - I attached some additional info to give you a better understanding of the DME fan relationship.

Edit 2 - It just occurred to me that I've seen some fan unit wiring where the three wires for the fan assembly have been touching one of the AC lines and as a result had the insulation melted. I'd check the fan assembly wiring (not on the fan - but in the engine bay) - especially the thin wire which is the PWM signal wire. Any interference (shorting, corroding, resistance etc) on the PWM line might cause the fan module to misinterpret the signal from the DME. - The INPA test attached will confirm if your DME is talking to the fan module correctly.

Edit 3 - Lastly - it would be worthwhile reading the PWM signal values on the 3rd wire. However you do not know what to compare the results to. The DME will request any of 15 fan speeds and send a specific signal to the fan module - that can be measured with an oscilloscope or even a decent voltmeter - however those values and their relevance and reliability are something I'm still working on unless anyone else has them recorded from multiple vehicles with different DME's for the e46? - Balidawg may have some insight on this.

Last edited by BMW-North; 08-04-2015 at 06:49 AM.
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