invisibled0

I have no heat in my car. After investigating the problem, it seems like the coolant doesn't get to the heater core. I opened the inlet and the outlet hose coming out from the heater core, only very small amount of coolant makes it to the heater core. I was thinking maybe the thermostat is not functioning correctly. Is this a safe assumption? If the thermostat is the problem, would it show up when I scan for engine error codes? Is there any way to test the thermostat. I also tried flushing the heater core. I have a 2002 B6 A4Q 1.8t.

Pete@ECSTuning

If your thermostat is sticking closed then it is not letting water circulate through your car. This will not throw a code, though you seem to be suffering from common thermostat failure symptoms. Thermostat failure is incredibly common on the B5 and somewhat common on B6 cars. 1.8t replacement is not terribly difficult though, nor is the thermostat super expensive. If you haven't done a timing belt or are near a timing belt interval you would do well to simply do a timing belt service and fix the thermostat while you are involved. If you are far from an interval, simply change the thermostat.

Here is a link to thermostats


Click here for Timing Belt kits

lyleswk

Wouldn't a bad thermostat result in the dash gage running high?

Pete@ECSTuning

Depends on if it is stuck open or closed. Closed, yes you would over heat. Open, no, you will be too cool. Too cool to create heat infact.

invisibled0

since the engine is not heating up, it must mean that the thermostat is stuck open. If it is stuck open, coolant should be circulating. Is this a good assumption?

lyleswk

Hmmm, I was assuming the temp gage behavior was normal (warms up to and stays at 80ish) and he just wasn't getting heat. Guess we will let him tell us

Side note: which sensor drives the cabin gage? Coolant flange or bottom of the radiator.

Pete@ECSTuning

Correct, if a thermostat is stuck open your car is effectively keeping itself "too cool" because it cannot limit the amount of water circulating. This sounds ideal, but it isn't. This will not just affect your cabin heat, this will also alter your fuel mapping because the coolant temperature sensor affects your fuel trims. Being perpetually cold will result in you NOT maximizing your power or fuel efficiency.

lyleswk

Did anyone see my other question: which sensor drives the cabin gage: coolant flange or bottom of radiator?

lyleswk

I stumbled across the answer to my sensor question...okay, i did dig a bunch. Bottom line, both sensors provide input to the ECU and the ECU controls the gage, engine trimming, fans, and thermostat (and maybe some other stuff). So, neither one directly controls anything directly, only providing input to the ECU. Having said that, it does sound like the thermostat is stuck open. Should be evidence by your gage never showing normal operating temp.

Learn something new everyday...

Pete@ECSTuning

Sorry about that, I got caught up in the first question. Way to go though! Good follow up.