FredR

Not sure how the a/c can be working when the compressor is not attached but...?

The hvac control head unit is a known problematic item particularly so the inbuilt relay. Whether that is a part of your problem I do not know but I would think it could well be. Greg Brown does a rebuild service and if it is the relay then Dwayne did one of his excellents write ups on how he went about this. Mine was giving me grief in that it was not latching the power supply to the a/c compressor but I had a spare head unit from my late S4- solved that problem.

The electrically operated solenoid valves are easy enough to test once out of the car or once you can get to them. The final elements of the system [the actuators] are most definitely problematical in that they are prone to the actuator diaphragm splitting- I have either two or three of them out of service including the recirc flap. In my case I do not need a heater over here so I have the heater disconnected from the coolant circuit.

One of these days I will deal with the system as a whole but having removed the hvac system from my late S4 I do not fancy taking on that job unless and until I absolutely have no choice. I think I would prefer a set of manual levers rather than that vacuum operated monstrosity! Testing the actuators is easy with a Mityvac kit.

soontobered84

Fred asked you if the AC was on. He is talking about the push button on the console just under the AC head unit. Is that button pushed?
You posted that the AC was working, but the compressor is not installed. Obviously, your AC is not working, however your AC blower could be blowing. Is that what you meant?

You could have a bad vacuum solenoid. I had the exact same problem with my GTS and the culprit was a single bad vacuum solenoid which controlled the vacuum to the heater valve. I think you may also have an additional problem in that the vacuum feeder line to vacuum manifold solenoids which provides all this vacuum may connected to the wrong place (ie drawing vacuum from the wrong source) Check your routing of the vacuum lines vs. the diagram on the inside of the hood.

Good Luck!

Rhubarb, where are you located?

FredR

In a rhubarb patch at a guess

Rhubarbbear

Yep, in a Rhubarb patch!! I'm in Bedford, UK. God knows why I wrote the A/C was working, there's not even a compressor attached - It's the result of this driving me mad! Spoke to Richard at Ritech today, who was very helpful. He thinks the solenoid has failed in the open position, so it's always passing vacuum.
I got a vacuum pump and all the lines appear to hold vacuum so that's good. I've now ordered another set of solenoids from Ebay which are apparently good, so when they arrive I can swap them and see if that fixes things. Took the MAF off while I was at it and ran it over to John Speake at JDS in Cambridge for testing. It was 10% out of kilter, which is a country mile apparently. He swapped it for a reconditioned one, so that will go back on tomorrow.

SeanR

You could always unplug the vacuum feed to the solinoid for the heater control valve and then check to see if the valve is open with the engine running. This will tell you if the HCV is plumbed correctly.

FredR

Bedford- that figures! I hear the locals get excited when the traffic lights change!
10% under read for an aged MAF sounds about typical. When it only reads 90% of the air flow you only get 90% of the fuel Porsche intended. In practice that is probably not as bad as it sounds given the stock tune runs relatively rich flat out with an AFR of something just over 12 so in your case you would probably have been somewhere in the low 13's at full throttle which is quite optimal for power delivery. When on part throttle it must have been running somewhat lean though assuming that error factor was liner across the board.

Given you have been speaking to Richard I take it you understand he is something of a 928 a/c guru in the UK.

Randy V

I had the same problem.

Turned out to be the vacuum solenoid on the white vacuum hose circuit. It's the one that cuts vacuum to the heater valve, allowing it to open and flow hot coolant into the heater core.

You can test those solenoids by applying 12V power to them on the bench. Ask Greg Brown for specifics.

They are NLA but 928 Intn'l should have a bunch of good used ones.

Rhubarbbear

soontobered84

Rhubarb,
You can replace just a single solenoid if you choose when you get the ones you ordered from EBay. Save the others for a rainy day!