My temp gage doesn't ever reach a happy medium, it stays right above cold all the time and my heater will not kick into "rapid" speed when on auto. Has anyone had this issue too? it's an 1998 Audi A8. The self-check feature checks out fine when initiated. *the attached photo is NOT from my car, I "borrowed" it from another forum, but it looks just like that after hours of driving*

 

Thermostat stuck OPEN, most likely...doesn't happen as often as stuck closed, but it does happen. Happened to me recently. When I removed it, it was open just a millimeter (enough to run a strong shower), then open more as it heats up, but never completely closed. 2 different German cars I have did the same thing...the 3rd one overheated with one stuck closed.
A major symptom of "stuck open" was short of heat in the climate control. Fortunately, they're pretty easy to change in a 32V motor, unlike the buried design in newer 40V motors.

 

the coolant temp sensor. It's on the back of the engine, straight down. Real common to fail and do exactly what you're seeing. About a $30 part and 15 mins to change.


Bob

 

But the sensor shouldn't cause decreased cabin heat...have you tried setting heat high manually? Should be able to easily flood the cabin with heat if engine is at normal op temp. If motor really seems to be fully warmed up and full heater output, then temp sensor could be the issue (?)

 

my bad, I glossed right over the lack of heat so thermostat is the likely cause. The other thing to check is coolant level. It would have to be really low to cause the issue you're having but worth a look.


Bob

 

Yup! T-Stat stuck open.

If the thermostat is stuck open then coolant continues to circulate through the radiator. The thermostat is supposed to close when the coolant reaches a certain low temperature (don't know exactly what it is. 180 degrees?) to trap the coolant in the engine. Then coolant temperature rises. When the coolant reaches the high temperature point (210?) the Thermostat opens up and lets coolant flow through the radiator again to cool it back down.

Other than gauge showing lower than normal, colder than normal heat from the HVAC system is a dead give-away that the thermostat isn't working right.

 

On the A8, it actually opens at 188-192 degrees. Most importantly, it stays closed until the coolant in the engine reaches that temp and closes temporarily when it drops below that.
Overheating is obviously not good, but underheating doesn't let the engine run at full efficiency. The engine needs to be fully warmed up to burn the optimum fuel mixture correctly...also effects catalytic convertor efficiency and life.