Grand Vitara V6 1999 Overheating only on Motorway

[jimmy5]

Not been on the motorway for a while, but noticed now that the temp creeps up from dial half way point into the hot and then the rad cap blows off. If you engage the fan heater on full the temp comes back down. The fan is on always as it should be. Not sure if the thermal switch that alters the speed is working though. The engine is completely rebuilt with new timing chain tensioners and I had a new water pump at the same time - say 10,000 / 1.5 years ago. Took the stat out and it opens ok in a pan of water in the kitchen. The only thing I am unsure about is wether the radiator is plugged / blocked. After overheating on a motorway run the rad is cold except round the edges where it is very hot. Surely this is not normal?? Top and bottom hoses also hot so it seems some water is flowing round?. The car is not loosing water except from rad cap blow offs. Oil has no water in it and coolant has no oil in it. Complete new gasket set at re-build also. Mileage 75k. Radiator looks OK but I think it may be the culprit, does anyonle have any thoughts on this? How hot are your GV RADS in the middle after a run - would love to know! You can drive round all day slowly and the temp stays normal.

[IanL]

At motorway speeds, there should be so much airflow that the fan is irrelevant. Are you sure the thermostat opened fully? If so, my money would be on a partially blocked radiator.

There are some places which will recondition your rad for a fraction of the cost of a new one. But I might just invest in a new thermostat first, because they are fairly cheap and so often turn out to be the problem.

[Mister Bitt]

Hi jimmy5!

You have done everything I would do in the circumstances and I cannot add much to the words of wisdom from IanL, but I note that you say 'The fan is on always as it should be.'

Do you mean the electrically operated fan in front of the radiator? The only time I'm aware of it operating (that's to say when I can hear it start up) is when the engine gets quite hot, say in a long queue of traffic on a very hot day. Lately (Winter/Spring) it hasn't seemed to have operated much.

I didn't know they were on all the time. Mine seems to be either on or off. Does the thermal switch actually alter the speed? Crikey!

I will give you some feedback about the temperature of the radiator as I've got to do a bit of a run soon.

Hope you get it sorted, please let us know how you get on.

Mister Bitt.

[IanL]

I believe the electrically driven fan in front of the radiator is for the aircon, and is controlled by that. If the aircon is off, it should not run.

The fan behind the radiator is provided to cool the engine coolant; it is belt driven through a temperature and engine speed-controlled clutch, so it will run slower if the coolant is relatively cool, and will "slip" when the revs rise.

[Mister Bitt]

Hello there!

As promised some observations on the state of my GV2000's rad after a 1½ hour motorway run. When I stopped I switched off and lifted the bonnet straight away.

The top hose was too hot to kep my hand in contact with it for any length of time. The bottom hose was cool, surprisingly so. The rad itself had quite a temperature gradient. The top ¼ of it was hot, but in the centre it was definately cool, a bit warmer at the edges.

They say it's never to late to learn, and I must admit I didn't realise that the aircon heat exchanger rad was in front of the engine rad.

And yes indeedy, the fan in front of them is activated by the aircon. So that's why it works in the warmer weather! I use it a couple of times a month during autumn & winter to keep the seals up to scratch. I switch it on at the end of a journey as I park and jumped to the wrong conclusion, i.e. that it was an additional rad cooling fan switching on as a result of slower air flow. D'oh!

Best Regards

Mister Bitt