I have a 2000 A4 Quattro 2.8 Auto with a heating/temp issue. I flushed the core with water and air pressure and the flow seems fine, both input/output hoses going to the heater core are the same temp to touch(hot). I noticed the temp guage is a little slow to get to operating temp but it does reach 12 o'clock on guage, dead center after 10 minutes or so. The heat blows but never seems hot, just lukewarm to a little hot.

I tried the easy stuff first and changed the coolant temp sensor last week but it did not solve the problem, the car seems to warm up quicker but the heat is still not as hot as it should be. When the car gets to correct operating temp with guage at 12 o'clock I notice when I drive on the hi-way the temp goes down on the guage to 10 o'clock but usually never lower, when I stop and go on local streets the guage creeps back up to 12 o'clock and stays there as normal.

One other thing not sure if it's related but my oil temp guage never works either, no matter how long I drive it always stays to the left on never moves which I think is odd.

From reading I was told that it could be either be stuck open thermostat or heater core...both expensive to replace.

Does anyone have a suggestion on how to test an open thermostat and heater core to see which one is the problem??

This sucks with winter around the corner

 

Replace the thermostat is very inexpensive. I've replaced mine twice. Paid $10 for the thermostat. The coolant was more.

 

I believe it's pretty easy on the 1.8 but on the 2.8 like OP has, it's behind the timing belt so it's a bit more of a process.

OP: Not sure how to test if a thermostat is opening immediately but I'm sure someone here does. There's got to be a side of the radiator (essentially "behind" the stat in the line) that will stay cool until it opens then immediately gets hot after a bit. If that happens it's fine. If every part of the line warms up evenly (and slowly), you'll know it's stuck open.

I just don't know how to go about doing that...

I do know that heater cores are a very common issue. I did mine last fall in fact. It's a bitch to do but I get hot air now so it was worth it.

 

Ah you are correct didn't pick up on that, my fault. Thank Audi for that one OP.

 

I find the temp gauge symptoms pretty suspicious. Once mine goes straight up, it never moves. If you want more data, check out this link telling you how to read the coolant temp from the climate control. If you set the CC to read value 51, you should see your coolant temp climb up to 90C and never move. If you don't get to 90C or it backs off after it gets there, almost has to be the stat is stuck open.


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I had a friend with a VAG COM and we hooked it up to my car, we went for a ride and got the temp up to 90c and monitored the temp. We we hit hi-way speed with heat on it dropped down to 78-80c, is that normal cause the car is cooling down by way of air hitting the radiator at 60mph so it might seem logical?

when we got to side streets with stop/go traffic the temp went back to 90c and stayed.I did notice the heat never really got hot , again more lukewarm.

So my friend suggested an old school trick and said let's block the radiator with a piece of cardboard and see what happens. We went for a ride , temp got to 90c, put on heat and the temp guage stayed at around 86-90c and heat seems slightly warmer but still not hot as it should be.

So I'm still stumped, not really sure it's the thermostat or core, any other suggestions welcomed

 

No, not normal at all. Yes, there is more air at 60 mph. But that's what a stat is for. It should close more to keep the temp at 90C.

I wonder if you might have a clogged heater core and a stat. Even when the outside air is in the teens, I still get heat when the coolant shows 60C or more. But you won't know till you swap that stat. When increased air flow makes your temp drop, the stat isn't doing it's job.

 

anyone use or heard of an inline thermostat, install by top radiator hose???

 

....so in theory if my temp is at 90c and the heat is lukewarm then the heater core is likely partially clogged. So I'm thinking my thermostat and heater core are both shot. To repair either one is going to cost more than what half the car is worth...not looking to drop 800-1,000 on a 14 year old car.

 

Yea if you pay for your car repairs, you just can't have a 14 year old German car. I only get away with it because I do all my own work.