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Overheating 540i. Cold Radiator? HELP ME PLZ

1783 Views 25 Replies 9 Participants Last post by  Jaakkova
I have searched but cannot find a thread that solves my problems without throwing money at it.

Backstory:

Replaced radiator with a cheap aftermarket one last fall because the old one was leaking. Was find until about last month when it started popping hoses off. 4 times total, 3/4 were right after I stopped the vehicle and turned it off. The little plastic nipple that connects the resevoir bleed screw to the upper radiator hose snapped. Replaced the coolant resevoir with a junkyard one. Then I was at a drive through, getting some nuggets as one does, and it started steaming from what i believe was the bleed screw, squirting coolant and steam down the radiator. Let it cool, topped off coolant and inched it home under 2k rpms to my place 1 mile away.

Replaced Things:

Waterpump
Thermostat
coolant resevoir (used)
Radiator (6 months ago before problems)

Causes:

Really need some help diagnosing. Maybe aux fan not working? i dont seem to be getting power to it but also my ac doesnt work. Clogged radiator not allowing thermostat to open? I turned on the car and let it get close to operating temp to see if the aux fan would come on, but it did not.I then felt the radiator and only the top 1/4 was actually hot. The lower 3/4ths were cool, doesnt seem normal to me is it?
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Can you say "cooling system overhaul". Use quality parts.
I would love to. But $

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Can you say "cooling system overhaul". Use quality parts.
Hmm specced it out on fcp euro, 287 for the rad with hoses and new bleed screw, and reservoir cap. Might have to

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I would love to. But $

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The problem may have been inadequate bleeding.

You don't mention the model year. Aux fan control has changed over the years.
You probably have multiple issues, but eliminate air in the cooling system first. For one, since changing of the radiator, you have leaks in the system 4 times. Did you properly bleed the system after each leak? If not, that's the first thing you need to do. Jack up front end off the ground to as high as your jacks will go, turn to ignition 2 position without turning not the engine but put your HVAC temperature setting to max high, like 90-degree F. Follow online bleeding instruction available. Once you are sure to get the air out, see if you have any additional issue, such as AC not working. It sounds like there is an issue with AC system and the Aux fan won't turn on.
Unfortunately, that's often the reality of owning these cars.
I have searched but cannot find a thread that solves my problems without throwing money at it.
Coolant reservoir (used), and Radiator (cheap aftermarket) : that's the whole story...
I agree with everyone else here. The cooling system is a weak link in these cars and there is a very specific bleeding process to get air out of the system. This is not a place to cut corners and expect a perfect out come.
Hi, so it's a 99. Unless you need to be a wizard to bleed it or something I'm pretty sure I did it correctly. I followed instructions from a YouTube video and double checked it with a procedure posted on a BMW forum. What do you think about the aux fan?

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The problem may have been inadequate bleeding.

You don't mention the model year. Aux fan control has changed over the years.
So I have decided I will replace the radiator, coolant reservoir, and all new hoses. That way every component will be new.

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The ac compressor was broken when I bought it and the belt was disconnected. Do you know the relation of the ac
System to the aux fan? Would a broken compressor prevent the aux fan from coming on once operating temp is reached? I have purchased a new aux fan by the way.

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You probably have multiple issues, but eliminate air in the cooling system first. For one, since changing of the radiator, you have leaks in the system 4 times. Did you properly bleed the system after each leak? If not, that's the first thing you need to do. Jack up front end off the ground to as high as your jacks will go, turn to ignition 2 position without turning not the engine but put your HVAC temperature setting to max high, like 90-degree F. Follow online bleeding instruction available. Once you are sure to get the air out, see if you have any additional issue, such as AC not working. It sounds like there is an issue with AC system and the Aux fan won't turn on.
On your MY, the aux fan is controlled by the engine computer. It provides a control signal to the control module mounted on the fan. It can vary the speed of the fan based on the radiator outlet temp sensor.

The fan will run when the outlet temp exceeds 120 deg. C. It does not depend on whether the A/C is on.

Slavishly follow this procedure from Jim Cash:

Bleed Cooling System BMW method

On the I6, you need to raise the front end of the car up as high as you can to get the air to the front of the engine.

*********************************

Cooling system bleed procedure (E39)

- with cold car
- top up coolant to proper level (even slightly higher will not hurt at this stage)
- leave rad cap loose

This stage bleeds air trapped in the heater system.
- turn on ignition switch to position 2 (no start)
- set temperature selectors on the climate control to max temp.
- manually set the fan speed to low (lower end of speed range)
- start engine (check that climate settings have not changed)
- after 5-10 seconds of running rev the engine 3-4 times to about 2500 RPM - hold there for about 5 seconds each time.
- shut off engine (should not run for much longer than about 30 seconds - you just want to shut if off before it starts to generate any heat and expand the coolant).
- inspect the coolant level and top up again if necessary.
- tighten the coolant fil cap.

This stage bleeds air from the engine, rad and fill tank.
- now warm up the engine (drive a bit if you wish)
- with engine running CAREFULLY / SLOWLY open the bleed screw. Hold a rag around the area to prevent any excess splashing. Keep the screwdriver in the slot of the bleed screw so you can close it quickly.
- you should have some air/steam escape - progressing to some bubbles of coolant.
- as soon as it progresses to the stage where you are seeing liquid coolant come out then close the bleed screw. Do not over tighten - it is all plastic threads and you can easily turn too far at which point the screw jumps back a thread and you have to re-tighten. Just make it snug with all coolant stopped.
- when the car is cold again re-check the coolant level and top up again if necessary.

NOTE - some of the 6 cyl models have 2 bleed screws - the second one is near the thermostat. In that case you should bleed at both locations (one at a time)

You may have to repeat the bleed procedure 1 or 2 more times over the next week. Can "probably" skip the stage for the cold start bleeding of the heater system.

If the level continues to fall below the correct level then have the system fully pressure tested - explaining to your service manager what you have already done.

It is very important that all leaks be sealed so that the system can operate at full pressure. The engine computer is programmed to keep the coolant temp at about 108C at "most" times (there are exceptions) and the only way it can maintain that temp is if the pressure can be maintained - system fully sealed.

Cheers
Jim Cash

Info from RDL:

Aux fan control later MYs - http://forums.bimmerforums.com/forum/showthread.php?2051678-Erratic-engine-temps
The sensor in the lower rad hose is used to control the auxillary fan in front of the radiator/AC condenser. There were 2 modes of control depending on model year. In earlier designs the sensor is a multi-pole temperature switch which turns relays on/off in stages for 3 possible fan speeds. For the later design the sensor is a thermistor providing a temperature signal to the DME which then controls the aux fan in 20+ speed using a PWM signal to the fan control module - no relays.

The notion in both designs is that the aux fan runs only when the radiator isn't able to provide "cool" flow to the engine; "cool" being approximately 80C. This results in a seemingly strange result that the engine could be overheating while the aux fan is not running. E.g. the thermostat fails to open, the engine overheats, the radiator outlet though is quite cool/cold because little or no hot coolant is released into the radiator, auxillary fan is still.

In both designs, A/C operation is also able to turn on the aux fan indpendently of the rad outlet temperature.

In total then it's an either/or for the auxillary fan to be turned on.
EDIT: more precisely I should have said either - or - or both.

Regards
RDL
Wow this was exactly the Information I needed, thankyou very much! I do have two questions.

1.) You said the fan runs when coolant temp reaches 120C... I don't understand. I thought the normal temp range was about 100-110C. Does this mean the fan only runs in extreme heat scenarios?

2.) I used the unlocked cluster and let the car heat up after I thought I had fully bled it. Once warm, Ktemp was seemed constant at about 107C. I believe it started steaming from I believe the bleed screw area right after I shut it off(or maybe it was after being at 107 for several minutes while running). This must then be because the system is not fully sealed, right? I suspect bleed screw then.

Sent from my G9 PRO using Tapatalk
On your MY, the aux fan is controlled by the engine computer. It provides a control signal to the control module mounted on the fan. It can vary the speed of the fan based on the radiator outlet temp sensor.

The fan will run when the outlet temp exceeds 120 deg. C. It does not depend on whether the A/C is on.

Slavishly follow this procedure from Jim Cash:

Bleed Cooling System BMW method

On the I6, you need to raise the front end of the car up as high as you can to get the air to the front of the engine.

*********************************

Cooling system bleed procedure (E39)

- with cold car
- top up coolant to proper level (even slightly higher will not hurt at this stage)
- leave rad cap loose

This stage bleeds air trapped in the heater system.
- turn on ignition switch to position 2 (no start)
- set temperature selectors on the climate control to max temp.
- manually set the fan speed to low (lower end of speed range)
- start engine (check that climate settings have not changed)
- after 5-10 seconds of running rev the engine 3-4 times to about 2500 RPM - hold there for about 5 seconds each time.
- shut off engine (should not run for much longer than about 30 seconds - you just want to shut if off before it starts to generate any heat and expand the coolant).
- inspect the coolant level and top up again if necessary.
- tighten the coolant fil cap.

This stage bleeds air from the engine, rad and fill tank.
- now warm up the engine (drive a bit if you wish)
- with engine running CAREFULLY / SLOWLY open the bleed screw. Hold a rag around the area to prevent any excess splashing. Keep the screwdriver in the slot of the bleed screw so you can close it quickly.
- you should have some air/steam escape - progressing to some bubbles of coolant.
- as soon as it progresses to the stage where you are seeing liquid coolant come out then close the bleed screw. Do not over tighten - it is all plastic threads and you can easily turn too far at which point the screw jumps back a thread and you have to re-tighten. Just make it snug with all coolant stopped.
- when the car is cold again re-check the coolant level and top up again if necessary.

NOTE - some of the 6 cyl models have 2 bleed screws - the second one is near the thermostat. In that case you should bleed at both locations (one at a time)

You may have to repeat the bleed procedure 1 or 2 more times over the next week. Can "probably" skip the stage for the cold start bleeding of the heater system.

If the level continues to fall below the correct level then have the system fully pressure tested - explaining to your service manager what you have already done.

It is very important that all leaks be sealed so that the system can operate at full pressure. The engine computer is programmed to keep the coolant temp at about 108C at "most" times (there are exceptions) and the only way it can maintain that temp is if the pressure can be maintained - system fully sealed.

Cheers
Jim Cash

Info from RDL:

Aux fan control later MYs - http://forums.bimmerforums.com/forum/showthread.php?2051678-Erratic-engine-temps
The sensor in the lower rad hose is used to control the auxillary fan in front of the radiator/AC condenser. There were 2 modes of control depending on model year. In earlier designs the sensor is a multi-pole temperature switch which turns relays on/off in stages for 3 possible fan speeds. For the later design the sensor is a thermistor providing a temperature signal to the DME which then controls the aux fan in 20+ speed using a PWM signal to the fan control module - no relays.

The notion in both designs is that the aux fan runs only when the radiator isn't able to provide "cool" flow to the engine; "cool" being approximately 80C. This results in a seemingly strange result that the engine could be overheating while the aux fan is not running. E.g. the thermostat fails to open, the engine overheats, the radiator outlet though is quite cool/cold because little or no hot coolant is released into the radiator, auxillary fan is still.

In both designs, A/C operation is also able to turn on the aux fan indpendently of the rad outlet temperature.

In total then it's an either/or for the auxillary fan to be turned on.
EDIT: more precisely I should have said either - or - or both.

Regards
RDL
!. No. That is one of the controls.

2. I can't see what you are observing. All I can do is to advise.

Did you exactly follow the bleed procedure? It is not rocket science.
Ok I don't understand your response on the first, can you elaborate? It will kick on before 120C?

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!. No. That is one of the controls.

2. I can't see what you are observing. All I can do is to advise.

Did you exactly follow the bleed procedure? It is not rocket science.
Since edjack hasn't replied in the last couple of days, I'll venture to help clarify

Regards 1)
First, one needs to be careful distinguishing between engine temperature and rad outlet temperature.
V8 normal engine operating temperature as reported by the sensor in the t-stat housing is the 100 - 110 C that you mention. I think edjack made a typo when he said "... outlet temp exceeds 120 deg. C ..." The actual trigger value is ~80C for your type of aux fan. And no, it doesn't take extreme heat to get the aux fan running. The aux fan will begin spinning slowly when rad outlet temp reaches 80C, at which point engine temp (rad inlet) might be 100C or could be 125C. If rad outlet temp continues to rise, aux fan speed is increased

Regards 2)
Engine temp of 107C is a good value for your V8 engine. However once a hot engine is turned off, coolant temperature will rise. This rise in temperature once the engine is stopped is termed "heat soak." While the engine is running the pistons, cylinder walls and cylinder head are all a good deal higher temperature than the coolant - that temperature difference is what drives the heat from the engine's metal parts into the coolant. Once the engine is stopped the cooling effect of water pump and radiator stops too. The hot metal continues to transfer heat to the coolant with the result that temperature rises ... and therefore so does the pressure. Apparently in your case that pressure rise is/was just enough to find a weak spot that didn't leak at 107C pressure but did leak at higher pressure. That coolant turned into steam once in the lower pressure of the atmosphere.

If the leak was at a bleed screw, probably a little tightening is all that's required. If the leak was at a tiny crack just beginning, a different story.

EDIT: bleed screws have been know to crack and leak. I've had one such and the crack was not easily visible. If the leak point was the bleed screw, try a new one since they are inexpensive and easy to change.
Wow this was exactly the Information I needed, thankyou very much! I do have two questions.

1.) You said the fan runs when coolant temp reaches 120C... I don't understand. I thought the normal temp range was about 100-110C. Does this mean the fan only runs in extreme heat scenarios?

2.) I used the unlocked cluster and let the car heat up after I thought I had fully bled it. Once warm, Ktemp was seemed constant at about 107C. I believe it started steaming from I believe the bleed screw area right after I shut it off(or maybe it was after being at 107 for several minutes while running). This must then be because the system is not fully sealed, right? I suspect bleed screw then.

Sent from my G9 PRO using Tapatalk
Ok thanks for clearing that up, it makes sense now. It appears now that my problems are

a.) An aux fan that won't turn on at the appropriate temp.

b.) A system that can not contain the pressure once the engine is shut off.

I did order the remainder of parts to fully refresh cooling system (hoses, rad, tank, fan temp switch, &brass bleed screw). Now I'm guessing the biggest culprits are the aux fan temp switch, and a leak somewhere in the reservoir assembly. I guess we'll see once I get the parts.

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Since edjack hasn't replied in the last couple of days, I'll venture to help clarify

Regards 1)
First, one needs to be careful distinguishing between engine temperature and rad outlet temperature.
V8 normal engine operating temperature as reported by the sensor in the t-stat housing is the 100 - 110 C that you mention. I think edjack made a typo when he said "... outlet temp exceeds 120 deg. C ..." The actual trigger value is ~80C for your type of aux fan. And no, it doesn't take extreme heat to get the aux fan running. The aux fan will begin spinning slowly when rad outlet temp reaches 80C, at which point engine temp (rad inlet) might be 100C or could be 125C. If rad outlet temp continues to rise, aux fan speed is increased

Regards 2)
Engine temp of 107C is a good value for your V8 engine. However once a hot engine is turned off, coolant temperature will rise. This rise in temperature once the engine is stopped is termed "heat soak." While the engine is running the pistons, cylinder walls and cylinder head are all a good deal higher temperature than the coolant - that temperature difference is what drives the heat from the engine's metal parts into the coolant. Once the engine is stopped the cooling effect of water pump and radiator stops too. The hot metal continues to transfer heat to the coolant with the result that temperature rises ... and therefore so does the pressure. Apparently in your case that pressure rise is/was just enough to find a weak spot that didn't leak at 107C pressure but did leak at higher pressure. That coolant turned into steam once in the lower pressure of the atmosphere.

If the leak was at a bleed screw, probably a little tightening is all that's required. If the leak was at a tiny crack just beginning, a different story.

EDIT: bleed screws have been know to crack and leak. I've had one such and the crack was not easily visible. If the leak point was the bleed screw, try a new one since they are inexpensive and easy to change.
Regards a)
How do you know the fan should have been running? I.e. have you confirmed that the radiator outlet temperature is greater than 80C when the aux fan isn't running? I've not seen anything in your posts that suggests a faulty aux fan system.

Regards b)
Correct about that. This seems to me the only problem you've identified - leaks. Work on getting a system that will hold pressure & not leak. Then see if there are any other issues.

FWIW, I wouldn't change the aux fan until there is proof that it is bad. It's likely the most expensive part that you've purchased. If there's any life left in the one now in the car you'll want to take advantage of it. Besides, there's no advantage to replacing it with the rest of the cooling system - there is no "while I have this apart" advantage.
Ok thanks for clearing that up, it makes sense now. It appears now that my problems are

a.) An aux fan that won't turn on at the appropriate temp.

b.) A system that can not contain the pressure once the engine is shut off.

I did order the remainder of parts to fully refresh cooling system (hoses, rad, tank, fan temp switch, &brass bleed screw). Now I'm guessing the biggest culprits are the aux fan temp switch, and a leak somewhere in the reservoir assembly. I guess we'll see once I get the parts.

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You might be right about the aux fan. However is it not uncommon to have a clogged radiator that won't allow flow? But if it was then it would not activate the temp switch for the fan. If the fan is in fact functional it would mean that either the temp switch is bad or the radiator is restricting flow to outlet pipe right?

Edit: I know the aux fan was not running at operating temperature.

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Regards a)

How do you know the fan should have been running? I.e. have you confirmed that the radiator outlet temperature is greater than 80C when the aux fan isn't running? I've not seen anything in your posts that suggests a faulty aux fan system.

Regards b)

Correct about that. This seems to me the only problem you've identified - leaks. Work on getting a system that will hold pressure & not leak. Then see if there are any other issues.

FWIW, I wouldn't change the aux fan until there is proof that it is bad. It's likely the most expensive part that you've purchased. If there's any life left in the one now in the car you'll want to take advantage of it. Besides, there's no advantage to replacing it with the rest of the cooling system - there is no "while I have this apart" advantage.
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