Have a 98 KH Verada and have had the water pump and timing belt replaced less than a month ago. Have also had new battery two days ago.

Have just noticed that the temperature goes right up to the red while driving not long after moving off. If the engine revs are over 2000 it drops down straight away but goes back up if the revs drop.

This happens even when stationery (ie in neutral, hand brake on and I rev it to over 2000). If the air con is running it loses its cool and pumps non chilled air until the revs are up again then the cool kicks in..

I checked under the bonnet and noticed only one fan is running. If I turn the air con off both fan stops. Turn it on again and the same fan starts up.

I know jack sh@$ about cars so any advice would be great before I take it to a mechanic and spend more cash.

Hey Mate,

I don't know much about Magna/Verada Mechanical but just check the Thermostat.. to see if it's sticking,

if that's all good check the fuses in the engine bay to see if the Thermo Fan one is gone.

Cheers Matt.

as you look from the front, left fan is the condenser fan, right is the radiator. I'd check a few things out first like the harness is connected to the fan. check that the radiator is full but don't crack the cap when hot, preferably do it cold. From what i can work out the ECU turns on the fan so will be relying on a temp sensor t activate it. It activates one of 2 relays in the box under the bonnet which then supplies power to the fan. If the gauge is going high and no fan that's a worry. A dud sensor can give a wrong reading, stuck thermostat can cause overheat fast, but since you say revving the engine causes it to drop, I'm wondering if the water is low.

By temperature goes up into the red then back down, you mean slowly, right? I assume the gauge isn't just jumping up to maximum and back down? An actual overheat would present with slower increases and decreases in gauge indications.

Is the coolant level full?

If the temperature is going up into the red, you need to stop driving the car. Every time it goes up there you are overheating the engine and doing long term fatigue damage to the alloy heads.

If coolant level is okay, sounds like issues with the fans. The fan will operate at three different speeds, depending on demand as controlled by the ECU. Most members seem to have luck fixing overheat issues by either replacing the relays which control the fans, or identifying one or both of the fans thenselves have failed.

Start by replacing the relays. You can temporarily swap the aircon and fan relays to see if it swaps which fan runs. If swapping the relays makes no difference and the same one of your fans is running, then you may need to replace the dud fan.

Once you confirm all the relays and both fans are operational, if it's not fixed, you need to start looking at
1. thermostat (DON'T run car with the thermostat removed, and replacing one which seems to be working isn't always guaranteed) Personally I'd just replace with genuine and be done with it
2. Coolant temp sensor
3. Pressure and flow test