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Discussion Starter #1
So this is a strange one. I've had my E39 M5 for just about 2 years now, and the car has never even shown any single indication of overheating, I have checked and verified that the KTEMP in the OBDII monitor reads appropriately for the thermostat, and other than putting off a coolant flush (because the car came with gasp green coolant fluid), I have had zero cooling issues.

Yesterday I leave the gym to go home, and prior to getting on the highway see a sick deep grey colored Audi R8 V10. R8 pulls out and is behind me, I get on the on ramp and roll quickly around and of course he's right on my ***. We merge onto the highway and the R8 takes off, and we both eventually end up in the left lane, R8 in front of me, then traffic comes to a complete standstill.

I drive about .5 - 1 mile in 90 degree 90% humidity disgustingness in 1st gear stop and go, with MAX AC on full blast. Already thinking about the heat being generated in my engine bay, I was already looking at my oil and engine temp gauges. Everything fine.

Until.... there is a tiny gap of traffic, Audi R8 guns it and emits the most glorious sound with pops and crackles as he lets off the gas, and I speed up and pull right behind. At this point, I see my oil and engine temp gauges immediately start to rise to the right! Like I could SEE them moving!!! I PANIC and smush myself into the next three lanes so I could go into the breakdown lane and turn the car off, but as I move over I am right on another exit for a highway going south. I figure it's far better to push some air through the car than to stop and turn it off because as soon as I got away from the R8, things started to stabilize.

I shut the AC off, put heat on 90 full blast fan and drove home like that, being super careful to not sit idling for too long as I didn't know wtf was up.

Now that things are ok and I saved a blown head gasket or even worse, I truly believe that my car was just stupid hot, on the verge of not being able to keep up and that little gap we drove through quick and my pulling right up to the R8's rear end must have shot 10,000 degree heat from that V10 DIRECTLY into my radiator fan, which sucked all that heat in and tossed it into my engine bay, essentially raising the coolant temperature so high it couldn't keep things calm.

Would you all agree that this is a plausible theory, based on the fact that my car has never ever given a sign of any cooling issues whatsoever? Seems legit..
 

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Sounds plausible. How high did the temps get? Did you get a CEL? Might be worth checking for any codes.
 

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Discussion Starter #3
No CEL, I would say the engine temp gauge was almost at the 3/4 mark but not quite and the oil temp was a bit past halfway mark. Good idea to check for codes.... I will do this. I really think my OCD attention to detail with focusing on the gauges before it actually started to overheat made me act quickly and save the day before things got worse.
 

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Do you have any history of the fan clutch being replaced? It's possible your cooling fan was not active during the times you were crawling in traffic.
 

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Discussion Starter #5
No history of fan clutch being replaced, and I'm positive my electric fan was blowing on top of the mechanical one. I hear the airplane fan sound whenever I get out of my car and it's in the 80's all summer long so that's definitely working. I think the fact that it was working is what yanked the heat out of each of that car's 10 cylinders and dumped straight into my engine intake!
 

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I truly believe that my car was just stupid hot, on the verge of not being able to keep up and that little gap we drove through quick and my pulling right up to the R8's rear end must have shot 10,000 degree heat from that V10 DIRECTLY into my radiator fan, which sucked all that heat in and tossed it into my engine bay, essentially raising the coolant temperature so high it couldn't keep things calm.
If you were racing F1 cars then maybe. Otherwise no hell no. Just my 2 cents.
 

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Discussion Starter #8
Bumping this thread as I am concerned I have a soft failing thermostat. Coupled with this strange overheating issue I felt was honestly due to sitting in traffic with MAX AC on a 90+ degree day and then coming up on a hot *** R8 V10 who had just gunned it, pouring excessive amounts of heat directly into my electric fan, I now have a cold temp issue.

Ambient temps outside this morning were the coolest we've seen in awhile, around 51 degrees. Car takes awhile to warm up at this temp like usual, but after about 10 minutes on the highway, my KTEMP sat solid at 77 C. When I got off the highway I did a 3rd gear rip and then stopped to get gas and it went up to around 79 and then back down. By the time I got to work, it was around 81. I have not had any strange overheating issue like that one time behind the Audi since then, and have been like a psychopath monitoring my KTEMP on the OBD monitor constantly. I think I might be making myself extra paranoid, but if work is needed, I need to do it before the winter comes. T-stat likely 3 years old by now, car has 100K and (GASP) I have not swapped the green coolant that came in the car yet to BMW blue. Was going to do that asap this fall, but I might as well do the T-stat if I am flushing all of the coolant since it's somewhat old. I just don't want to touch anything that is working properly, been there, done that before, MANY MANY times....

Pics coming in the next post, please chime in with any advice here, thank you all!
 

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Discussion Starter #9
This is after 10 minutes on the highway on the way to work this morning.






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Discussion Starter #10
Here are some other shots of driving around town last week





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If you're concerned, replace the t-stat for starters. It's a cheap part and not hard to replace. It's also a great time to flush that **** coolant out (you will need to open the block drains) and flush with some distilled water first, then fill with Blue BMW. The coolant is not contributing to the issue now but compatibility with the aluminum internal components could be an issue long term. For the extra $10 the blue **** costs per gallon, it's worth having the right stuff in there. I'm a proponent of using whatever fluids you want as long as they meet the requirements. Engine oil, trans, diff, powersteering. Whatever floats your boat. When it comes to coolant in BMW's though, I stick with BMW. There's a reason they tint it blue and it's not readily available aftermarket. Some will claim it's the best marketing scheme out there... maybe.

My theory with your current situation is that the inner seal came out and is now riding on the t-stat. If you had cool weather and a highway cruise it will probably run too cold. 78C is (IMO) still considered normal for highway cruising. That's well within a sensor's accuracy +/- error range.

The next theory is that your radiator is half plugged with ****. If it can't flow, it's going to start running hot.

IMO, what happened isn't normal. My car started to creep above normal only a handful of times in the 9 years I've owned it. Each time was due to some type of problem. Once recently was at the track, it started getting hot after too few number of laps. Turned out the sealing ring popped out and was jamming the t-stat so it couldn't open or flow all the way. Another time was in Death Valley, roughly 110F, AC blasting, crawling up a pretty steep grade behind a slow car. Got home from that trip and investigated and found the radiator (although only 4-5 years old) had a ton of **** blocking fins. Removed and cleaned it out and it's been great since.

Good luck
 

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Discussion Starter #12
You offer great advice here Technician! I will say that I am well versed in BMW repairs as I did all the work on my E46 330i in the 12 years I owned it. But from what I have read, replacing the T stat is not an easy job due to the 3 o-rings. Seems like there should be ok access to do all of this though. I am not afraid, but I kind of want to do the water pump and t-stat and all of it at once.

I have had the BMW blue coolant ever since I bought the car, I don't know wtf is wrong with me, I need to evacuate that green asap to save my aluminum. Off to another building now for an hour drive, I will do KTEMP all the way there and report back on how it's reading.
 

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Discussion Starter #13
More pics after an hour drive, Tstat seems fine I guess...



These two were in some stop and go traffic




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What's the ambient temp? T-stat does seem to be working fine.
 

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Discussion Starter #15
It started out around 50 this morning and got up to low 60’s at the end of my drive..


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Plenty cool enough to expose a bad t-stat or one that's stuck open due to a damaged seal.

I would investigate the radiator area. If you're thinking to change the coolant, just remove the radiator hoses to start dumping the fluid and just plan to remove the radiator. You can then flush the radiator with water. Once you pull it out you will immediately see if it has lots of debris caught. It wedges between it and the condenser, so it's hard to see any other way.

Sounds like a nice Saturday afternoon or Sunday morning project!
 

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Discussion Starter #17
That's a really really good idea. I am going to put this on my list to do asap, maybe even this weekend! Thanks for your replies here bud!
 

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The thing I noticed is the gauge is not in the exact same position for the stated OBC temp reading. If I saw that I would likely change the temp sender first and closely inspect the plug and connections. There can be issues there, but if you are trying to diagnose something it would help to know what the real temps are and that they are accurate.
 

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Discussion Starter #19
Good point sailor. What do you think about these new images from my way back? Was reading the oil temp this time.





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Discussion Starter #20
Actually, when I look back through all my KTEMP pictures here, the value does appear to match the coolant temp gauge deflection accurately. Oil temp gauge also seems accurate based on reading.
 
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