Car overheated, but oil/coolant levels OK

Lobster (Lobster ) gurgled happily, sounding much like they were saying:

Is the cooling fan working?

Could a sheet of paper or something have flown up and partially blocked the air intake?

There is a chance it is the thermostat, thermostats are usually cheap and easy to fit, so worth a try, at the same time drain and flush the cooling system.

I have also seen this problem on several cars where it is NOT the tehrmostat, it may be caused by a radiator fault, either blocked cooling air flow, missing fins over a large area, internally blocked, internal flow dividers broken etc. are the ones I have seen.

For local use the cooling is sufficient, but beyond a certain loading or speed the heat starts to build up.

I have been using a car for 4 years that does this, it has never actually boiled, just gets very hot if you use it hard, especially in hot weather, I can see that the fault is the radiator as it is almost bald of fins over a large area.

So a likely fix for you, if it is not the thermotat, is a new radiator/coolant, it might be worth flushing the system just in case it is a bit blocked and will clear, or just continue to use it, and be aware of the problem and drive accordingly.

Mrcheerful

================================== I have a vague feeling that this can sometimes happen in very windy conditions. I've no real idea why it happens (nor sure that it does actually happen) but it's possible that a very strong following wind could counteract the normal airflow caused by forward motion.

Cic.

Well, looks like it isn't! I've just driven the car for a journey of 4 miles, and on my return the temp gauge had just reached the red zone, but no warning lights were on yet. I checked under the bonnet before switching off, and the electric fan was definitely off - surely it should have been working by this time?

Is it governed by the same thermostat as controls the coolant flow, or another one?

Thanks David

Lobster (Lobster ) gurgled happily, sounding much like they were saying:

Definitely.

It's not governed by a 'stat, but by a temp switch - usually a different one to the gauge.

If it's hitting red in only four miles, though, that sounds like probably a stuck 'stat. You've got some serious diagnosis to do - and fast, before you cook it. I'm suspecting something wrong inside the wet bits.

Is the rad getting hot? All the way up and down? What's the coolant like? No airlocks? Being pumped round correctly? At the very least, you need to change the coolant and flush it all through.

But it's also worth making sure the fan's working. Identify the switch, short it out, and see if the fan starts. If not, feed it power directly, and see if it goes round. If it does, trouble-shoot the wiring.

Then pray the head gasket's survived.

================================== Start by checking the cooling fan and thermostatic switch. Trace the wiring from the fan to the thermostatic switch (probably on the side of the radiator) and test the fan by direct connection. If it runs consider replacing the thermostatic switch - can be a bit expensive.

Since the problem occurred originally at a speed that would be unlikely to require fan cooling it's quite possibly a faulty thermostat - not the above thermostatic switch for the cooling fan. It could also be a faulty water pump. Replace the thermostat and see if that improves the situation.

Cic.

check fo a blown fuse

Bizarre Dwarfs boy!!!!

Well, did some diagnostics over the weekend and found that the cooling fan came on instantly when the stat was bypassed, but otherwise was dead as a dodo. So the thermostat looks like a highly plausible suspect.... given that it's highly inaccessible and I don't have a spanner or socket that will fit anyway, I think I'll pass this one over to my tame mechanic to sort!

Many thanks for the advice, all.

David

The fan will only operate when the water is extremey hot (over about 95 degrees C) In the winter it can be hard to get some cars up to cut in temperature by just letting them idle. In any case though the fan should not be running at motorway speeds in normal use (your original problem)

Oh. Hmm.

But when I did my test drive, when I stopped, the temp gauge was at the threshold of going into the red, which I would never normally see - ie the engine was much hotter than usual, even in summer, and I've certainly been aware of the fan still running when I've switched off.

Wouldn't I have expected the fan to have been running following the test drive?

David

if it got hot enough at the appropriate place (ie where the thermo switch for the fan is) There are many things that can cause it not to get hot enough at the switch or it may be that the switch has died although they are generally very long lived, removal and testing in hot water is the right way forward.

================================== It's possible that you have two problems. The thermostatic switch (which switches the fan on / off) might be faulty and might have been so for some time. Depending on your driving conditions there might have been no call for the fan and so the faulty switch could have gone un-noticed.

At some time recently the cooling system thermostat could have developed a fault and this could have produced the initial problem you saw during your motorway journey. Your later short test journey which showed over-heating could be the final failure of the thermostat.

Thermostats are quite cheap (about £10-00) and this would be the first thing to replace. You could do other things suggested, such as radiator flushing at the same time.

Cic.

Follow-up:

The above was pretty much right on the button. It was the main coolant thermostat which was duff - because of that it wasn't circulating hot water through the radiator, therefore of course the cooling fan switch was never getting hot enough to trip. So thanks all for the advice.

[BTW - in the end I took the job to my usual mechanic to sort out. Cost me 72 GBP + vat labour (ie plus parts): don't know what his hourly rate is (it's a back-street, non-franchised operation which I've used for years) but does that sound a bit OTT for this job? It's almost the same labour charge as I'd pay him for a full 12-month service. Said it included time for diagnosis etc (although the problem turned out to be exactly what he said at the outset would be the most likely cause).]

David

================================== I think you should look for another garage for future use. The thermostat should have been first port-of-call.

Cic.

Oh I'm sure it was - as I said, the guy even told me up front that the stat was the most likely culprit. It's not his workmanship or diagnostic ability I'm questioning, it's just his bill. Which I assume from your reply that you would concur is high, then?

David

================================== Yes, I would say so. Thermostats *can* be difficult at times because of possible corroded fixing studs or bolts (those holding the cover) but that doesn't appear to have been the case with yours since your mechanic suggested the cost was largely due to time spent in diagnosis. Unless there is something very unusual about access to your thermostat (and assuming no difficulties with corroded / tight bolts) then he should have been able to remove it and do a basic test within a few minutes - say an easy half hour at most.

It would be worth a quick check to see how easy / difficult access is for your particular car. When I did mine recently (elderly Peugeot 306 diesel) it took about half an hour in total and I'm not a professional.

You appear to have paid about £100-00 all-in for a replacement thermostat (about £10-00p) and very little actual labour time. I'm not suggesting that you accuse the mechanic but a little research as I've suggested might give you second thoughts about your mechanic.

Cic.