E39 Overheating -HELP HELP

My 97 528 started over heating about 3 weeks ago. The viscous coupling had gone so it was replaced just over a week ago. I took the car on holiday,
driving in various road conditions, and there were no problems. Suddenly on Sunday after having driven about 40 miles the guage rose and I stopped the car. The AA came out and filled the expansion tank and bled the system. The problem went away - until this morning when it happened again. I filled it up, bled it again and ran it on idle for quite a while - no problems - after cooling checked the water level - OK. Did the same again - OK. Drove the car for 10 miles and it overheated again. Filled up etc agaoin and the problem was cured.
The exhaust gas test has been done - nothing found. No oil in water. No water appears to be getting into the oil. No weird exhaust emmisions.
What can be causing what appears to be a pressure build up - could it be the thermostat? COuld air be getting into the system somewhere? The water pump has been replaced.
Can anybody help.
Thanks
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My 1954 Ford V8 had much the same symptoms. It turned out to be a cracked head. You may only see the water go away when the engine is under load. Thus, the only result will be unexplained water loss. You may not see strange exhaust emmisions because steam is hard to see. You may not see water in the oil.
You should get a compression test done because that will check the rings, valves, and head.
Jim
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Major ouch if it is.
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Yes, you are certainly correct about the high cost.
The only time that the old Ford lost water was while it was being driven. There was never any trace of water anywhere, and the engine never leaked while it was idling.
I believe that the crack happened as a result of delayed valve repair. However, it might have been a flaw in design. 1954 was the first year of production of the Ford OHV V8.
Jim
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Bognor Regis, W.Sussex, England, UK
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The E39 has a cooling system that isn't designed for longevity. The radiator, water pump and thermostat all start to cause problems in the 60-90K miles range (lower end for V8's, higher for I6's). You've replaced the pump? Probably look elsewhere. That coolant is going somewhere (although it can take a couple runs to get all the air out of the system after a drain and refill).
The radiator plastic will develop cracks, particularly in the upper hose fitting. Check the thermostat and its housing (I think there's an aftermarket aluminum housing for the M52), a preventative replacement would be a good move if it hasn't been replaced already.
Check all hoses. If they're original, they're of the age where leaks can develop.
If the fan clutch has failed, it'll overheat in stop and go traffic, then cool when moving at speed. If you've been running the A/C on a warm day (mid-March?) an aux fan failure can cause the temps to go up.
A pressure test on the cooling system would be a good idea.
A cracked cylinder head is a possibility if one of the overheating episodes was significant (it doesn't take much on these cars). Tell-tale is water vapor in the exhaust, particularly on start up (personal experience there, albeit with an E12). Or it could be the head gasket (the less expensive failure). We'll hope you parked it sooner rather than later when the temps began to soar.
The temp gauge on these cars doesn't really reflect actual engine temperature. The normal straight-up indication covers the entire spectrum of acceptable temps (it won't move for minor variations). When the coolant reaches a certain level of "hotter than normal" it will finally climb into the red. This can be quite rapid, accompanied with the instrument panel warning light. When this happens, the car has to be parked ASAP (you don't have time to drive 5 miles to the next exit) or the engine will be damaged severely and expensively.
R / John
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These days you'd use an exhaust gas analyser to check for that presence inside the cooling system.
--
*Consciousness: That annoying time between naps.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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It could just be a small leak somewhere, from the new waterpump or from the rad etc etc caused by the initial overheating. (Are you *sure* it was caused by the fan in the first place?) I guess if somebody put cold water into a dry overheated engine then it could have done some damage.
Run it at idle or high idle and feel the hoses to see what's going on with the thermostat, waterpump etc. ...or drive it from cold a short distance and then check for water loss before it's warmed up. If you put neat anti-freeze in the car you can sometimes smell it in the exhaust and/or it'll come out a different colour to regular steam if there is a head gasket or head problem.
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Thanks to everyone who replied. It appears that it is not losing coolant but is bubbling and losing it through very high pressure out of the expansion tank cap. If the car is operated for short distances or left overnight etc. no coolant is lost. I've now replaced the thermostat and its housing. The originals were still in situ.
I drove it 15 miles and everything wasa fine. Fingers crossed.
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