Any setting I put it on the heat is only lukewarm at best. Although, when the fan is turned off and the temperature selector is all the way over to hot, I get lots of hot air coming in through the vents, more than with any of the fan settings. Someone at work said the previous owner could have put in a lower temp. thermostat. And why do I get hot or cold air (depending on where the temperature selector is at)coming in through the vents with the fan off?

 

Not the thermostat. That is the MOST misunderstood piece of the engine. All that does is open when the engine reaches a specific temperature. It does *NOT* control the engine temperature, nor does it make the engine run cooler. People who think that are fooling themselves.

 

i have to disagree with VW on this one. lukewarm heat can be caused by a number of things including low engine temperature due to missing, malfunctioning, or low temp t-stat. also by a malfunction in control of the blend door under the dash. also by poor coolant flow through heater core (incl stopped up).

first thing to check is your normal engine temperature after driving 5 miles. is it up around 190 or so (good for heat), or down low at 150-160 (bad for heat). if its low, you need a t-stat.

next thing to check is backflushing the heater core with a garden hose. run water through it both ways and out onto the ground (not back into the radiator). when you have strong, clear flow both ways - your done with that. you'll get a sloshing sound for a while due to trapped air, but if you let it idle for a while with radiator cap off, that will help it escape. the slosh should disappear after a couple of days. keep an eye on your temps and make sure it doesn't overheat due to a big air pocket or something. you'll need to remove or cut your heater hoses to do this flush. its ok to cut them, and then reclamp them back together using a pipe or plastic tube thats sold at auto parts store. also not a bad idea to flush the entire engine block and radiator and refill with fresh anti freeze.

as to why does warm air come out of vents when fan is off ? convection flow. the heater core is hot and warm air rises and sucks cool air in its place. also, w/o the fan blowing cold air over it, the air that is moving feels warmer.

i know VW is going to disagree with me, so here's my argument. thermostat is a variable valve that cracks open a certain temperature (say 180) and gradually opens to its full position at a higher temp (say 190). so assuming a properly working cooling system, 180 to 190 degree coolant leaves the engine and passes through the radiator and reenters the engine at about 110-120. as the engine cools the thermostat closes up some to compensate. as the engine warms back up the t-stat opens more. so the t-stat is contantly cycling through various stages of open/closed, holding that magic temperature, which is visible on the temp gauge. With a 195 stat, my engine ran approx in the 190-210 range. With a 180 stat with a hole drilled in it, it considerable down the gauge, around 170-180. now on the other hand, if your engine and transmission are generating temperature beyond the capacity of your cooling system, the your t-stat goes wide open and stays open and you can only hope for the best.

 

I have to agree with Mr Vaughan. The thermostat is there to keep the engine at a certain temperature. If it is stuck open and the weather is cold enough, the temp may not ever come up correctly. I have a 180 in mine and I have to block 1/2 the radiator in the winter to keep the temp up to 180.
My suggestion is to do as dvaughan says and flush the heater core and system, change the thermostat (either to the stock 195 or to a 180), and see what you get......

 

before changing your t-stat, watch your temperature gauge. if its sitting at 190-195, then your t-stat is working fine and there is no benefit in changing it. it doesn't hurt anything to change it as a maintenance item, but it won't fix the immediate problem.