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Discussion Starter #1
Hello everyone. I have bought a 2002 540i for very cheap ( 400$ that was just sitting around with 90K miles ) I did a bunch of repairs but one thing I am stuck on is the car running HOT. Tstat is 106c OEM btw. I believe the car should be at 86c running normally. I get heat in the vents as well.
New Parts:

Water Pump
Radiator
Upper and lower hoses
TStat
Coolant reservoir tank with new sensors

I have had one pump die on me which was brand new but aftermarket so I installed a new good brand one. However, driving on the highway the temp goes up to 110c and sits there unless I keep the rpm around 3-4K then it drops to mid 80C from my memory. However, keeping RPM high causes a coolant leak and a slight smell of it but I dont see it anywhere. Could the hoses at the back of the motor be clogged? I assume them being clogged because when at higher RPM the temps go down. Could it be a head gasket? I do not see coolant in the oil however and the motor runs like a Swiss watch, perfectly smooth and strong.

What am I missing here? Should I do a coolant flush with a special cleaner? Possible plastic sitting clogging the rear hoses?

Thank you for any and all advice.
 

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Your leaking on top of the engine most likely the valley pan. Its one of those you dont see the leak for because it collects in the valley pan.

I would start by changing the thermostat also unless you changed it and know it is good. Is it throwing any bmw codes, was there a known hot issue before or is this new?
 

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You may have air pockets too in the coolant, have you properly filled it and burped it?

I experienced same issues while my valley pan leaked, never fixed it. Got rid of it before i got to it. I think i still have the hardware, oem replacement, should you decide to go that route yourself. Id verify first there is coolant on valley pan. Duckduckgo it.
 

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M62 should be running that hot, designed that way for efficiency. S62 runs cooler in the 80s.
This is correct. M62s run hotter, even on the highway.

Pressure test the system, fix all the leaks, and bleed it properly. If the gauge stays at 12 o'clock under normal driving conditions, you're fine.

And stop watching the KTMP value. You're unnecessarily freaking yourself out.
 

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Discussion Starter #7
You may have air pockets too in the coolant, have you properly filled it and burped it?

I experienced some issues while my valley pan leaked, never fixed it. Got rid of it before I got to it. I think I still have the hardware, OEM replacement, should you decide to go that route yourself. I'd verify first there is coolant on valley pan. Duckduckgo it.
I bled many times and also used my friend's bleeder for the coolant so that’s not it.

I will check the valley pan, but would this explain higher temps than normal? When I’m not on it hard, I don’t smell coolant. Why would temps drop when higher rpm, the pump is pushing more coolant? Does it almost point to an obstruction? New Tstat.
 

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Discussion Starter #8
This is correct. M62s run hotter, even on the highway.

Pressure test the system, fix all the leaks, and bleed it properly. If the gauge stays at 12 o'clock under normal driving conditions, you're fine.

And stop watching the KTMP value. You're unnecessarily freaking yourself out.
Even with Tstat being 106c? It shouldn't get to 110c then. Why does the temo drop to 90s when I'm at higher RPM?, Is that also normal? I apologize for my ignorance. The system was pressured tested and all seemed fine. Ill keep driving and check on the coolant lever each morning when cold.

Thank you.
 

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A leak will probably cause a pressure issue. The cooling system is supposed to operate under pressure, it lowers the boiling point of the coolant. I usually let the car sit for a couple hours after driving, even a day sometimes, and I slowly twist the expansion tank cap. If air is released when I turn the cap the system is holding pressure, but again, unlikely with a leak.
 

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Even with Tstat being 106c? It shouldn't get to 110c then.
Why would it not? 106c is the opening closing temp for the T-Stat. less than 106 it closes and coolant stops flowing sort of. There is a head bypass system to get cab heat early. The real question is why is it running cooler than the T-Stat temp. M62 cooling system is complex because it is also a high pressure system. You really need to be talking to guys that have had lots of experience with the M62, because it is all in the details. Have you posted your questions in a specific 540 forum?
 

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A leak will probably cause a pressure issue. The cooling system is supposed to operate under pressure, it lowers the boiling point of the coolant. I usually let the car sit for a couple hours after driving, even a day sometimes, and I slowly twist the expansion tank cap. If air is released when I turn the cap the system is holding pressure, but again, unlikely with a leak.
No, it RAISES the boiling point. That is one of the primary reasons to pressurize the system. For every psi, you roughly increase the boiling point about 3F.

Regards,
Jerry
 

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Discussion Starter #12
No, it RAISES the boiling point. That is one of the primary reasons to pressurize the system. For every psi, you roughly increase the boiling point about 3F.

Regards,
Jerry
To clarify. So if I have a small leak the temperature will rise 3F for every 1psi. Correct?

Thank you.
 

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Discussion Starter #13
A leak will probably cause a pressure issue. The cooling system is supposed to operate under pressure, it lowers the boiling point of the coolant. I usually let the car sit for a couple hours after driving, even a day sometimes, and I slowly twist the expansion tank cap. If air is released when I turn the cap the system is holding pressure, but again, unlikely with a leak.
I opened the cap and didn't hear any pressure being released which would point to a slow leak.

Thank you.
 

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To clarify. So if I have a small leak the temperature will rise 3F for every 1psi. Correct?
It raises the boiling point of the fluid or coolant so for every 1 PSI, the coolant boils at a 3 degree higher temp. If the coolant boils then the temps can get much hotter, think steam. It would take a lot of math to try and predict what temps might be seen.
Again I would be reading Bimmer Forums 540 or 740 threads the details are surprising and the results a little unexpected. Fluctuating temps does not seem to be out of the norm, but I have no experience with the details they are talking about.
 

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Discussion Starter #16
It raises the boiling point of the fluid or coolant so for every 1 PSI, the coolant boils at a 3 degree higher temp. If the coolant boils then the temps can get much hotter, think steam. It would take a lot of math to try and predict what temps might be seen.
Again I would be reading Bimmer Forums 540 or 740 threads the details are surprising and the results a little unexpected. Fluctuating temps does not seem to be out of the norm, but I have no experience with the details they are talking about.
Thank you again, I will register on the other board and seek advice, but yours is much appreciated. I did order all parts for the valley pan cover and necessary seals. This would be the last possible area causing the leak and looks like HG is ok. Happy sunday!
 

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To clarify. So if I have a small leak the temperature will rise 3F for every 1psi. Correct?

Thank you.
I don't know because the system is no longer closed.

I would take a guess that you would not increase the boiling point (at least not very much depending on the size of the leak) because it is now just an open system. To the extent the system can increase pressure, then you will get some increase in boiling point.

Regards,
Jerry
 

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Ok, I know a LOT about the miserable 540i cooling system, so listen up! Firstly, the other posts are correct that it runs ridiculously hot compared to almost all other production cars. And no, 106C is not at all outside its normal operating temperature range; the T-stat doesn't even open up until either 101 or 105 degrees (depending on exact model year)3. In fact, here's a cool little snapshot from a tool I used when editing my gauge to reduce the stupid 'buffer' zone in the middle. Stock, the gauge will stay in the middle anywhere between 75 all the way up to 115 degrees:
945801

Now, just because BMW thought this was ok back in 1999 or whatever doesn't mean we need to agree 20 years later, and many of us (myself included) think that's just crazy to let a poor car run that hot all the time. And fortunately, there's fix for those of us with that mindset! Grab yourself this 90deg instead of 105deg thermostat, pop it in, and be happier longer with a less-stress coolant system!!!
 

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Discussion Starter #19
Ok, I know a LOT about the miserable 540i cooling system, so listen up! Firstly, the other posts are correct that it runs ridiculously hot compared to almost all other production cars. And no, 106C is not at all outside its normal operating temperature range; the T-stat doesn't even open up until either 101 or 105 degrees (depending on exact model year)3. In fact, here's a cool little snapshot from a tool I used when editing my gauge to reduce the stupid 'buffer' zone in the middle. Stock, the gauge will stay in the middle anywhere between 75 all the way up to 115 degrees:
View attachment 945801
Now, just because BMW thought this was ok back in 1999 or whatever doesn't mean we need to agree 20 years later, and many of us (myself included) think that's just crazy to let a poor car run that hot all the time. And fortunately, there's fix for those of us with that mindset! Grab yourself this 90deg instead of 105deg thermostat, pop it in, and be happier longer with a less-stress coolant system!!!
Thank you for the info. I noticed that at 114c the temp needle moves to almost 3/4 gauge. I will purchase this TSAT even though mine is brand new. I'm doing the Valley pan and the two coolant pipes this week, parts on order. I also live in the hot climate of Puerto Rico so lower temps will sure help. Thank you!
 

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Sounds good! Also check out the info here if you're good with a computer and are interested in re-programming the temperature gauge to give you more of a REAL indication of temperature by reducing that huge buffer that keeps the needle in the middle pretty much the whole time; especially helpful to let the needle move upward sooner with a lower temperature T-stat in there!
 
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