I did cross post this from another website so thought i would post it here as there tends to be a little more activity. I iwll also post a response I got from the other site which has mor info. Thanks ..

Here goes....

Although I am a nueb on this site I have been working with this car for over 2 years. I did a search and found a few similar problems but no solution. I will try to be brief.

Problem : No heat from vents at idle, increase rpm and heat blasts out. ..simple enough

Atempts at repair.

1 Repaced thermostat , water pump, and coolant, bled coolant umpteen times with no results.
2. Replace cap from coolant syatem, replace heater hoses from fire wall to engine ( what a freakin nightmare )
3. Actually removed entire dash and replaced heater core and checked flapper boxes, everything functioning correctly and new core installed. Same problem.
4. Bled coolant sytem again.
5. Used thermal laser and found that coolant temp going into core just not hot enough , increasd RPM and coolant line temp increased by about 20 degrees.
6. Engine gains and manintains normal temp on instrument panel. cAn actually indicate a lower temp on the highway and be blasting out the heat , and a higher temp while idling yet blow cold.
Checked head gasket for leaks, no pulsing at the expansion tank , engine runs strong, no coolant usage at all, and each time I bleed no air comes out.
7. Bled by overfilling expansion tank untill fluid came out of bleed hole , caped off bleed screw and returned expansion tank to normal level.

The Audi 90 has no heat control valve, just straight from the engine to the firewall.

Other than this problem the car is SOLID, but it is undrivable in Canadian winters in bumper to bumper traffic. Occupants will freeze to death. CAr has to be parked for half a year.

HAs anyone experianced or has a resolution to this or is this a design flaw to which ther is no resolution ?

Thanks in advance

PS since this is my first post thought I would atatch a pic. I have practically rebuilt this car from scratch and this is the only problem that eludes me.

follow up post after a thermostat was recomended..

Thanks for the response guys .... and just to recap since I was not clear on a couple of points.

I replaced the heater core and both hoses that run to the heater core, one comes from the coolant pipes that come from the head and the other goes to the return pipe to the water pump.

I replaced both the water pump and the thermostat. the original thermostat I tested as fine. I don't think it is an engine heating problem given that the engine comes up to temp fine and regulates fine. If I am running along the highway the temp is actually in the 80 -90 range and will melt you if you out it on full blast. idling the engine goes up past 90 say 90 - 95 or so yet blows feakin freezing. I do believe it has to be a blockage somewhere but I am running out of places.
Do you guys have ample heat at idle. ?

Also what was tha about the plug and what does it do ?

I almost want to belive that the pump isn't running fast enough at idle to draw the fluid through the core, like there is a path of least reisitane somewhere else. This one has me totally stumped.

On a somewhat realted note do I have to pull the engine or intake manifold to check the pipes that my hoses fasten to ?

 

Interested to see responses. I have searched and determined it may be a recirculating flap problem.

 

i can't think of anything else really. Manual or automatic climate control?

 

closing and then when speeding up air pressure forces it closed?

 

Well that is a good thought. it feels pretty darn tight against the pump and I can not even turn it 90 degrees at the longest throw . It had this problem before I ever got the car and ( I know the PO ) I have since replaced the belt when I did the water pump and t stst twice.

it is a manual heat and the cables completely close the doors, checked that out when I did the core.

pretty strange one.

 

The flap is closed. even if i increase rpm in my driveway it gets really hot. Maybe I just need water pump with a smaller pulley :-)

 

Interesting, I need to check my flap here soon.

 

Remove both hoses going into heater core at firewall.

Fabricate hose to allow you to flush heater core using garden water hose.

Flush heatercore backwards flow. See what comes out of heater core and observe how much flow passes thru the heater core...

Post results here.

 

the heater core is new though

 

Yes the heater core is new.... I back fushed the old one and it was perfect , but I ripped the dash out and put in a new one anyway since I was getting so frustrated... I think the answer may be in the temps. If any of you guys have a laser temp guage and could check your temps of the hoses at idle and at say 2,000 rpm it would tell me a lot. If I new better I would say the system is blocked somewhere but where ? isn't this nuts ? I have a thermal probe also so I can take thermal temps at idle and at 2000rps along with anbient so you guys can see what I am dealing with.