SQLGuy

Basically, it's hot here, or at least it was yesterday, and the GTS's A/C is not cold enough.

I ran the blower with A/C off today, and measured vent temps. Side vents are 20F hotter than ambient (using digital food thermometer to measure). Bottom of the center vent is 15F hotter than ambient, but top of the center is only about 8F hotter.

Here's what I've checked/fixed so far:

1. Had a bad actuator on the HCV. Replaced that - just the actuator, not the valve. I can see that the actuator is pulling full closed.

2. Replaced torn diaphragm in comb flap actuator.

3. Reconnected disconnected vacuum line at actuator next to the steering wheel (not sure why it was disconnected).

1 - 3: No vacuum leaks in HVAC lines, all actuators and flap motion verified with Mity-vac.

4. Verified that seal at top of firewall is sealing air inlet from engine compartment. Verified air flow into blower with a smoke stream. Engine mounts are good.

5. A/C has a new compressor, a new receiver drier, and a new front expansion valve. It's running R12. (unfortunately, the receiver/drier the shop installed does not have a sight glass, so I can't verify that they charged it sufficiently to cover the rear A/C option... cooling drops noticeably when rear A/C is enabled)

6. Cleaned out front evaporator with Simple Green, a brush, and a hose.

7. I was able to look at most of the foam seals while I swapping the dash recently. Other than the one at the front of the center console (which I replaced), they all looked pretty - even surprisingly - good.

I'm expecting that the HCV is leaking, but I wanted to run this by the group in case there are other things you've seen that resulted in these symptoms.

Jim M.

It sounds like your heater valve is bypassing hot water. It may be closing completly but the seal may have deteriorated. Do the outlet temps rise as the car warms up? Also what are you hi and low side R12 pressures. If the cooling drops with the rear A/C on it may be low on R12.

SQLGuy

Well, it's only $14 for the Four Seasons mostly metal valve from Rockauto, so I ordered one. Hopefully that will help with the heat gain issue. I think I need to resolve that before I can see how well (or badly) the A/C system itself is cooling and go back to the shop that did the R12 work.

Bill Ball

Yes, if the vents are 10-20F warmer than ambient air temp, the heater valve is leaking. You're not getting full heat (completely opened valve), but your getting some heat from that leaky valve. The only time the vents might be above ambient is right after starting the car, especially if the dash is heatsoaked from sitting out in the sun. Once underway, after the dash has cooled to ambient, the vents should match outside temp.

PHIL928

Yep, I replaced that valve and cable tied it closed, now it's blowing ice cubes in the middle of the summer (40+ degrees Celsius)

StratfordShark

I would also make sure that the setting motor is moving to full cold when you select a temp below ambient on the slider.

If it doesn't then you need to diagnose whether motor problem, or a fault in the sensor string (external sensor, internal sensor, hvac controller sliding resistor) input to the motor.

Very recently I suggested to another owner that his "too hot" problem was likely internal cabin sensor thermistor drifting out of spec in high direction. That's what it turrned out to be (if it goes too high, then the setting motor is always being "told" that cabin is too cold).

WallyP

If a heater valve is installed backwards, coolant flow will push the valve flap off of the seat, allowing coolant flow to the heater. On the stock valve, black goes toward the engine.

Bill Ball

These faults create an all-or-nothing situation where you get full heat or no heat.

dr bob

Jeff--

Do your temp tests again immediately on starting the car with cold engine, so no hot water circulating. If they are closer to what you expect (ambient), start the AC and test the temps again. If the temps increase with the engine coolant temps, you can blame a leaking HCV.

SQLGuy

A/C was not in the mix for the tests at the start of this thread. Tests were done with A/C off, car fully warmed up, and car having been run long enough that heat soak from a previous run, or just sitting in the sun, should not have been an issue anymore.

The main thing I was trying to find out is wherther there are other things, besides a leaking/bad HCV could cause such additional heat from the side vents.

For instance, someone mentioned the setting motor, but if the HCV is closed, it shouldn't make any difference whether air went through a "cold" heater core or a "warm" evaporator - so, out of scope for what I'm trying to test.

Emickelsen

Funny, I was having the same problem. Found my heater valve was backwards. Ordered a new (and more stock) one, new HVAC control unit (internal relay bad), checked for vaccume leaks, did it all. Now everything seems to work great, and it blows real cold at idle/low RPM. At higher RPM's it goes hot again. Could my R12/freon (I dont know which one the car has) just be low?

GregBBRD

The heater core will always be warm when the engine is hot, even if the HCV is in perfect condition. There is enough "backflow" to the heater core that it will never be "cold". The setting motor has to close the flap between the heater core and the evaporator and the foam on that door has to be good enough to not let air past.

Otherwise, you will always get hot air mixing with the cold air stream.

SQLGuy

That's pretty annoying. My existing HCV is installed correctly. I suppose it could be leaking... I didn't replace it (yet) because the number I found referenced when I searched here (84707) is actually a hand-operated valve, not a vacuum-operated one.

Is there an article you know of, or can you comment on what's involved in replacing the foam on the mixer door? I looked for and asked about this before swapping my dash, but didn't get any definite suggestions on it. While I had the dash out, I checked out as much of the foam as I had access to, but didn't see where the mixer door is.

I have torn into VW HVAC's before, and they actually use a door made of perforated metal. Once the foam is gone, the door is totally useless to redirect air, as all of it goes through the 3/8" or so holes. Hopefully 928's don't have such a setup...?

SQLGuy

Well, after shopping around my local parts stores and finding only all plastic valves, I went back and found and ordered the correct Rockauto part (74671). I also tightened up the mixer flap link about 4 turns.... we'll see if that makes a difference. When I did something similar on my S4, it helped a bit for cooling in the summer, but resulted in too much cold air leaking through in the winter. I really don't feel like pulling that dash again...

Has anyone tried adding HCV's on both sides of the core, to really isolate it?

WallyP

The standard 928 HCV closes with vacuum, so it slowly opens as the vacuum bleeds off when the car is parked. There was some discussion about putting in another valve in series that opens with vacuum, so hot coolant circulation is stopped. Search should find the discussion, with the vacuum arrangement.