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Overheating 540i. Cold Radiator? HELP ME PLZ

1875 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.


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:

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


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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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.

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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.
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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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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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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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With a well functioning cooling system the auxiliary fan will not usually be running except in very high ambient temperatures. The belt driven main fan (with temperature controlled viscous drive) should be able to provide all the cooling necessary. Actually, an aux fan running in anything less than high ambient temperature is a sign to first check the main fan viscous clutch (assuming the A/C is off and not forcing the aux fan on to keep A/C pressures within limits.)

Nothing in your posts suggests a cooling system that won't keep up with the cooling needed. 107C engine temp is normal for your V8 engine. There is no evidence of a cooling problem due to pump, rad, main fan or aux fan. The only problem identified is a leak.

To repeat a point from my prior post - the steam you saw does not indicate an overheat. Rather the 107C coolant, perfectly normal, was liquid inside the pressurized cooling system and keeping the engine at normal temperature. It flashed to steam only after the little bit of it was released into the lower pressure of the atmosphere.

Regards a clogged radiator
A) internally clogged to impede coolant flow - engine will overheat, rad outlet temp will be low/cool since any coolant that does get through the radiator will be in the rad a long time and get lots of cooling effect. Aux fan will NOT be activated.
B) fins blocked to impede air flow - rad outlet will be high since too little air flows over the fins & tubes to cool the coolant. The aux fan will be run when rad outlet reaches 80C in an attempt to push more air through the blocked fins. If the aux fan can't push enough additional air through the rad, the rad outlet temp rises until it's too hot to keep the engine within normal and the engine overheats.

To satisfy yourself regards the aux fan, you could use a thermometer (contact or IR) to measure rad outlet temp. Only if it is over 80C and aux fan is stationary is there any reason for concern.
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