Hot and bothered - Riggers
If you can solve this you're a genius (it's baffled everyone else I've asked).....

A few months ago my 1991 Fiesta 1.6S started overheating. It's fine when I'm on the move, but as soon as I come to a halt or get stuck in slow-moving traffic the temperature soars until the fan comes on. It then gets stuck in a cycle of heating up and cooling down (as the fan switches on and off) until I'm back on the move. It only takes a couple of minutes to go from normal temperature to the red zone.

The car runs fine apart from this, and I suppose I can live with it (until the day the fan fails and it all goes bang!), but there's definitely something wrong as needle never used to budge. It's also bugging the hell out of me as I've spent a fair bit of time and cash trying to fix it.

Before any suggestions are put forward, here's what I've already tried:

New thermostat, new temperature sensor (it's definitely overheating - not electrical), new radiator cap, new radiator, new coolant, new water pump, back flushed engine (no blockages) and bled system for air locks.

It's not losing any coolant and there's no water in the oil, or oil in the water.

Any thoughts?
Hot and bothered - Railroad
There isn't much left to do. You say it's definitely overheating, but is that just because the temperature gauge tells you so?

Using a thermometer or a multi-meter with temperature facility, place the probe into the cooling fins on the radiator, and again onto the thermostat housing. Run the engine until the fan cuts in. The temperature could reach over 100º before the fan cuts in but this will not be abnormal so long as the system is circulating properly, and the radiator is dissupating heat. The radiator and top and bottom hoses should all be warmed up, with the bottom hose slightly cooler than the top. Also you may see the reading fluctuating slightly. This will be as the thermostat opens and closes.

Compare the thermomter reading with the temp gauge. Remember that the pressure in the cooling system is regulated by the temperature. The hotter the engine the higher the pressure. As the engine cools the pressure will reduce. Most pressure caps lift at about 15psi, but in theory this should rarely happen because the engine won't normally get hot enough for the pressure to reach the cap rating. So if the cap lifts often and the temperature seem normal, suspect a cylinder head or gasket fault.

You can buy cylinder head leak testers quite cheaply too. You use one to sniff the coolant vapours in the radiator or expansion bottle. If the liquid in the tester changes colour then carbon monoxide is detected in the cooling system. If this is the case you have a problem with the cylinder head and/or gasket, this may not even seem to be a problem now as the gasket may just have become weakened, but you can be sure it will get worse.

Also does the fuel gauge read accurately? If it doesn't the fault could be the voltage stabilizer behind the dash. This will give you unsteady or inaccurate temp gauge readings too.
Hot and bothered - Peter D
This sound like a water pump problem. Speak to ford and find out which pump is fitted I I have removed pumps with the nylon pump impellor loose on the shaft due to a crack in the nylon. Gone back to the site when you know what pump is fitted. Regards

Hot and bothered - Riggers

I've already changed the water pump.

Spent an afternoon up to my elbows in oil. When I finally got the old pump off - confident it was goosed - I was a tad irrtated to see it looked to be in perfect condition.

Anyway, while I had the opportunity, I decided to fork out 20 quid and fit a new one (well, you never know).

Result? Complete waste of time, money and swarfega.
Hot and bothered - Riggers
Thanks Railroad

I'm sure the engine is definitely overheating, as it gets noticably hotter to the touch. Also, when I'm stopped and it's raining, the rain water on the bonnet begins to steam after a while!

I don't have a multi-meter thingy so can't check the actual temperature.

The top and bottom radiator hoses feel to be the same temperature - hot - which doesn't seem right.

The fuel gauge is fine. I'm pretty sure it's not an electrical fault.

Your head gasket theory might be right. Hot exhaust gases leaking into the coolant? But if that's the case, wouldn't coolant then leak the other way through the gasket when the engine's switched off? It's certainly not losing any coolant.
Hot and bothered - Railroad
I think the only way you're going to get to the bottom of this is to pressure test and temperature test the system to see if it actually is overheating. Do you know anyone with a cooling system pressure tester? Where do you live?

If you have one, remove the pressure cap. Do this when the engine is stone cold ie. after it's been standing overnight.
Connect the tester to the expansion bottle.
Pump the system up to 15-20psi.
Leave it for 10 minutes or so whilst checking for any external leaks and for a noticeable drop in pressure.
If there are none then carry on.
Release pressure.
Top up coolant level, re-connect tester and start engine.
As the engine warms up the pressure will begin to rise.
When the thermostat opens the pressure will fall slightly before rising again.
Remember that pressure is relative to temperature, so by the time the fan cuts in you'll probably see that the pressure reading is somewhere around 15psi (the cap rating) or less. If it goes way over suspect possible cylinder head trouble.
If the pressure rises and falls as the fan cuts in and out, and stays under or around 15psi overall, then the engine is not overheating.

Remember to take extra care when removing the tester when the engine is hot to avoid scalding yourself. It's adviseable to leave it and remove it once the engine has cooled.

If you don't have a pressure tester feel the top and bottom radiator hoses manually. You will have to guess. You should feel pressure when the engine is hot, but they shouldn't be rock hard.

Good luck and let us know.
Hot and bothered - Cyd
Have you had the car "tuned" recently? Could it be running weak, especially at low throttle & idle? A weak mixture burns considerably hotter than a normal one and the excess heat will cause a build up of heat around the exhaust valves. this heat has nowhere to go but into the water jacket.

Easiest way to find out might be to go to your nearest MoT station and ask them to take a CO reading for you (they'll probably charge a fiver). Compare this to spec (ask the tester - he'll probably know if you don't already).

Come back to us with the result. If it is weak you'll need to work out why.
Hot and bothered - smokie
Maybe I'm missing the point here, but couldn't this be normal? Standing in traffic will cause the engine to get hotter, then the fans cut in. That's how it's designed isn't it? And without air circulation round the engine compartment the whole area will obviously get very hot. I know my car positively glows after a good run...standing back a few feet from the bonnet you can feel it.

The fact that it didn't used to do it - well maybe it USED to be bust (say, the needle) and it's corrected itself...

I should add that the views above are entirely non-technical!!
Hot and bothered - RichardW
As I see it the only reason you think it's overheating is that the gauge says so. If it was overheating then the fan would run all the time. The fan is switched by the water temp somewhere in the circuit (usually the rad outlet). To my mind your system is working 'normally' - however it appears from the gauge, and the steaming bonnet (did it do this before?), that 'normal' may have shifted. Only reason I can think of is that the switch for the fan has had a bit of a schism and is no longer switching at the right temperature. You should be able to test this in a pan of water, and Haynes should give the cut in/out temps.

However, as has been said already, you need to get a 'second opinion' from an accurate source as to the actual temperature. An air-con specialist may be able to help here as they may have stuff for testing the efficiency of the air con.

Hot and bothered - terryb
I'd have thought the obvious thing would be a dodgy switch on the fan, so it cuts in too late and cuts out too early. Must be heat activated somewhere along the line.
Hot and bothered - daveyjp
I had a 1980 vintage Fiesta 1.1 which did exactly this. I changed thermostats etc and even fitted a switch to manually turn on the fan, but nothing would stop it going in to the red. Unfortunately I never sorted it out before it failed its MOT for exxtensive rot in the floor and had to be scrapped.
Hot and bothered - Nortones2
Two other aspects not yet mentioned: 1. hose condition. have heard of soggy hoses causing flow restriction. 2. airlock: bit unlikely. However, as this is a brain-storming session, all ideas should be considered:) Good luck.
Hot and bothered - Cliff Pope
Whenever I have felt top and bottem radiator hoses I have always initially been surprised that the top one can be too hot to touch and the bottem pretty cool. Then on reflection I realise that is exactly as it should be - the radiator is cooling the water.
The fact that yours both seem pretty hot is a bit worrying. That to me suggests either that the water is not circulating, or that the radiator is not cooling. As the radiator is new I'd try the thermostat. You say it is new - is it the right one? Is it in fact actually working? Is it fitted the right way round?
What happens if you remove it completly?
Hot and bothered - Claude
In your original message you said you had replaced the Temperature Sensor. Did you mean the sensor that feeds your dashboard temerature guage or did you mean the sensor that switched the fan on and off ?
As you appear to have replaced every other bit of equipment that might be at fault I suggest that the fault may lie with whatever \'temperature sensor\' you DIDNT replace! Worth checking. As someone pointed out previously all you need is a kettle and a thermometer to check those items.
Hot and bothered - Riggers
The sensor I replaced was the one that screws into the thermostat housing. It's the one that feeds the dashboard temp gauge. It's also got wires going to the fan, so I assume it triggers that too.

I think I'll try the hot water/thermometer test. If it doesn't show up an electrical fault, it's going into the garage.

Thanks for all the advice guys.

If I ever get to the bottom of it, I'll let you know!

Hot and bothered - king arthur
Does the temp gauge actually go into the red? At what point does the fan cut in? Has there been any change in your driving behaviour around the time you first noticed this (e.g. getting stuck in traffic for longer)? And, how long have you had the car?
Hot and bothered - Plantman
I agree with someone who mentioned that the fan cuts in too late and switches off too early.

Mine did this and would only switch the fan on a few mm before the red. got it replaced and now it switches the fan on if the needle goes slightly above half way !
Hot and bothered - John S

I wonder if you do indeed have a problem with this car. I recall that’s exactly how my 1.6S Fiesta behaved. In the warmer weather (it was less obvious in winter and I note you imply this ‘problem’ has come on recently), the car behaved exactly how you describe. The car would usually run with the needle in the middle of the normal zone. Stop in traffic and the gauge would rise fairly quickly to about ¾ of the way up the normal zone, fan would cut in and temperature would drop back to near halfway, and so on. As soon as the car moved away, things quickly returned to normal. I’ve seen this on other Mk 3 Fiestas too. It seems that’s how the fan thermostatic switches are on these engines, unlike some others which maintain a temperature closer to the normal running temperature when stationary. If the engine coling system was faulty it would also give problems when the car was being driven, the engine was under load, and there was far more heat to be dissapated than when at idle.


John S


Value my car