21 replies to this topic

[tmsmini]

Also check for hairline fractures in the expansion tank with small coolant leaks and make ure the expansion tank cap is on correctly and holding pressure.

[mrducati]

Poor water circulation, the combustion chambers reach several hundred degrees plenty hot enough to boil water rapidly if it isn't continually pumped over to the radiator & replaced with cooler.

This makes sense, but it is a bit strange that this happens AFTER I shut the engine off. Does this mean I need a new radiator? I have used cemical radiator cleaner and I could not feel any coldspots running my hand over the radiator so I thoght it was OK. Is there any other places than the radiator that could restrict the circulation?

A leaky head gasket blowing combustion gasses into the water jacket would be one way of messing with the circulation though, so keep an eye how quickly the cooling system pressurises (squeeze the top hose) and look for other signs like bubbles and mist in the header tank long before it warms up.

I don't think the hoses gets really hard at all, and certainly not when very quickly. Also I have a new head gasket (although I'm not confident I have done this correctly).

Also check for hairline fractures in the expansion tank with small coolant leaks and make ure the expansion tank cap is on correctly and holding pressure.

If I remove the cap when this happens (I know, dangerous!), it overflows violently so it must be holding some pressure, right? There is some water comming from the cap eventually, but I guess someting has to give sooner or later.

[Darkscamp]

for the boiling on after switching off. is your aux fan working ok as i've heard the fan switch connections can corrode or the switch itself can be faulty. also check the aux fan fuse.

a club members fan on their mpi used to run on for a good 30seconds after the ignition was turned off.

[mrducati]

for the boiling on after switching off. is your aux fan working ok as i've heard the fan switch connections can corrode or the switch itself can be faulty. also check the aux fan fuse.

a club members fan on their mpi used to run on for a good 30seconds after the ignition was turned off.

The fan does come on while driving, but I've never heard it running after I have turned off the engine even if I hear the water boiling.
How hot is the cooling system designed to be before it starts to spill water everywhere? If the fan cuts in at 105 C how mutch warmer will it be before the system says goodnight (assuming it works as intended) I feel the window between where the fan cuts inn and overheating is very small.

[Ethel]

When you stop the engine you also stop the water pump so water in the head doesn't get moved 'n has longer to boil. The system is pressurised because it raises the boiling point (pressure cookers) - about 120 degrees Celsius at 14psi, I think MPI pressure caps are even higher?

I suppose the cap could be leaking off pressure

[Sprocket]

The pressurised rad cap is only there to prevent boiling over when the engine is turned off and in heat soak. As Ethel says, when the engine is turned off, coolant flow stops and the high temperatures in the combustion chanbers soaks into the coolant, boiling it, as it boils, the pressure in the system rises, as the pressure rises, so does the boiling point, and hopefully, the temperature does not rise above the boiling point of the final heat soaked temperasture of the coolant.

It is a common problem on the MPi for the rad cap to leak. New caps are available and not that expensive.

Get it on a diagnostic and see if the coolant temp sensor is infact reading the correct engine temperature, as measured by a calibrated thermometer. Aslo make sure that the thermostat is in the correct part of the thermostat housing, above the sandwich plate and NOT below it; an old mans trick that one.

[mrducati]

quote name='Sprocket' date='Jul 24 2010, 09:02 AM' post='1889485']
The pressurised rad cap is only there to prevent boiling over when the engine is turned off and in heat soak. As Ethel says, when the engine is turned off, coolant flow stops and the high temperatures in the combustion chanbers soaks into the coolant, boiling it, as it boils, the pressure in the system rises, as the pressure rises, so does the boiling point, and hopefully, the temperature does not rise above the boiling point of the final heat soaked temperasture of the coolant.

It is a common problem on the MPi for the rad cap to leak. New caps are available and not that expensive.

Get it on a diagnostic and see if the coolant temp sensor is infact reading the correct engine temperature, as measured by a calibrated thermometer. Aslo make sure that the thermostat is in the correct part of the thermostat housing, above the sandwich plate and NOT below it; an old mans trick that one.
[/quote]
I'll order a new cap and see if that changes the situation. It seems logical given the explaination by you and Ethel. The termostat was correctly mounted. Now the car was succsessfully used as a weddingcar despite all this one hour ago. I must say I was more nervous than usual, but I thought it would work alright since it was onely a 3 km drive.
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