Heat gauge drops while coasting down long hill

[hunter1234]

I have a 2004 Audi A4 1.8 turbo Quattro. I have replaced the thermostat, both temp sensors, flushed the heater core. My problem is that I have plenty of heat, although my temp gauge is slow to come up in the morning, once the temp gauge comes up to center position if I coast down a long hill the temp gauge drops. I noticed this morning that if I do not run the heat and coast down a long hill the temp gauge stays dead center and does not move, once I turn the heat on and coast the temp drops. This has me stumped. I know when I flushed the heater core a large amount of chunks and fine debris came out. Could this be a radiator issue? I know its not a faulty sensor as I have put 3 different ones in over the last week thinking maybe a new one was bad. I have bled the air out every time I flushed or changed the sensors so may times I am an expert on bleeding air now. In summary, heat off gauge stays dead center, heat on gauge drops while coasting down a long hill? Any help will be greatly appreciated. I have been working on this non stop since I bought the car 2 weeks ago

[hunter1234]

Can anyone help me with this?

[schowe]

Maybe your replacement thermostat is faulty? Did you buy on OEM one or aftermarket?

[iHaveBoost?]

I think that's normal. Going downhill means less stress on the engine, it can just coast. So there's a lot of extra air coming in, but it the injectors are shut off. So no combustion = no heat. Thus cooling the engine down a bit downhill.


I'm sure someone will correct me if I'm mistaken. But if it isn't normal, I'd replace the thermostat. Sounds like it's stuck open. Sometimes they're defective...

[freeloader700]

..... but it the injectors are shut off. So no combustion = no heat. ....

The injectors are still spraying at when the car is idling



OP, like he ^^ said though, when you are coasting down hill there is little to no stress on the motor creating no new heat just the ambient heat of the car running. Factor in the air that is not cooling to motor. You said when the heat is off it stays in the center, with the heat on it drops. When you put the heat on, it is extracting some heat. You could still have an air bubble as well

I think its a combo of normal cooling/extracting of heat + possible air bubble + defective coolant temp sensor

[imnuts]

The temp guage on my car doesn't move whether I'm coast or not once it's up to temperature, even when it was 10F outside and I had the heat on. I'd say you have a problem somewhere, possibly an air bubble.

[lemon]

thermostat

[rocky.ca]

t h e r m o s t a t

[Charles.waite]

I 3rd thermostat. Unless you're coasting down a mountain, the temp shouldn't really be falling in normal operation.

[old guy]

My guess is also a defective replacement thermostat. Try this: Unplug the thermostat heater and see if the temperature still drops under those conditions. You will get a code for open circuit to the t-stat heater but you can clear it later or it will eventually clear itself after you plug it back in. If it cures the problem it means the ECM is forcing the t-stat to open even though it isn't necessary. That indicates a sensor problem. If it still drops in temperature that pretty much confirms a defective thermostat.

When you are off throttle going down hill there is a complete fuel cut-off to the injectors but there is still heat being generated from the engine braking. My 97 Passat VR6 had an oil temperature gauge and during a long downhill with nothing but engine braking I would actually see a 5-10° increase in oil temperature. Once the t-stat closes all heat absorption goes to the oil rather than being dissipated through the radiator.

[non_quattro02]

I recently replaced my heater core and now I have the exact same problem. The car is now taking longer to warm up and if I am coasting on the highway, the temperature needle drop a little. It happens only if my heater is on. I was going to replace the thermostat once it got a little warmer outside. Let me know if you find the issue.

[Nollywood]

Thermostat installation is crucial. The bypass valve must be at the 12 'o clock position. If it is, and your engine's cooling via the heater fan (if temp is set to hot) or even by the air generated by forward motion, then your thermostat is defective.

Was it an OEM or aftermarket stat?