[RossX5]

Engine is overheating - no coolent leaks, leak down test done and passed at BMW store, water pump replaced, clutch fan replaced, checked for air in the system at BMW store.
The BMW store are recommending i replaced the Aux/AC fan. That hasn't worked for ages and wasn't overheating then when driven in hotter temps.
I feel the Aux/AC fan will just mask a overheating problem. Any ideas on what it could be?

 

[andrewwynn]

Don't have enough info.

If ambient is high enough aux fan can be needed.

If ambient < 80f overheat possible if not moving fast enough eg traffic jam.

Test your fan clutch by stopping it while idle it should not take long to spin up and especially once car is warm the clutch locks based on temp conducted from the water pump it's attached to.

I would bet on the fan clutch as I had to replace one of mine recently.

I also bought the proper fan clutch wrench after I broke my thermostat in half using the smack with a hammer method that's worked for years.

 

[RossX5]

It has had three fan clutches in the last 1500k.

I took the radiator out and it was clogged with crap between the radiator and ac-condenser. That reduced the air flow. I also took out and tested the thermostat and it didn't open, so replaced that.

I'm unsure how the bmw store missed this.

Anyway I'm going on a long drive soon and I should find out if the overheating issue is fixed.

Oh and I bought and fitted a after market aux/ac fan.

 

[RossX5]

The amibant air temp has been between 5c and 15c

 

[tttomttt]

There is a combustion gas test that can be performed on the coolant to determine if combustion gases are entering the cooling system through the head gasket. It is a cheap DIY test that only takes a few minutes. Take your time when doing it to get the correct results.

 

[andrewwynn]

T stat not opening is 95% of the problem. You can pull up real-time ECT on the hidden OBC menu to verify.

 

[Doug Huffman]

T stat not opening is 95% of the problem. You can pull up real-time ECT on the hidden OBC menu to verify.

This case will be the first that I recall of a wax-pellet motor failing shut. A wax-pellet motor is very powerful and robust. Capable of 4,000 Newtons, ~900 lbf. The biasing shut spring is about 1/3 as powerful.

 

[andrewwynn]

I have seen them break both ways but the stuck shut was from the frame breaking so said motor pushed air.

 

[Doug Huffman]

motor pushed air.

That would be a problem - entirely different.

 

[andrewwynn]

I misspoke when I said "air" it was really pushing: pulling "nothing" as it broke free from the cage and did nothing. I have witnessed a stuck closed t-stat not broken obviously like that one maybe the can holding the wax split and leaked I've no idea.

It is exceeding rare ive changed dozens of thermostats and was very surprised to ever see one not open. (and even the one that physically broke I think opened a little)

 

[RossX5]

I had the t-stat in boiling water over 100c and it didn't open. The over heating issue seems to be fixed after replacing it, cleaning the radiator and replacing the aux fan. Batter to be safe than sorry

But I was surprised about the amount a crap that was between the ac condenser and radiator. If you have an older bmw its worth a look to keep the air flow.

There is a combustion gas test that can be performed on the coolant to determine if combustion gases are entering the cooling system through the head gasket. It is a cheap DIY test that only takes a few minutes. Take your time when doing it to get the correct results.

The leak down test came back at 2% so its all good on that front.