Temp gauge fault

Davy

i have a 1997 VW passat and the temp gauge was faulty at time when i bought it (2years old at the time), but now doesn't move at all. i haven't taught about it till someone i know was selling there 03 passat for newer model, but there temp gauge is faulty as well, so obliviously a VW fault not fixed but what surprised me, there mechanic told them it can cause the car to use more fuel??
i don't know much about cars,, but never heard that before, anyone know about this?

Mr.Diagnostic

davton wrote:

there mechanic told them it can cause the car to use more fuel??

The mechanic was wrong in this case.

Davy

Mr.Diagnostic wrote:

The mechanic was wrong in this case.

any idea where he was getting this idea, or mixing it up with something else?

Mr.Diagnostic

davton wrote:

any idea where he was getting this idea, or mixing it up with something else?

On a lot of modern engine management systems (EMS) there is only one temp sensor on the engine. The instrument cluster uses the same signal as the EMS for engine temp. On such a system the gauge reading wrong could be caused by a faulty temp signal and in this case the EMS would get the wrong signal too and would fuel at an incorrect level. This type of fault would be very rare.
Well you did ask!

On your Passat the temp sensor has 2 seperate parts, one for the EMS and one for the temp gauge, thus the gauge signal/reading cannot have any effect on the fueling.

BTW the most common cause of temp guage faults on these is the cluster itself.

Davy

thanks for such detail,, answers my Q

cpoh1

Mr.Diagnostic wrote:

The mechanic was wrong in this case.

His mechanic is actually right. Ive seen a lot of cars (including one passat) that had a faulty thermostat stuck open ensuring a cold reading on the temp guage even after prolonged use of the car. The colder engine meant more fuel was burned and less mpg.

Mr.Diagnostic

cpoh1 wrote:

His mechanic is actually right. Ive seen a lot of cars (including one passat) that had a faulty thermostat stuck open ensuring a cold reading on the temp guage even after prolonged use of the car. The colder engine meant more fuel was burned and less mpg.

That would be true, if the fault was the engine running cool but the op said his friend's gauge is faulty and his friend's mechanic said that this would cause it to use more fuel.

Davy

Mr.Diagnostic wrote:

That would be true, if the fault was the engine running cool but the op said his friend's gauge is faulty and his friend's mechanic said that this would cause it to use more fuel.

well do you think its enough to get it checked out?

Mr.Diagnostic

Is the car running without fault?
Does the heater blow as hot as it should?
Does the heater blow hot air within a few minutes of a cold start?

If the answer to all the above is yes then there is no need to worry about the stat. You could always measure the temp of a rad hose after the engine is warmed up to check.

If you are still worried then it is an easy matter to check.
The only concern is if your gauge is not working and you suffer an overheating problem you could damage the engine. If it is the gauge at fault then the cluster will need to be changed. New ones are expensive but used ones can be fitted.

Davy

ye its fine to all those,, the engine does does seem to get hot very quick, like after a short journey the bonnet would be fairly warm. strangly, i notice the gauge go up, hasnt moved in months but didnt work for long. prob just check it at next service