temp gauge like a yo yo

[rye]

hi everyone
recently bought a v plate 99 a4 v6 been no problems till today.I drive about 7 miles to work,temp gauge normally up to 90 degrees after a few mile as did today then a few mile further gauge shot down to 60 stoppped there for a while no warning lights lit runnig as smooth as ever got to work sat for a while gauge went to 90 checked coolant level ,all pipes hot no pressure in system.Could not fault done exactly the same on return home any suggestions or common faults?

 

[Siena]

To be clear: Does the needle move to the cold section when doing a constant speed? Or just random? When it moves to the cold zone, does the heater still blow as hot as it should?

If it's speed related, then I'd suspect the thermostat is either jammed open, or missing altogether.

If it's just random, whatever the speed, I'd suspect the temperature sender is at fault, (most likely) or the instrument cluster's faulty (least likely).

 

[John@ECM-TECH]

Sounds like the thermostat is sticking to me, the cold air these days really chills the engine making these issues more apparent

 

[mwarrey]

Common problems are the sensor, and the the water pump. If the car IS overheating it is most likely the water pump which is bad news as this is expensive to change. If you haven't had the timing belt changed yet, its a good time to do both. If the car is not overheating its likely to be the sensor which is good news as this is easy to change and very cheap in comparison.

 

[John@ECM-TECH]

the temp sender is defo ur least expensive option to try, I've never heard a water pump go on a 1.8T but when mine was on it's way out on my old VR6 it sounded like a bucket of nails and it was loosing water.

The norm is to change the waterpump during the cambelt service.

 

[mwarrey]

I've never heard a water pump go on a 1.8T.

You need to spend more time on here! this is sadly a very common problem. To save a few pence VAG decided to change the metal impeller on the waterpump to a plastic one. Eventually this becomes detached from the drive shaft and causes overheating.

 

[John@ECM-TECH]

You picked me up wrong m8, I meant me personally!
I was thinking that Icould hear it (and see the pully wobbling) on my vr6 so it may be that you could hear the waterpump going on the 1.8t.

 

[mwarrey]

Ok, thanks for clearing that up. No, you won't be able to hear it has gone, the bearing usually outlasts the impeller.

I think the problem is with the sensor and this is where I would start.