Luke warm heat 92 Eco

[92EcoDiesel Jetta]

Last night in 12F ambient, driving on the hwy for an hour I was getting lukewarm heat. Last winter I had great heat.

92 Eco with AC.

I pulled over at a rest stop and noted the following:

1. Coolant temp gauge in the middle

2. Upper rad hose hot, 180F per IR thermometer

3. Lower rad hose cold

4. Upper part of radiator fins hot

5. Bottom of radiator fins cold

6. heater core hose in hot, 180F per IR thermometer

7. heater core hose out hot, 160F per IR thermometer

I think all of the above is normal for 12 F ambient and the thermostat and heater core is OK.

Why am I getting lukewarm heat?

Per the Bently, the amount of heat is controlled by blend door. I assume it blends the ratio of cold fresh air vs hot air from the heater core? Am I getting too much cold air because the blend door is out of adjustment or can it be a vacuum problem to the servos?

I don't want to rip the dash apart to find where the problem is. Is the blend door cable on the heater box accesible without ripping the dash apart? Maybe it's just a matter of adjusting it?

[cyrus #1]

I think you are on the right track with the blend door theory.  You should see more of a temperature drop than 20° across the heater core.  Something must be causing a lack of air flow through the heater core.

The other thing that happens in MKII's is the foam that seals the blend doors tends to disintegrate over time.  Have you had any black bits come through the vents?

[R.O.R-2.0]

you probably need to re-foam your heater box..

i would be willing to bet that all the foam has fallen off your blend doors..

its a real bugger to pull the dash and heater box just to re-seal the blend doors..

and you might as well replace the heater core while you are in there, because they are cheap, and it takes another 5 minutes..

[mystery3]

Something must be causing a lack of air flow through the heater core.

All the foam that used to be on the blend door?

[92EcoDiesel Jetta]

Bits of foam has been coming out of the vents for the last 3 years. What if I close off the fresh air inlet under the rain tray? Will that work as a temporary measure to get more heat by blocking out cold air coming in? It's easy enough so I'll try it.

[TylerDurden]

First I would check the blend doors. They can be re-covered in-situ with good tape (metallic, gorilla... not crap duct tape).  I take out the center vents and the console... the front duct can then be extracted. There are a couple of screws on the bottom of the dash that come out to flex the dash a bit.





I would not cover the fresh air inlet. Recirc will only cause major fogging of the glass. (You can always tell the guy on the highway with re-circ active, lol.)

I did the blend doors in my 89 and the heat still sucks. The core must be plugged.  Pulling the airbox is a PITA. I have heard guys have split the evap section off and removed just the heater section, but that can't be any fun either.

[92EcoDiesel Jetta]

Thanks Tyler! Were those round holes in the blend door once covered in foam and they need to be recovered? I guess I don't understand how the blend doors work.

[92EcoDiesel Jetta]

Why didn't VW use a solid piece of metal instead of swiss cheesing it with big holes???

[TylerDurden]

This is a hacked photo (not my original pic) showing airflow in the heat section.

Blue= cold air
Red = hot air
Purple = blended air
Yellow = door movement & core



The door closer to the firewall directs air to and/or around the core for temperature adjustment.

The door near the front directs the blended air the the vents or to the flapper on the left.

The flapper on the left directs up to glass or down to feet.

[R.O.R-2.0]

the heater core CANT be blocked with foam, because the foam is AFTER the core, thats why you see it blowing out the vents..

[srgtlord]

As for the swiss cheese... I bet it saved VW a nice chunk of change for not having to put the extra metal in.

[TylerDurden]

the heater core CANT be blocked with foam, because the foam is AFTER the core, thats why you see it blowing out the vents..

Hmmm... when the door is blocking the core for cold operation, it must be before the core, so loose foam could get blown into the fins.

[scrounger]

92 ecojetta: That infrared thermometer sounds like a nice diagnostic tool. Could you post a bit about it.

TylerD: I can separate the back of the heater from the inside of the car without taking out the dash? I think it was mentioned in another thread yesterday or the day before. What approximate angle was the flow diagram taken from?

Thanks guys.

[8v-of-fury]

Just your basic IR thermometer.

[scrounger]

Jeremy:
Looks pretty cashy. Must be a pretty common tool then?