Topic

Idle won't make any odds.
No coolant would be a lower boiling point. (Couldn't say if low enough to be a problem )

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Hi Daz
When the you say the engine starts to overheat have you any signs of coolant loss when the engine has cooled down?

Maybe try and wrap some kitchen paper around the expansion cap and tightened with an elastic band , if you are losing any coolant from here then you will see on the paper.
I recently had my head gasket replaced by rough luck racing ( Jon Norris) who pressure tests every expansion cap he buys/ uses and told me that even some new caps can be faulty.

It could be that when the heater is set to hot the engine heat is escaping through the vents and into the cabin. But when the blower is set to cold the heat can not escape and is then causing the coolant to boil.

Just a long shot but maybe worth a try


Steve

there doesn't seem to be any coolant loss.

I've taken it out for a drive today & when the heater is set to cold it acts exactly the same way as it does when it's stationary, but when set to hot the temp hovers between 88 & 92. I cable tied a rubber glove over the expansion cap & after 20 or so miles driving it had inflated a very small, almost an unnoticeable amount (I'm assuming from air escaping from the cap)

Here an off the wall sort of idea.
If the only symptom of overheating is indicated by the temp gauge the car may well not be actually over heating.

The gauge is electric which relies of the temp sender to give a variable voltage signal to the gauge by varying the electrical resistance according to coolant temperature.
Now lets consider what circumstances that could cause a false reading when the heater is set to cold.
As the heater valve is cable operated, maybe it's actually acting as an ground cable depending on it's position.
This could be due to some damage to the insulation of the wiring rubbing against the cable or perhaps a ground wire has a dirty or bad connection causing the current to earth via the heater cable.

When the obvious causes aren't the culprit, the one has to look to less obvious and obscure causes.

Can you beg, borrow, buy, bludge or steal an inferred heat temperature reading gun? Use this to get a better idea as to if the car is actually overheating, by pointing it at the radiator, taking a reading and comparing it to the gauge reading.
Most OEM gauges fitted to cars are rubbish built to a price, they're not calibrated scientific instruments, and even if the were calibrated properly at the factory they would still need to be periodically recalibrated anyway.

Cobber wrote: wrote:

Here an off the wall sort of idea.
If the only symptom of overheating is indicated by the temp gauge the car may well not be actually over heating.

The gauge is electric which relies of the temp sender to give a variable voltage signal to the gauge by varying the electrical resistance according to coolant temperature.
Now lets consider what circumstances that could cause a false reading when the heater is set to cold.
As the heater valve is cable operated, maybe it's actually acting as an ground cable depending on it's position.
This could be due to some damage to the insulation of the wiring rubbing against the cable or perhaps a ground wire has a dirty or bad connection causing the current to earth via the heater cable.

When the obvious causes aren't the culprit, the one has to look to less obvious and obscure causes.

Can you beg, borrow, buy, bludge or steal an inferred heat temperature reading gun? Use this to get a better idea as to if the car is actually overheating, by pointing it at the radiator, taking a reading and comparing it to the gauge reading.
Most OEM gauges fitted to cars are rubbish built to a price, they're not calibrated scientific instruments, and even if the were calibrated properly at the factory they would still need to be periodically recalibrated anyway.

I've taken the temperatures using an OBD reader & torque, which I believe get the temps from the ecu using the fan sensor, whereas the gauge uses the coolant temp sensor, the temperatures on torque correspond with the readings on the gauge. From what I've read, and again I'm happy to be corrected because it'd make my life easier using the two different sensors and getting the same results throws out the temp gauge being faulty.

Clutching at straws here............

So if it is over heating, then the only other obscure thing I can think of, is the water pump, the pump impeller may be slipping on it's shaft when the heater valve is closed, caused by the increased resistance to flow.
Although impellers slipping on the shaft does happen, I would think it extremely unlikely to end up with the situation where the increase in flow resistance due to the heater valve closure would make the difference.

Cobber wrote: wrote:

Clutching at straws here............

lol that's exactly what I've been doing for weeks now, it's had a brand new water pump too. Everything seems to be doing exactly what it's supposed to, rad heats up properly, pipes are hot in the right places.

It doesn't overheat at all if the stat is removed, but I've tried four stats now, so I think that's ruled out, it's been bled several times by the usual mobile mechanics & I've bled it at least 20 times, if I had hair I'd have pulled it out by now.

Clutching at straws No 2

I have searched for an old post and can't for my life remember where I seen it. But I'm sure someone had a problem with over heating and tried a number of expansion caps and changed a load of different parts to no avail.

In the end it tuned out to be a hairline fracture in the expansion bottle. Maybe the threaded part of the bottle has a slight split in it and when the cap it tightened the split is sealed. But when the system is under full pressure i.e. when the interior heater is turned off , its then causing the split in the expansion bottle to open enough to allow the coolant to boil.

As you have checked everything else and been thorough I don't know what else you can check.


Steve

I've got to fit a new expansion tank onto a friends TF over the weekend, I'll "borrow" his old one for testing purposes

Can someone tell me if the rubber pipes at the rad end are supposed to pressurize or not? From memory when I felt them they felt a bit soft when the car was up to temp.

The hoses should feel under pressure when the car is up to temp.

From what I've read so far the fans are cutting in and out at the correct temp with the heater circuit open .

The problem is that with the heater circuit closed the fans are not cooling enough to reduce the coolant temperature. In fact the coolant temp is continuing to rise!

As the flow through the heater circuit is enough to keep the car in the normal temp range there must be a reasonable flow from the water pump.

Logically then, there must be a restricted flow through the radiator and/or underfloor pipes. Under normal circumstances air flow whist the car is moving is enough to keep the coolant within the working range,so again in your case this is not happening.

The question is why! Is there some sort of blockage?
I would try flushing the cooling system through,especially the rad and underfloor pipes.

They should vary slightly in temperature and should be reasonably firm but still squeezable if that makes sense

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