This thread is of interest to me as well.
Rear A/C nice and cold. Front however is blowing about 10 deg. warmer.
Recirc flap (in the pass footwell behind the waffle grid) is working as it should
Heater valve actuates immediately, so I assume it's working as it should
Upon engine startup, I noticed something in the center vent quickly move up or down; however the upper comb flap seems to remain down regardless of setting: A/C on or off; foot or panel, defrost or not. With the center vent diverter trim removed, I can lift it upward with my hand. This does nothing except open the vent more for freer airflow. I assume that this comb flap is simply a diverter, but is this flap being down an indication of another valve/flap not working as it should & letting warm air in?

 

Paul,

With those temps it sounds like the heater control valve should be your top priority. You can force it closed and zip tie it in place but then you won't get any heat in the winter. Still if you need a quick fix, might be a good approach. If you want fix it and keep your heat, check if the valve is getting vacuum with a might vac, if it is but is not operating, replace the valve and the short hose attached to it (about $35), if it's not, pull out the vacuum manifold and start tracking down the source of the leak. If the manifold isn't getting vacuum, often the slitter near the brake booster that feeds it is leaking. It's actually fun work once you understand how it all functions.

Some have also reported valves that close but still leak hot water because of internal warpage, so if you think the valve is original, might not be a bad idea to replace it anyway.

 

"My A/C doesn't work, but right now I'm more concerned with the incessant searing heat coming out of the center console at the ****er and window switch panel. Would that be engine heat sneaking up through the firewall, or a vent problem? "

One cause is collapsed motor mounts, which allow the engine and torque tube to drop enough to pull the foam seal away from the floor pan, which lets very hot air into the console.

 

Does this problem go away when you install new engine mounts, or does some insulation somewhere need to be replaced or resealed?

I replaced my collapsed motor mounts at the same time as I was doing the A/C work.

Thanks,
Paul

 

The molded foam is pretty tough, and will usually reseal very well when the mounts are replaced. Remember that this particular problem causes hot air from the shifter and console, not from the HVAC.

In your case, it appears that (as Dan says) the refrigeration system is working fairly well (with one possible exception, the front expansion valve), and the problem is in the air flow/coolant flow/temp control area.

Some possible problems include:
- Uncontrolled coolant flow. Heater valve faulty, installed backward (black side should be toward the engine), not getting enough vacuum to hold closed.
- Airflow flaps faulty (foam bad) or maladjusted.
- Hot air getting into the HVAC inlet, usually due to leaking/missing hood seal.
- Faulty front expansion valve.
- Crap blocking the evaporator.

There are other possibilities, but these are more common.

 

A real diagnosis includes looking at pressures in the system as well as the temps. They work together to help with the diagnosis.

The IR temp gun is not the right way to measure air temps. Grab a cheap thermometer from the grocery store, one with a metal probe and a dial on the end. A fancy one has a digital readout but the dial modle is more than adequate. I have one for ambient, one for the center vent, and one for the footwell so I can see how much the temp changes.

Evaluation is done with engine at engine 1500 RPM.

There are some great books on auto AC systems. Your auto parts store should have a Chilton's or Haynes book that will teach you a lot about the basics. The factory training supplements included on the JM CD manual set are good, but assume that you've had the basics first, so get the Chilton's and read it first. The refrigeration portion of the 928 HVAC is very generic. The vents/flaps/actuators fo distribution and heating are specific to the car.

Read Wally's excellent HVAC primer, available on the 928 Specialists website.


Remember also that virtually all the stuff you read on internet is not subject to any editorial review for accuracy. Including this, probably. IMHO, the basic Chilton's auto AC manual is superior to almost any on-line DIY amateur how-I-did-it write-up. Most amateurs, and probably most AC "professionals", have just a small slice of the total knowledge needed to do serius performance work on the refrigeration systems. Among the group here, Wally knows his stuff, as does Dan Prantl in Florida. There are likely a few more in the hiding woodwork, but the list is not that long.

 

Hi Wally,

I have a few of questions about the expansion valve, since you mentioned it:

1. The new ones I installed were Rein brand - same as what was in there OE. How likely is it to get a bad one new?

2. If it were bad, wouldn't the tech, and I, have seen this in the pressure readings, or would that have been "hidden" by the operation of the rear A/C? I would think that if I took pressure readings with the A/C on but the rear A/C off, that should tell me if the front EV is blocked... correct?

3. On the VW Golf I used to have (on which, BTW, the A/C worked REALLY well after I repaired and converted it), the expansion valve had a little foam case to insulate it from engine compartment and ambient heat. In the 928, not only isn't there such insulation, but Porsche put the thermometric element of the EV right next to a heater hose. Isn't this a bit of a problem?

Thanks,
Paul

 

1. It is very unlikely that a new expansion valve is faulty, but not impossible of course.

2. A properly functioning rear A/C would mask a defective front expansion valve if it was running and the solenoid was not closed due to the rear evaporator reaching very low temperatures. Yes, if you want to diagnose the front expansion valve, make sure the rear A/C is off and check running pressures.

3. It helps if the expansion valve is insulated, but not to a very large extent. There are many other car makes that actually have a plastic molded case with insulation inside that snap over the expansion valve. I guess Porsche didn't deem it necessary at the time. You may be able to modify one such case from another car and make it fit though, but I doubt you would see significant gains in A/C performance because of it.

Dan
'91 928GT S/C 475hp/460lb.ft

 

My vote is for it being the heater control valve. In my '88 S4, I had cold air from the rear A/C but hot air up front. The HCV was shutting, or so it seemed. I zip tied it shut all the way and now I have a cool car (I thought it was cool before anyway). Good luck!

Joe

 

Hi Wally,

I have a few of questions about the expansion valve, since you mentioned it:

1. The new ones I installed were Rein brand - same as what was in there OE. How likely is it to get a bad one new?

Possible, but not likely.

2. If it were bad, wouldn't the tech, and I, have seen this in the pressure readings, or would that have been "hidden" by the operation of the rear A/C? I would think that if I took pressure readings with the A/C on but the rear A/C off, that should tell me if the front EV is blocked... correct?

The system pressures should have given you a good indication of a faulty expansion valve.

3. On the VW Golf I used to have (on which, BTW, the A/C worked REALLY well after I repaired and converted it), the expansion valve had a little foam case to insulate it from engine compartment and ambient heat. In the 928, not only isn't there such insulation, but Porsche put the thermometric element of the EV right next to a heater hose. Isn't this a bit of a problem?

I don't think that it is a major effect.

As I said earlier, and others have said, I think that your problem is in airflow, coolant flow, or temp control. The heater valve is the first suspect, but you need to consider the other possibilities.

 

In my case, I'd think bad front e valve, but since I replaced them both a few years back and the original front wasn't nearly as bad as the rear...I'm stumped.

 

With the A/C off, and the temp lever set full cold, how cool or warm is the air you get from the front vents?

 

Depends. Usually ambient plus five to -maybe- ten degrees, after the engine is warmed up.

-----

When you first start the car, engine cold, after sitting in the garage overnight, what's the center vent temp with AC on, 1500 RPMs, fan on 2? There's no coolant heat added, no hot-soak heat from sitting in the sun. Should be less than 40º, hopefully lower 30's. If it gets warmer then as the car warms up, start looking at the heater valve for open or leakage.

 

Are you asking what it is in my case, or asking what it should be?
This is an interesting question, because I'd say that the center vent is blowing a good 15 - 20 deg. more than outside temp w/the a/c off; lowest 'heat' setting. Maybe I'm getting some kind of hot air mix that I shouldn't be. I've always attributed it to a big, hot V8 permeating thru the firewall.

 

I was asking you, because, if, for whatever reason, you're getting "extra" heat from the HVAC system, then you probably don't have a bad expansion valve.

A lot of people have said that, even if their opening and closing correctly, these heater valves tend to warp internally and then don't seal well.