1997 e39 540i overheating... need help with diag

[csaunderse39]

Hi I need help. I have a 1997 e39 540i that overheats. Here's the back story I had a leak in a hose and replaced it than bleed the system and it ran fine for a month. Than I found out the water pump went bad so I decided to do a top end rebuild( valve cover gaskets, valley pan, coolant tube clean and all coolant related flush.) Put new radiator, aux fan, switches and sensors in it and put it back together. It sat in the garage for about two months after repair was finalized but I would go out to it every other day look for leaks and start it and let it run for a while to get up to temp to see if it would over heat and it never did.so I finally got my drive way cleared of all my other project and decided to take it to work. Well it drove to work fine stayed at normal temp ( unsure what that is, my cluster was changed by the old owner and the new ones pixels are bad.) But middle on the gauge, yes I understand that, that could still not be operating temp. But it was fine to work which is about a 30 min drive from home. Now Idk what happens or if something broke but it was a 92 degree weather day and the car is black but let's just say it baked all day. I went to leave and not even 5 min down the road it started to over heat. Didn't matter how fast I went, if I shut off the a/c or turned on the heat. It started to climb. I pulled over let it cool down than checked the coolant level seemed fine. And didn't see any leaks. Checked oil for mixture still fresh and clean. Got it towed home and pulled back in the garage and started diag. I ran a compression check, all cylinders within spec. But I kinda figured they would be, because seven though the temp went up it still ran with no misfire of any sort. I topped off the coolant and started the vehicle and for about 45 minutes from a cold start it runs fine than it starts to climb in temp. Sometimes if I turned the heat on it would come down, but soon. Go back up. I noticed that the aux fan kicks on when the a.c. is on but not when the temp starts to climb. I tested this by jumping the wires and forcing the fan to kick in high. So I replaced the sensor that is supposed to contr that just recently and no change fan still won't kick on when temp starts to climb.( Also when I forced the fan to come on it still had no affect on getting the temp down it still climbed even when on high fan speed. I'm at a loss and I'm not scrapping the car. Only other issues with the car is the cats are rattling possibly about to cut them out and straight pipe it. If you have any on-site please I could use some guidance. Thanks in advance to anyone who attempts to help

[Zchild]

I just experienced a similar issue in which you're describing and it turned out my thermostat and clutch fan decided to fail at the same time. I changed out the thermostat and clutch fan and all of my issues disappeared. A few weeks prior to that I changed out my Auxiliary fan as well because it failed. Anytime I do a water pump I generally do the thermostat as well. Just my two and a half cents worth...

[geargrinder]

Just being a pedantic wanker here, that's not a 'top end rebuild' which is a term mostly applied to jug-cylinder type engines where you do everything north of crank/crankcase... and at the very least would be expected to mean head work... but anyway...

With Zchild for the most part... Did you do the thermostat same time as the water pump? How's your visco/mechanical fan? Seems like you've run it enough to be sure there's no mystery bubbles in there - I6's are trickier in that regard but w/ the V8 if you run a few temp cycles and drive it around, even worst case it usually gets the bubbles out after a couple times.

Given the odd Aux fan not activating thing - a longshot could be the dual temp sensor (at the thermostat). Perhaps its a bad cluster reading and the cars not actually overheating? Use the cluster function to read KTEMP and search and read the part about checking KTMP w/ key in pos 1 vs 2 and seeing if readings are same. Run the car up to the 'overheat' point then shut it down and quick run that check and see if one sensor is way off from the other.