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Discussion Starter · #1 ·
So I googled myself down a rabbit hole trying find a similar overheating issue as mine but nothing that matches. In the last week temperatures have ranged between 100F - 107F, I’ve been running my AC in my 540i all summer with no issues in 80F - 90F temps. This last week in the higher temperatures when driving home from work which is a 25 mile commute and I get stuck in stop and go traffic or picking up dinner going through drive thru, the temp gauge jumps from the 1/2 mark right in the middle up to the 3/4 mark near the red. The first time it happened was in a drive through, granted Habbit Burger takes about 20 minutes to get through the damn drive through. It was over 100 out, I was in the car with my son waiting with the A/C on and the temp jumped up to the 3/4 mark. I put the car in park turned the AC off and idled the car at 1500 rpm getting the fan spinning until it cooled. It came down to the 1/2 mark after 30 seconds and stayed there the rest of the way home with the AC off. I did an experiment and I drove with the ac off when it was over 100 out and I didn’t have any issues. Then ran it with AC on got caught up in traffic and it got hot again, turned AC off, got moving and cooled back down.

I can hear the auxiliary fan for the condenser and the fan clutch both working. I suppose it could be the fan clutch getting weak, it might be original and maybe it’s only partially working which is why it’s partially overheating and hasn’t gone into the red. I’m not low on coolant, that was flushed and changed a couple of months ago. I had it at a bimmer shop about a month ago and the mech told me the water pump was leaking. I have no coolant on the ground but I know it can still be weeping a little even without the evidence. Is it possible if there’s a tiny leak at the water pump that a bubble could’ve formed in the cooling system causing reduced flow? I heard trapped air can be an issue on these cars. I would assume it would run hot regardless of the AC bring on if that were the case though even in cooler temps. I don’t think it’s the thermostat because if it were stuck it would either be hot all the time or take forever to get up to temp.

Anyway I’m thinking it’s the fan clutch so I opted to covert it to an electric fan. Just wondering what you all think about my overheating issues and thoughts about the electric fan conversion.
 

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What year 540? There were 2 different aux fans and control circuits.
If your aux fan is working 100% it could be a weak clutch fan and some air in the cooling system.

I replaced the clutch fan years ago with a 16” Flex-a-lite fan that I have mounted inside the shroud.
The fan controller keeps the fan running all the time around 33% speed. The controller will ramp the speed up if needed.
I don’t like on/off fan controllers, the clutch fan spins all the time for a reason.
 
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Not a bubble caused by a weeping water pump.

Outside (ambient) temps have almost nothing to do with how hot or cool your car runs. You have a different problem.

Fan clutches work great...you should test yours with a roll of newspaper..If you can stop the fan with a hot engine with the roll of newspaper....get a new fan clutch, but not a cheap one or one made by Meyle..Also don't replace it with an elec. fan..

You can test your aux fan (in front of radiator) with INPA and other 2-way scanners. You should test it.

If your fans work as they should you may have an obstruction or a faulty water pump.
Any chance anybody ever used stop leak or similar in the past? I have pulled hardened stop leak the size of my thumb out of E39 cooling hoses. Why did you flush your cooling system?
 

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Discussion Starter · #4 ·
What year 540? There were 2 different aux fans and control circuits.
If your aux fan is working 100% it could be a weak clutch fan and some air in the cooling system.

I replaced the clutch fan years ago with a 16” Flex-a-lite fan that I have mounted inside the shroud.
The fan controller keeps the fan running all the time around 33% speed. The controller will ramp the speed up if needed.
I don’t like on/off fan controllers, the clutch fan spins all the time for a reason.
It’s a 2001, I bought the fan from a small company out of California that sells on eBay. It’s a dual fan set up and comes with a controller designed to be a drop in. Not sure What quality the controller will be so I might upgrade it depending. I also think I’d prefer the fan to run at a slower speed and ramp up when needed, better on the power draw I’d think.
 

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Discussion Starter · #5 ·
Not a bubble caused by a weeping water pump.

Outside (ambient) temps have almost nothing to do with how hot or cool your car runs. You have a different problem.

Fan clutches work great...you should test yours with a roll of newspaper..If you can stop the fan with a hot engine with the roll of newspaper....get a new fan clutch, but not a cheap one or one made by Meyle..Also don't replace it with an elec. fan..

You can test your aux fan (in front of radiator) with INPA and other 2-way scanners. You should test it.

If your fans work as they should you may have an obstruction or a faulty water pump.
Any chance anybody ever used stop leak or similar in the past? I have pulled hardened stop leak the size of my thumb out of E39 cooling hoses. Why did you flush your cooling system?
I’ll try the newspaper trick but I think I’m going to swap out for the electric fan either way, at least to try it. It was something I was planning to do since I purchased the car last year. I’m looking to improve the mileage a bit, I also plan on putting an under drive pulley and didn’t want to risk under powering the mechanical fan.
Im wondering about the water pump now that you mention it, once I take a look to see if it’s at all wet around the seams of the pump I’d change the pump out if that’s the case. I had to replace the alternator which is why I changed the coolant, wasn’t a true flush but I ended up having to do it twice because a week later the small coolant hose behind the alternator ruptured. However I do admit I am not using BMW coolant, I’m using all vehicle green Prestone coolant. Possibly could be the issue not bring up to par with the bmw coolant. I have no idea if anyone ever put stop leak in it but I do know the radiator and expansion tank were replaced 45k mi ago so maybe someone did something back them. I bought the car last September so it wasn’t terribly hot out but I didn’t have any issues.
 

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Discussion Starter · #6 ·
If your aux fan is working 100% it could be a weak clutch
I feel like the aux fan is running harder than it ever has and stays at what sounds like 100% power most of the time. I pulled up to my parents house a few days back and my dad commented on me coming in for a landing because it sounded like a piper cub lol. Didn’t think much of it at the time but it wasn’t overheating then. But I was coming from work and it was only a 10 mile drive, just doesn’t seem like it should’ve needed that much assistance from the aux fan because it shouldn’t have been that heat soaked yet especially because it’s almost all freeway driving.
 

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Unlock your cluster and run test number 7 (Google is your friend on this). That will tell you engine temps during your drive. Post those temps. The temperature needle is buffered and really doesn't tell you or us anything.
 

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Discussion Starter · #8 ·
Unlock your cluster and run test number 7 (Google is your friend on this). That will tell you engine temps during your drive. Post those temps. The temperature needle is buffered and really doesn't tell you or us anything.
Will do when I get home, I was going to grab my scanner to get some real-time temps on my way to work this morning and on the way home but forgot it.
 

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Removing the manifold (engine?) cover might help you out when temps get this hot. My E38 is nakey (I just never put them bacl) to keep temps down. That manifold one is like a baby mattress.

Are you running both electric and "conventional" fans? I know the condenser is electric (boogie woogie). so you have two electric cooling fans + the fan clutch/fan off the WP?
 

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Discussion Starter · #10 ·
Removing the manifold (engine?) cover might help you out when temps get this hot. My E38 is nakey (I just never put them bacl) to keep temps down. That manifold one is like a baby mattress.

Are you running both electric and "conventional" fans? I know the condenser is electric (boogie woogie). so you have two electric cooling fans + the fan clutch/fan off the WP?
It’s got the AUX condenser fan and the stock fan clutch set up currently. I ordered an electric fan set up that should be here next week, hoping to install sometime next week. I’m going to pull that engine cover off on the way home to see if that helps, you’re right they’re pretty heavily insulated.
 

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Discussion Starter · #12 ·
Got a link to the fan and controller you purchased?
It comes with the kit I bought, I’m not sure what kind it is yet other than it allows you to adjust for the temp you want. Im converting my Explorer to an electric fan and bought a Davies Craig controller for that one, I like how it’s set up plus it’s already wired to kick the fan on high when the AC is on. What brand is the the controller you used on your car? I like the fact that it has the ability to run at a lower speed constantly.
 

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I built a PWM speed controller, I can set the minimum speed from 0-100%.
Typically it‘s set to about 30-40%. A thermistor senses radiator air flow to increase the speed if needed.
 

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540s don't overheat when the stock system is running correctly. I would have fixed whatever was originally ailing the car, rather than try to bandaid it with an electric fan conversion.
My car has overheated exactly two times...once when the expansion tank let go, another time when the aux fan failed. Suprisingly, on the second overheat, it finally dawned on me I hadn't heard the aux fan operating in months....it was original to the car, 15 years old and 250k miles at the time.

As one other stated, the ambient temp outside doesn't really have that much to do with whether the car overheats...both of my incidents occurred in the 50F or 60F temps...If you watch your operating core temp of the engine when you unlock the cluster, you will see that regardless of outside ambient temp, the cooling system can keep temps pretty consistent, regardless if it is 30 degrees out, or 108 degrees outside. That is what it is designed to do.
 

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Discussion Starter · #15 ·
540s don't overheat when the stock system is running correctly. I would have fixed whatever was originally ailing the car, rather than try to bandaid it with an electric fan conversion.
My car has overheated exactly two times...once when the expansion tank let go, another time when the aux fan failed. Suprisingly, on the second overheat, it finally dawned on me I hadn't heard the aux fan operating in months....it was original to the car, 15 years old and 250k miles at the time.

As one other stated, the ambient temp outside doesn't really have that much to do with whether the car overheats...both of my incidents occurred in the 50F or 60F temps...If you watch your operating core temp of the engine when you unlock the cluster, you will see that regardless of outside ambient temp, the cooling system can keep temps pretty consistent, regardless if it is 30 degrees out, or 108 degrees outside. That is what it is designed to do.
I was always planning on swapping out the stock fan for an electric one. I’m doing so because I’m trying to get a bit more mileage out of it, take some weight off the water pump and a little load off the engine. I know it’s a small gain but added with my other things I plan to do it ought to get pretty good mileage on the freeway once I’m done. I wasn’t planning on doing the electric conversion this year but this overheating issue has sped up my timeline. I’m really think it’s the clutch on the fan, I’m sure it’s original so it’s 21 years old at this point. If the water pump broke I’d imagine that I’d be full on overheating all the time as no coolant would be flowing through the system, right?
 

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I was always planning on swapping out the stock fan for an electric one. I’m doing so because I’m trying to get a bit more mileage out of it, take some weight off the water pump and a little load off the engine. I know it’s a small gain but added with my other things I plan to do it ought to get pretty good mileage on the freeway once I’m done. I wasn’t planning on doing the electric conversion this year but this overheating issue has sped up my timeline. I’m really think it’s the clutch on the fan, I’m sure it’s original so it’s 21 years old at this point. If the water pump broke I’d imagine that I’d be full on overheating all the time as no coolant would be flowing through the system, right?
Yes, probably if the water pump impeller was fully non functional or broken off completely. But you can "never say never". Anytime you start down the rabbit hole of modifying or re-engineering the cooling system, you open up a can of variables. Generally, I have replaced the entire cooling system every 100k miles (radiator, expansion tank, fan clutch, water pump, thermostat, thermoswitch, rad cap, belts, pullies and tensioners. Occasionally I have had to replace individual components because they failed or were damaged early, such as a radiator leak, or water pump leak. I'm currently at 338,400 miles or there about.
 

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Discussion Starter · #17 ·
Okay so I pulled a code when I got home and it was bmw trouble code 8D DME: Activation electric fan. Perhaps I have a bad sensor. Haven’t had a chance to research it yet, maybe I’m not overheating
 

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Okay so I pulled a code when I got home and it was bmw trouble code 8D DME: Activation electric fan. Perhaps I have a bad sensor. Haven’t had a chance to research it yet, maybe I’m not overheating
I would think your aux fan is bad.
 

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Discussion Starter · #20 ·
I would think your aux fan is bad.
Was thinking that as a possibility too but that still doesn’t explain why my temp gauge went near the red. A couple of things I’m wondering though after reading a few other forums. Based on the temp gage sitting right where it’s supposed to when the engine is at full temp, it has never shown any signs that it was going to overheat with the A/C off. The fan still runs at 100% but it is almost 100F so maybe the fan just runs that hard to keep things regulated. I sat in my hot driveway running diagnostics with my foxwell scan tool tool for a good 15 plus minutes of sitting and idling after a 25 mile drive, never once looked like it wanted to overheat. As far as my A/C system is concerned I think I might have a slightly blocked expansion valve because I believe the low side is running at a lot higher pressure than it should, i can’t remember the numbers off the top of my head but I hooked gauges up to it a while back when I thought it wasn’t working. My ambient air temp sensor was taken out along with the underbody panel it goes into at some point before I bought the car. When the bare wire gets wet it’ll register all kinds of weird temps all below freezing until it finally defaults to -40F. The car must know that -40F is a default because when it jumped around to all those other temps below freezing I couldn’t get the A\C to turn on.

The A/C seems to cool the cabin down just fine but I have noticed at slow speeds or in stop and go traffic it almost feels like the ac is turned off for a few moments although the cabin blower fan is still circulating air. Is it possible the A/C system is making the engine hot because it’s overworking?
 
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