Just dropped my car off for a service and its 3rd belt change 220,000 miles, getting the water pump changed again 2nd one just in case.

I have noticed recently that once the temp gauge reaches normal temp, that if the car is sat idling the temp drops back to between a quarter and its half way point, should it do this or could the thermostat be on the way out??

Edited 20 November, 201213 yr by skippy41

The dash panel shows any coolant temperature between 79°C to 112°C as '90', so the temperature is lower than 79°C which is wrong. Even at idle the engine will warm up until it's in nominal range - it won't get cooler unless the thermostat is faulty.

Sat idling a diesel will generate hardly any heat. In cold temperatures when you've got the heating on you're drawing heat from the cooling system, this can cause the temp gauge to drop off as the coolant is getting colder without any heat from the engine to warm it back up again.

The dash panel shows any coolant temperature between 79°C to 112°C as '90', so the temperature is lower than 79°C which is wrong. Even at idle the engine will warm up until it's in nominal range - it won't get cooler unless the thermostat is faulty.

It gets to normal half way so 90,but if the heater is on number 2 when idling it drops

I normally have it set to 22 on the dial but lately it does not feel like that temp in the car, so have turned it up to around 24 as the air coming out of the vents feels cool at 22

Edited 20 November, 201213 yr by skippy41

It gets to normal half way so 90,but if the heater is on number 2 when idling it drops

I normally have it set to 22 on the dial but lately it does not feel like that temp in the car, so have turned it up to around 24 as the air coming out of the vents feels cool at 22

Could possibly be a dodgy thermostat.

The temperature will drop slightly under those circumstances as does ours but I wouldn't expect it to be as much to move the guage down.

The only time I've seen the temp drop again is after a very short run when it just got up to temp then I came to a standstill. The level dropped then climbed back up again.

Phil

Get the thermostat changed anyway, they are dirt cheap.

Does it drop back just the once? This could be the thermostat opening and the initial in rush of cold coolant from the radiator into the engine block.

The nominal temperature range (displayed as '90') is chosen carefully so the possible effects mentioned don't affect the gauge reading (the ECU sees the actual temperature - it is the dash insert that processes the data).

The nominal range runs approximately from 6°C below the start of thermostat opening, to 6°C above the stage 2 fan cutting in.

When the thermostat starts to open, you only see 1 or 2°C drop at 0°C ambient as the flow is limited. The thermostat starts to open at 85°C and isn't fully open until just over 100°C.

At the upper end, the temperature may continue to rise by several degrees at 30°C ambient after stage 2 cuts in.

This is normal operation, so the temperature is displayed as nominal.

Good news thermostat ok,

Full service,fbps,timing belt and water pump, all in £409

Good news thermostat ok,

Full service,fbps,timing belt and water pump, all in £409

Thats a good price - I assume that wasn't a mainsteeler?

Thats a good price - I assume that wasn't a mainsteeler?

No but an ex VW Mechanic who was paid off when the VW dealer closed 3 years ago, he started a small garage on his own, and only charges £25 an hour, he charges VAT on the parts only as he has to pay it.