Ed Scherer

Did you clean the front evaporator? Those have a history of being clogged up. See SeanR's thread A/C not cooling well? Evaporator cleaning for a rather extreme case. Also see the "How To" HVAC blower removal and clean-up + Cleaning the Evaporator.

Edit: ahhh... I see that Maine just beat me by a few minutes on this suggestion.

dr bob

IR guns give very poor air temp readings. Instead, grab a [couple] little digital thermometers with stick probes, us thm in the vents. From the Grocery store kitchen section, Wal-Mart, etc.

Then it's not the heater valve leaking by.

You can test that from the engine bay, right where the little check valve connects by the brake booster. Use your MitiVac or similar, you should be able to pull (and hold) a vacuum there. Takes some pumping since you are also pulling air from the accumulator in the fenderwell, but that really tests everything at once. If you can't get or hold vacuum there, go to the rubber manifold by the chackvalve and do each branch one by one to find the leak. If you can't hold at the HVAC tube there, but can at all the individual solenoids, only then should you suspect that rubber manifold at the solenoids.

docmirror

MIne is the same, and I've decided that the specs on the evap expansion valve in the front is set higher than the one in the rear. Since the one in the front is always on in AC mode, they set the exp valve pressure/temp gradient slightly higher than the rear. Also, there is natural heat soak in the firewall from the hot engine radiant heat that isn't present in the rear.

It's just never going to be as cold in the front unit unless the exp valve is replaced with a lower one, then you run the risk of freeze clog. I've cleaned the fins, tied off my heater valve, made sure the recirc flap was up and tight, and it's still ~10-15F warmer in the front. So, I run the front unit on 3, and the back unit on 2 until I get the right temp then I lower the back unit to 1 setting and the front adjusted to the best conditions.

safulop

Yeah the heater control valve on these cars *sucks*. As in, doesn't hold water even with good vacuum. I'm on my 3rd or 4th replacement in 4 years, I lost count.

To zip-tie it don't you have to remove the air box? Yeesh.

MainePorsche

Would be hard if you didn't.
Airbox removal is a piece o' cake.

awilli6

I've tested all vacuum parts using Pirtle's & Dewayne's guide. I will clean the evaporator. Onece I get the solenoid manifold changed out and clean the evaporator & recheck for leaks, if it still doesn't cool correctly, I'll get the freon amount professionally measured.

Good info to know about expansion valves. If I can't get it colder, it'll be strictly a winter, spring, fall car. Memphis heat to much w/o good air.

docmirror

Cleaning around the temp sensor in the evap may have a decent affect on lowering the temp setpoint of the exp valve. If it is the variable type that senses the evap core temp(and I think it is) then having it clean could allow the setpoint of the exp valve to go down near freezing. It sounds like from the link from Sean several people have improved their front cooling by cleaning in there.

safulop

Oh man, I used my 944 daily driver with inoperative AC for 4 years in Chicago and then a few years in Fresno before I finally got the AC fixed. You've got to make some sacrifices for an old Porsche!

I remember when my neighbor in Chicago was flabbergasted that I had a choice between spending extra money on painting the car or getting the AC fixed, and I chose painting the car.

dr bob

That freeze switch into the evaporator is on-off with a little hysteresis. Doesn't really change the way the expansion valve works. Since it shuts off the compressor when the front evap gets too cold, it also cuts off flow to the rear evaporator at the same time.


For sure I'd look at how clean the front evaporator is. They can get full of dust, and that turns into a mud cake if you live in a humid climate where a lot of the cooling is spent condensing moisture. SeanR's thread on evaporator cleaning is a great guide.


At the same time, take a look at the sight glass in the top of the receiver/dryer, and see if there's a pretty solid liquid stream flowing there with the system engaged and engine speed about 1500 RPM, fan speeds on 2. There should be only a small amount of bubbles/foam passing through under the glass. If there's a lot the system may be undercharged or have air in it. Charging only by the sight glass isn't a great idea, as you really need to have pressure gauges to read high and low side pressures, and thermometers to tell you how and how much heat is being moved.


FWIW, I seldom if ever bother to use the rear air, even when I was in always-sunny SoCal with a black car and a greenhouse riding in the back all the time. It's there for local comfort of rear seat passnegers, and does not appreciably speed the extraction of heat from the cabin vs. front system only. The system is condenser limited, that is the limitation on total heat moved is at the condenser in the front of the car. Turning on the rear system will always reduce the flow and therefore the heat removal capacity at the front system. Meanwhile, the rear system has no heat migrating through the firewall or absorbed through the dash. No heater coils right next to it to share migrating engine coolant heat. No possible misadjusted or non-sealing blend doors or fresh-air doors bleeding heat into or around the AC evaporator as the front system does.


Greg Brown has shared a few tips on airbox maintenance over the years, and has identified some of the failing blend door seals as offenders in the war for cold AC. He's done a few hot-climate system mods that add more condensing capacity with heat exhangers and fans under the headlights, and spends time rebuilding and resealing the airbox in front to make sure everything works as originally intended there. These cars were developed with testing in seriously hot climates, and the factory had a bit of experience with trying to sell rear-engined air-cooled cars with no or at best marginal factory systems installed. They knew that folks in many of their markets were sensitive to HVAC performance, so the 928 system is in fact overdesigned for the size of the (subcompact) car cabin. Getting everything back to at least the way it was delivered is a big first step in any AC performance effort.

docmirror

Not the freeze switch, the TXV has two springs in it, and there is a capillary tube in the evap core. As the temp changes on the capillary, the TXV changes the amount of freon that transits the valve, thus changing the amount of cooling in the evap. If the TXV is set for a different opening pressure, and the springs are rated differently than the rear TXV, it will not produce the same 'chill' factor. I didn't look up the PN for the front and rear TXV to see if they are different, but my best guess is - they are.

Most TXV are not user adjustable, but one could find a repl TXV and capillary with a more aggressive flow rate that would cool the evap core better. Just a few thoughts, but I haven't dug into it enough to say for sure.

dr bob

Hmmm. The TXV on my car has a very short cap tube into the return side of the block. Never makes it actually into the evaporator, only reads discharge temp into the suction line.

It's been a while since I had mine apart, about 18 years, but my feeble memory is that the configuration of the expansion valves is the same front and rear.

Ed Scherer

Since I just cleaned my front evaporator a few hours ago, here's yet another example of just how bad those can get. I'm pretty sure I've seen worse examples (well, better examples of extreme fouling!), but this is still pretty bad. The two giant dust bunnies in the foreground were a bit surprising. Where the heck did those come from? I somewhat suspect mice might have been involved, but is there actually a path that will get them in there?

(see also my separate post on this task)

awilli6

Wow! Thats ugly.i looked at mine briefly and it appears to be relatively clean. I'll get a can of foam and hit it anyway this weekend. Thanks for the responses. When the part arrives, I'll get it put back together and report back.

safulop

That's interesting, considering the owner's manual states or implies that the rear AC system "increases the cooling capacity" of the car. Or something like that.

Alan

At relatively moderate temperatures where you need AC it does - but at extreme temperatures the condenser becomes more limiting. It might technically still provide more actual total cooling - but it won't feel that way in the front seat with warmer air from the front vents...

Alan