E39 overheating

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Hello,
Two nights ago I was driving along and looked down and notice the temp guage was in the red. Pulled over immediately and waited for it to cool
down. Started it up and drove for about two miles, everything was normal and then it heated up again. Had to pull over and let her cool down a few times before getting home.
Next day drove to work. Worked fine for about 5 miles, then heated up again. Did not go into the red, however it constantly went up and down.
Took it to the local mechanic today. He told me it was the fan clutch. He replaced the fan clutch. Drove it home today, half way there you guessed it....temp was heating up. It did NOT go into the red. It got to about the one o'clock position. Temp went up and down constantly.
Got to my driveway. Temp was running hotter than normal but did not go into the red while the car was idling in the driveway.
Here is whats interesting!! I turned on the heat, and moved the vent control to the heat position. The temp immediately went down to normal!
Then as an experiment I turned the AC on and moved the vent to the cold position. The temp guage started going up. Then I turned on the heat again and moved the vent to the hot position, then temp went down!
HELP! What could it be?
Yes the radiator has plenty of coolant.
Do you think it's the thermostat?
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This is bad.

It's not the fan clutch. If it were the fan clutch, it would be running hot at idle but not at highway speed.

This is reasonable. You're putting another heat exchanger in the chain along with the radiator.

The thermostat is the first thing I'd check, but I'd also check the water pump and the flow rate through the radiator. A clogged radiator will cause just this sort of issue.
How many miles are on it and when was the last time the water pump was replaced? --scott
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wrote:

Scott did not want to mention it, but overheating the 525i can cause serious (fatal) damage to the head.
There is a chance you've done a bad thing. If you have water control issues from this point going forward -- you have to repeatedly fill the radiator but there are no signs of leakage -- then this suggests that the head is cracked. A cracked head is never good. Sometimes it's worse than others, but it's never good in the grand scheme of things.
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I haven't had to fill the fluid. It has has no leaks.
I don't believe there is any head damage, The other night when it went into the red I pulled over immediately. There was no boiling or steam.
I even took the radiator cap off - no steam.
Today the temp fluctuates up and down, but hasn't gone into the red.
Do you think it's safe to drive for the weekend?

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I cracked my head, and it took 6 months to discover it.
If your car went to the Red, that's a very bad sign. You DO NOT need to do anything immediately. Just keep an eye on the coolant, and if you find that you're filling more than you formerly did, then this is an indication that the head is damaged.
You said it overheated multiple times, and got to the red at least once. That's what happened to me.
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No. Pull the thermostat out immediately and check it. It's okay to drive the car without any thermostat in the housing for a short while. So if the thermostat proves to be bad, leave it out while you drive to the auto parts store for a new one.
If it's good, test the radiator or take it to someone else who can. You might want to risk driving it short distances but you're taking a chance when you do because the consequences of overheating ANY of these engines are pretty severe. --scott
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No. The consequences could be very expensive.
The E39 has a strange temperature gauge which doesn't read actual temperature. It's controlled by the engine ECU which does - and translates this into almost just a warning light. It will show the engine warming up - then stay exactly in the middle over a wide range of actual temp - the range that is normal for any engine. Go above this and it rapidly climbs and the warning light comes on. There is a way of making the dash readout the real temp - but you'll have to Google for that. That might give clue as to what's happening.
The obvious thing to check first is the thermostat - these are cheap enough to just replace on spec. You might well need a hew housing too, though as these are plastic and sometimes break on removal. The next thing to replace is the waterpump. Both of these require draining of the system to some extent - so refill with the correct anti-freeze and follow the bleeding procedure.
If it's overheating when cruising with the AC off the fans don't matter as the air flow through the rad is enough. Using AC in slow moving traffic can cause overheating if the electric aux fan doesn't work - and this is a very common fault. The aux fan should run all the time the AC is on - and also if the coolant temperature goes high with it off. Is yours working?
The viscous coupled fan can normally be heard working at low speeds if you rev the engine - it makes more noise than normal as the coupling locks up at high temperature and makes it run at full speed rather than just idling round.
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London SW

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Are you absolutely certain about that?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Sun, 30 Aug 2009 11:46:57 +0100, "R. Mark Clayton"

The e39 auxiliary fan runs whenever the AC is on, regardless of temperature.
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wrote:

I'm with Dave on this. On my 528 the aux fan comes on with the AC. Even with a cold engine. Mike.
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No - replace the thermostat immediately - this is $5 part and an easy DIY.
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Are you absolutely sure about that price?
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Ones to fit a BMW?
Don't think we know the E39 model in question, but the supplier I use - the cheapest in the UK - charges between about 12-55 gbp depending on model. So a dealer will be much more.
There was a time when most cars used the same design of thermostat. Doesn't seem to be the case now.
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Later models of the E39 (with the 2.8L engine) use the electrically- controlled thermostat. They cost $75-$100. The days of the $5 thermostat are gone.
FloydR
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In which case isn't a wiring fault more likely?
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It's the freaking (electrical) actuator that fails. It's exposed to heat and coolant; bound to eventually fail.
FloydR
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Thanks for all the great replies.
I think I made some progress with my problem.
I forgot to mention when the mechanic replaced the fan clutch her replaced a hose for the radiator, therefore flushing the system and putting new coolant in...
As I stated before, "after" it was fixed the temperature climbed frequently but never in the red.
Now, two days later it apears to be working fine.
Could it be my mechanic didn't bleed the system properly?
I'm going to be watching it very carefully until I'm convinced it's fixed.
Thanks for all your great input
Jeff
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Yes, that's possible. I'm not sure it would cause the symptoms you report, but it is certainly within the realm of possibility.

My car had no apparent problems, except that it drank water. Watch the coolant consumption.
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