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Discussion Starter #1
Sorry its long winded, but here's the background etc. plus the question...

My 328 started to get hot the other day and the computer started whinging about low coolant, so my better half who was driving at the time pulled over before it got onto the red and had a nosey under the bonnet. She found the header tank looking empty and ended up calling out the AA. 2.5 hours later an AA man arrived who couldn't find any external leaks, couldn't find any evidence of coolant in the oil and so filled the system up and ran it at reasonably high revs for a bit - the temp guage came up above its normal half way, and before it got to red they switch it off. They gave up and transported it home to our local garage (I was away at the time, so the garage were asked to sort it out). The garage had it overnight and kept leaving it idling and checking it every so often, and again the coolant kept going down. They did a dye test on the coolant to check for head / gasket issues and it came out clear. Again, they couldn't find any external leaks or evidence of it in the oil. Eventually I got home, so they gave it back having not found anything, except that it had taken quite a lot of coolant, and they hadn't seen it come out anywhere.

I fired it up the next morning (coolant level was ok before and after I started it), warmed it up to normal operating temp (guage came up to half way and seemed to be behaving) and I then took it for a hard fast run up several long hills to see what happened. The guage stayed at half way, the heater was giving out very hot air (on demist - max heat) and it all seemed ok, except when I got back (15 mins of hard fast driving, mostly up steep hills) I got hold of the radiator hoses and the bottom one was still cool, whilst the top one was very hot. The coolant in the header tank was a normal level and the temp guage was still normal. I felt that the thermostat should have opened quite a while ago, so given the difference in temp between the two hoses I presumed the thermostat was stuck closed (although it was new less than six months ago - made in Germany by Motorad), and going over the sequence of events it seemed likely that a stuck closed thermostat could have caused the issues described.

I bought a new (Genuine BMW) thermostat and have today come to fit it. Before fitting, I put both the old and the new in a pan of hot water on the stove and watched to see the results. They are both rated at 92 degrees, and both seemed to open and close without any problems - if anything the Motorad one seems to open more than the Genuine one, but they do both open and close freely. A bit bemused I checked my manual, and read this:

"Thermostat Quick Check
To check if the thermostat is opening and coolant is circulating
through the radiator, allow a cold engine to reach operating
temperature (temperature gauge needie approximately
centered). Shut off engine. Feel the top radiator hose. If the
hose is hot to the touch, the coolant is probably circulating correctly.
If there are any cool areas in the hose or radiator, coolantflow
to the radiator is probably restricted . Check for afaulty
thermostat or a plugged radiator.
NOTEA
thermostat that is stuck open will cause the engine to
warm up slowly and run below normal temperature at
highway speed. A thermostat that is stuck closed will restrict
coolant flow to the radiatorandcause overheating ."


Now that says that if the top hose is hot, the coolant is circulating correctly - mine was, but my bottom hose was cool. Looking at the casting & thermostat housing I can see that the thermostat controls the flow to the bottom hose, so my bottom hose being cool when it shouldn't be says thermostat... But it works on the stove, and like I said seems to work better than the Genuine one. Any ideas?????
 

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Was the cooling system bled correctly?

How to bleed the cooling system.
1.Jack the front of the vehicle up onto stands as high as possible,
2.Turn the ignition on to Aux 2, Engine OFF, heater on fan setting 1 and temperature fully hot,
3.Remove the rad cap/expansion cap,remove the bleed screw from the radiator and IF you have them the bleed screw(s) from the thermostat housing/top hose, return pipe or EGR thermostat.
4.Fill the radiator/expansion tank up and keep pouring till there are no more air bubbles coming out from the bleeds, using a funnel that fits tight in the expansion tank neck and filling it full of water will help push any troublesome air out, when good you can then install the bleeds and tighten down(don't snap them),ensure the rubber O-rings/seals are still on the screws/on the bleed holes.
5.Suck some water out so it's not full to the brim, start the car up and check coolant shoots out of the small hole on the inside of the expansion tank neck when the revs are held up, if it does then replace the cap and then run the car up and check for heat from the vents when the car is warm, then carry on and check if over heating still persists. You may have to bleed troublesome cars again afterwards.(but very rare).
6.If all is good then check the water level again in the morning when cool.

or

Ignition on, heater set to hot, bleed screw out.
Pull the top hose off the radiator. Pour coolant SLOWLY down the rad hose a litre at a time. When the coolant is building up in the hose, hold it above the level of the rad hose stub and keep adding it. When coolant starts to dribble from the rad, refit the hose. You may find that coolant starts to overflow from the rad cap orifice - if it does, just screw the cap back on. Once you've done that, the engine and radiator are full of coolant - the little air pockets that might be left will come through the bleed screw.

After the vehicle is bled you should check that when the revs are held up you get a flow of coolant shooting out of the hole in the inside of the expansion tank neck, this will help tell you if the water pump is circulating, then run the car up and check the lower radiator hose gets hot.
 

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Discussion Starter #3
The system originally had no air in - it was bled properly when I fitted the engine six months ago and has behaved perfectly until this incident occurred.

At the point my wife pulled over with the overheating I suspect the reason it was low was because it had blown out of the expansion cap (for reasons currently unknown, but thought to be the thermostat stuck closed). The AA then topped it up, ran it for a bit (no doubt with air trapped in) and it started to overheat, so it went to the garage.

The garage tried to bleed it as best as they could, and given that when I got it back from them the coolant level didn't change and the heater worked and the temperature stayed normal, despite the thrashing I was giving it, I'd say that most of the air was out thanks to the garage's efforts. I didn't try to bleed it when I was test driving it, but then I didn't get any overheating - so it seemed to be sorted out. The only problem I experienced was that the lower hose was still cool well after it should have got hot, but as mentioned the thermostat seems to work fine on the stove.

If there had still been trapped air, it wouldn't be in the lower radiator hose, and it would not have ran at a consistent temperature whilst I thrashed it. But I don't get why the thermostat works well on the stove, but then was obviously still closed on the car (after the test drive). I'm going to fit the new one regardless, but I'm not convinced its any better than the old one.

Also, 92 degrees seems very high - is this the correct thermostat rating for a 328?
 

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BMW issued a 92 degree stat for Nikasil engined cars so they would heat up faster and run hotter to prevent bore wear..
I would be whipping the water pump out, replace it if it's shagged and refill via the top hose. Once the heater is blowing hot, it's bled so from there you need to check for leaks. If all is okay, I would get it pressure tested to make sure.
 
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