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My 1991 BMW 318IS has suddenly begun to overheat quite badly. On the way up to London the needle was just off the red zone on the temp. guage and this was with the heating on full blast! The inlet side of the radiator is very hot and the outlet side and pipe is stone cold. This suggests to me a thermostat failure but these are pretty rare aren\'t they? It could perhaps be a broken water pump - again quite rare. The problem is much worse at high speed than just cruising around town etc. Any ideas/help would be much appreciated. There is no gunk or \"mayonnaise\" in the dipstick or oil filler hole and no noticeable water loss. thanks guys, Matt
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Matt,
I assume that your engine is the M42 16-valve. John Burns says on his BMW website that these are prone to failure of the front part of the head gasket (known as the Timing Case Profile Gasket) but he also says that this leads to total coolant loss so it doesn't sound like your problem. Ref: www.unixnerd.demon.co.uk/bmw.html
The fact that the problem is worse at higher speeds than around town would seem to exclude the cooling fan clutch as the cause. Failure of this component shows up more when the car is stationary or slow moving.
Unlike the 6-cyl M50 engine, I don't think yours has the dreaded Tufnol water pump impeller and I haven't heard of water pump failure on your model. However, the overheating seems to be due to coolant non-circulation so I would still check the water pump.
I would also check the thermostat. As you say, these are very reliable but they do sometimes fail, often 'closed'. Another possibility might be a collapsed hose inner lining.
I'm sure others will have different ideas to offer.
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dizzy,
Many thanks for those pertinent suggestions. It is the M42 engine in my car. I have heard about the head gasket problems but as you suggest this doesn't seem to be the problem. Hopefully the problem is the thermostat as this shouldn't be too difficult to change. I will keep you posted!
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some early water pumps had plastic impeller blades that used to snap off and jam up, it is possible that this has happened if the pump has not been changed before
dave
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Dave,
Are you sure the M42 engine suffered from this water pump problem ie. 16valve twin cam similar to that in the E36? If so what would the signs be - there is no unusual noise or vibration etc. and no apparent leakage. Thanks!
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Dave, the 6-cylinder M50 engine had a plastic (Tufnol) impeller but I don't think the M42 engine did, though I can't be 100% sure on this.
Matt, thinking further -- if the water pump impeller had totally failed (as the plastic ones can at around 100,000 miles) I'm sure you would have experienced boiling and a distinct loss of coolant rather than simple overheating. If the impeller has suffered erosion or corrosion (as metal ones can when the specified antifreeze/conditioner concentration has not been maintained) this could cause the symptoms you have described. However I think this would be gradual rather than sudden so it does seem unlikely. I would guess that full or partial failure of the thermostat remains the prime candidate.
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yes im sure about the plastic impellers, bmw went for them when they introduced the m40/2 as cost saving, like the cambelt. which has also proved as a bad idea as they have gone back to chain driven again!!(i know the m42 always was chain driven before you point it out)
dave
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Thanks Dave. Sorry to have questioned what you said.
I'd only heard of problems with plastic impellers on the M50 but the M42 is of similar vintage so I'm not surprised that it also has the plastic impeller. I wonder if Matt's impeller is just starting to break up, or do they always fail without warning?
It should be quite an easy job to take the water pump and thermostat off the engine to check them both out. Nice little job for Easter! The existing rubber 'O' rings should be re-usable if it's not convenient to buy new ones.
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Thanks for all the help guys. I replaced the thermostat last night and hey presto - it was absolutely stuffed and the car is now running at just below mid-temp despite me hooning it around to try and get it to boil! Good diagnosis chaps! By the way, anyone intereted in tuning cars and decent exhausts have a quick look at my post in the classified section here - if you mention Honest John and your name I may do you an even better deal! Cheers guys,
Matt
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