I have a perplexing issue that's buggin the heck outta me. My 545i will throw up temperature warnings when driving. When it first happened, I had the fear of a major non-warranty repair. I pulled over, popped the hood, and, cautiously, removed the cap off the expansion tank, expecting superheated steam and fluid to be ready to burst fourth (I staged the process to relieve any pressure encountered. If you are not familiar with overheats or general automotive mechanic repair, I'd never recommend you doing this). To my surprise, I was able to remove the cap with no fanfare. Fluid level was fine, can see a nice coolant flow when engine is running, hose pressures are equal at temperature (which suggests a working thermostat), etc. I changed the coolant temperature switch located on the radiator at the lower radiator hose (grommet shrinkage can lead to leaks on these). When I run the AC, the auxilliary fan cycles on and off. I hooked up my old scanner and checked for trouble codes (the check engine light is NOT on. Looking for stored event data). All clear. Checked the live running PIDs(engine parameter data, including intake temperatures, coolant temperature, O2 sensor voltages, battery/car voltages, etc). Coolant temps from cold rose to normal and were solid @ 210. Ran the car on the freeway and monitored. Temp remained steady @ 210. After about 25 minutes, the overheat warning gong went off. Yet the temps remained the same. After an initial series of warnings, it magically stopped and didn't occur again. Has anyone dealt with this? I'm dropping the car off tomorrow at the dealer, but post warranty work costs moolah and I'm fearing the dent dimply diagnosing the problem could make. Thanks in advance everyone!

haven't had it, but could still very well be a leak somewhere. best advice would be what you're already doing: having it checked out early and hopefully preventing a "overheated engine" notification. could be just an error.. how many miles? last time you flushed the cooling system? you're probably right on the thermostat because a SES light should have followed (correct me if i am wrong on this). any trouble driving when the warning appeared?

No overheated coolant, no active or stored fault codes, and the fan and pump appear to be working - probably a glitch in the coolant temp sensor or faulty connection.

Awhile back, my Camaro would on rare occasion run at very high idle even though the car was warm and never displayed a fault code. I finally plugged in my diagnostic scanner and watched the sensor values. Lucky for me the car started acting up and the idle jumped to 1700rpms when the engine temp guage read 180 degrees (normal). What I discovered was the coolant temp sensor was showing only 60 degrees, which was not enough to throw a code but definitely enough to make the idle run so high. As soon as I touched the sensor connector the idle dropped back down to 650rpms. Although it didn't look bad, I ended up replacing the connector - problem solved.

If this was a one time incident I wouldn't worry much. But if its recurring then you may have to spend some time like I did observing and possibly logging the data to identify an anomaly. I would think that these cars actually store sensor paramaters that can be retrieved by BMW techs for analysis, but I would confirm this before bringing it to the dealer.

Thanks guys. @ aw3964 - the car now has 128,000 (my how they pile on over the years lol). No coolant leaks. System was flushed about a month ago. The car drives normally even with the error. It's just annoying to hear that "bing" going off. I just dropped her off @ the dealer for the booster line recall issue. I've asked them to diagnose this as well ($170 bucks just for the diagnostic...sigh). Cool thing is they gave me a shiney new 328 loaner. Not quite the F10 I was hoping for, but better than the ole stank PT Cruiser the 'other' area dealer gave me :nono:

@ pukka - Mna, I did test after test. High speed, low speed, long idle, etc all while monitoring the OBD PID maps. All within normal parameters it seemed. As you guys have suggested, I'm leaning toward a faulty temp sensor. My SA mentioned as much, but then he says possible computer fault :doh: . According to the information stored in the key, the car reports a steady running 178 degrees - well within normal temps. Sux to have to go through this routine, but that's the price to pay for such an advanced car. I'm seriously considering either 'downgrading' to a 335 (not so bad a downgrade lol!), or hustling a great deal on a 2010 550 from a dealer who wants to move em out as the F10s gain favor.
No more first-year high technology cars. My first F10 will be an 11 or 12.

Well here's the latest on this. I took the car in for recall work and requested diagnostics on this temperature warning issue. It was a head scratcher for the techs. The final verdict was stated to be pressure loss due to hose corrosion. The car expects to see 15psi in the cooling system. Pressure loss can trigger the warnings, I was told. Replacing all the cooling system hoses, at an approximate cost of around $1400, should cure the problem. :getlost: Soooo, I'm going to systematically start replacing the main hoses and clamps, then move to the secondary and minor system hoses. Sux. On a different note, I picked up a Dayco 3 year warranty serpentine belt and replaced the existing original in 10 minutes. The old belt has missing ribs, brazing, and was ready to fail it seamed. Id recommend all 545 owners replace the belts for piece of mind.

Your model must not have a/c as it would be a little foolish not to replace the ac belt as the same time. This one will take a little bit longer than 10 minutes if you don't have the right tool and the OE type stretchable belt ($10 online or $45 at the dealer) I do not recomend the after market ac belt as it is not design to stretch over the compressor, with the real possibly of bending the shaft of the compressor.

Oh if you have mileage on the car, you may as well change out the plastic reservoir too.

Let's not be a smart arse - the serpentine belt is a show-stopper if it breaks. Frankly, the AC compressor belt simply is not. Time dictates the amount of work I had to perform today. You are correct in the sense that it is best to change them both at once. Of course, it is also best to change all hoses, the expansion tank, associated clamps, etc at this time as well. But alas, my warning references belts, which, in the plural, suggests both the AC belt - for modern BMW 545i cars so equipped (which all modern BMWs are if I'm not mistaken) - and the serpentine belt. However, only the serpentine belt will leave you stranded on the side of a road should it fail. And with the ease of install, the relatively swift time frame to change, and the potential tow savings, it's not at all foolish to focus on that belt.

Sir, I'm no novice when it comes to automotive mechanics. I won't go and list my full resume' here, but I have worked in industries from computing, to anachoic testing at Edwards AFB. From high performance engine builds to civilian and mil spec turbines, including the ETOPS rated CF6-80C2 and CF6-80E1A2 turbofans. I'm no stranger to engine mechanics, dangers, risks, etc. However, in my particular case, the AC belt inspects fine - no cracking, good pliability, no rib loss, good traction, no glazing, and performance within specs. Again, while I appreciate the need to replace the AC compressor belt - the failure risks you list are very real should a worst-case scenario occur with that particular belt - the serpentine belt is GUARANTEED to leave you stranded should it fail. Again, if one doesn't have the immediate time, cash, or knowledge to replace both belts, again, my recommendation is to change the seprentine belt. However, if the ac belt is or becomes visibly frayed, or shows signs of possible failure, by all means if you don't have the time to replace it, simply cut it off. It won't stop you from driving (though you do lose the dehumidification function of your defrost). Let's not get into another tit-for-tat on this thread about belts. You raise very valid poiints and I appreciate and respect the reasons for them. But you state you are a dealer tech - what are your thoughts about the random overheat warnings? The dealer states the car monitors the pressure in the cooling system, and pressures below specs are triggering the warnings.

Sorry if it seems like I?m attacking you it is not my intention. I?m just going by experience, as sometimes, theory as to what will happen and what actually happen in real life can be very different. There are people on this board that are very inexperienced and will take what people write literally as the way to do things. I would rather tell someone what to do correctly than to tell them what works for me per say .


With as sensitive as some of the computers are now, it is possible that the code is due to pressure loss from the hoses, but you mention it was noticeable enough you can see it by removing the cap and not a have a large pressure drop and no coolant loss. The only way I can see that as being possible is a bad cap or very very soft ballooning hoses. I am only going by the information you have given without doing any diagnoses myself.