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Hi, I'm posting on behalf of a friend, who is struggling to get his BMW roadworthy again. The head gasket recently failed (believed to be because of a failed head bolt), so had the head skimmed, pressure tested, and rebuilt. The head is now back on, with a complete suite of fresh gaskets, a new water pump, new cambelt and tensioner, and a new thermostat. The cooling system has been flushed (to get rid of any emulsified water), and refilled with great care taken to chase out the air - the heater matrix was filled, then the block, then the radiator, then the header tank was filled until water came out of the thermostat housing bleed valve (highest point in the system). The car was started after a lengthy prime of the oil pump (at least 1 minute without ignition). It fired into life instantly, and seemed to be running well; water temperature gradually rising, no fluids leaking from the engine, and nothing untoward from the exhaust. However, just when it was thought all was fine and dandy, the temperature didn't plateau and kept rising. My friend kept running the car until the 3/4 mark on the temp gauge, which the car reached fairly quickly, at which time some screaching noises were emitted from the engine, so he shut it off pronto. The screaching noises were identical the noises experienced when the car originally broke down. The thermostat has been removed, and checked in a pan of hot water, and definitely works. If anyone has any good ideas what might be wrong with the car, please don't be shy to suggest them, as we're at a loss, and would welcome any ideas no matter how daft they might seem!
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Has the gasket gone again after this test? Was the head and block checked for cracks before it was rebuilt? I think these BM heads were prone to going a bit porous and cracking.
As it seems to have been a thorough rebuild, and you are 100% sure you bled all air successfully, there can't be a great deal of other possible causes. On the other hand, some engines once they are disturbed never seem to settle down again quite the same.
I hope you find the cause.
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The cooling of these engines is an absolute pain - I have had lots of problems with them in the past and they are best avoided to be honest.
Without seeing and hearing the engine its just conjecture - however the following springs to mind:
1. IIRC there is a very thin pipe from the header to the rad for 'self bleeding' - is this clear?
2. Is the system pressuring excessively (hoses go hard soon after starting and/or bubbles in header) - head could be porous or cracked (I know its been pressure tested, but....)
3. Is the fan working OK?
4. Are you sure that there are no blocakges in the system (rad, for example) and that water can freely circulate?
Its not rocket science - if coolant is circulating and there is no air/gas in the system then it won't overheat.
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Did you open the heater controls before you filled the cooling system?
wizard
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Thanks, for the replies. Below is a copy of my friend's post on another forum, which might help with ideas:
"I've recently re-fitted the head to my '88 325 after the head gasket failed due to an as-yet unknown cause. It all went back together fine but the engine is overheating immediately (when left idling it carries on past the middle of the gauge - I haven't driven it at all).
When filling the cooling system I:
- pulled the hose which runs from the head to the heater off the head then filled from the header tank until water came out of the hose - i.e. water runs through the heater OK
- filled the radiator through the hose that connects it to the thermostat (until water came out of the bleed valve on the thermostat) - i.e. radiator isn't blocked
- removed the brown gauge sender and filled through this until water came out of the back of the head
- topped up through the header tank
Thermostat has been checked in a pan of hot water, however I've now replaced this with a thermostat which is permanently open (for testing).
The hose to the top of the radiator gets very warm but I think this is just conducted from the thermostat - the top of the radiator is cold.
When the temperature rises there is a screeching noise coming from the water pump/front of engine area so I have replaced the water pump with a brand new one (although the old one actually seemed fine) but the noise happens with this one as well.
Have I missed anything obvious? Any thoughts? "
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Sounds like the water is not circulating. Has he flushed the rad and block. Try back-flushing the rad.
Not sure about the screeching noise - these cars have a viscous coupling on the fan which have a reputation for failing - its not that is it? (When hot, try stopping it with a rolled-up newspaper - with obvious health warning!).
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