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Matt, Engineer
Category: BMW
Satisfied Customers: 21715
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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My BMW Z3 M-coupe was serviced by a BMW agent in March

Customer Question

My BMW Z3 M-coupe was serviced by a BMW agent in March for an overheating problem. They fitted a new waterpump and thermostat but it was just the same. As winter temperatures set in , the problem disappeared. Yesterday with the warm day temperature and extended in town driving, the engine started missing quite badly, more so at idle and revs up to +/- 3000. The water temp also went up close to the red mark and oiltemp between 105 an 110 degrees C. The previous repair job cost me more than R20000 and before going back to the garage and hearing their explanation, I would like to know what could be a cause. The car has 76,000 km on the clock and is a year 2000 model
Your advice would be very much appreciated
Submitted: 1 year ago.
Category: BMW
Expert:  Matt replied 1 year ago.


1st reverse flush the cooling circuit (remove top hose to rad and use a hosepipe to push water into the engine until you get clear water out of the radiator hose) and top up with 30-50% fresh antifreeze. Also check that the small diameter bleed hoses that run from the top of the radiator and the top of the engine aren't blocked as this allow the air to vent back to the header tank automatically.

Also its possible the belt is slipping so its worth replacing its especially if the rubber ribs are cracked or it looks shiny on flat side of the belt - while its off check that the tensioner and other pulleys all spin freely the exception to this is the AC pump ( if fitted)

I'd also suggest ensuring that you've got all the air out of the system as this sort of episode can leave pockets of air trapped and prevent the coolant from circulating properly.

So check that the small hoses that run from the top of the engine and radiator that run back to the header / expansion tank are free flowing and not blocked

If these are OK then remove the tank pressure cap and run the engine from cold with the heater set to full hot

As the engine starts to warm, squeeze the large hoses running to the radiator to help displace any air - all the time keeping an eye on the level in the header tank, making sure it doesn't drop below the minimum and open any bleed screws built into the top cooling hoses - closing them off as soon as they start to gush water

Once the engine is up to temperature the tank should be quite full. Switch the engine off and leave it to cool

Then top up to the mark once its all cold and replace the cap

Expert:  Matt replied 1 year ago.


do you still need help?

Customer: replied 1 year ago.
Hi Matt,Thanks for staying in touch. This overheating problem baffles me. Before I took it to the local BMW agents in March for the overheating problem our daily outside temperature was high ( 32+ Celcius ). After the new waterpump and thermostat + new fanbelt and tensioner, it was still overheating for a few days but the winter sarted setting in and the temperature remained normal until the other day that we had our first really warm day. I was doing a lot of city driving and noticed that the engine started missing espessially at idle and low throttle up to about 3000 revs. I may be mistaken but lately the oil temp seems to be rising quicker than before. The last two or so days I have been driving around to try and get the temp to go up again but it remained normal. What has happened is that the misfire is getting much worse and is constantly present at low to medium throttle. I have arranged with the garage to take my car in for diagnosis and treatment. I hope they can locate the problem this time. The car is otherwise in perfect condition cosmetically and mechanically (it has done only 76,000 km = +/- 51,000 miles and is 15 years old ) and I care for it like a baby.If we find the problem I will let you know.Thanks very much,
Kind regards,
Expert:  Matt replied 1 year ago.


OK one other thing to check which can produce overheating and misfires is a lack of fuel pressure

and could be a lack of fuel pressure from the electrical pump at the tank.

If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,
Ideally measure the pressure at the injectors and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

Customer: replied 1 year ago.
Hi Matt,I dont think the misfire is connected to the overheating. It only started very recently. If it was one of my older generation engines, I would have thought it was a cracked distributor cap or sparkplug wires shorting. A loss of fuel pressure will not fit the misfire being present at lower revs and low throttle but absent under full throttle and high revs. I will see what the BMW agents have to say.Kind regards,
Expert:  Matt replied 1 year ago.

Hello Peter

you can get low fuel pressure at low rpm and not at high as the fuel pump is controlled by a seperate module that will change the pump speed according to load

this is unlike the older engines where the fuel pump runs flat out all the time

so its quite possible to have a blocked filter for instance and low fuel pressure at low rpm and when the revs come up the increased duty on the pump is enough to overcome the blocked filter

and there's no distributor on these engines as its all coil on plug

Expert:  Matt replied 1 year ago.


do you still need help

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