A/C Diagnosis - evaparator valve troubleshooting

[warlock]

Hey folks, new user here, started with a 944 S2 back in '95, and then a 993 sometime in the late 90s. She's been running fine, and I've gotten more adept at R&R thanks to lurking around here.

So I decided it's time to see why my a/c quit last summer. The compressor engages, but the vents just blow air at ambient temp. Looked to see if the condensor far was running and it's not!! Narrowed that down to the infamous ballast resistor and jumpered pins on the relay to get the high speed fan blowing. Still no change. Bummer, that would have been nice.

So I got a set of gauges and got to work. Here's the readings and steps I took with ambient temp of 71 F:

Engine Off: Low=32, High=32
Engine Idling: 4, 95
Engine @2k rpm: -4, 105 (low side was reading a vacuum)

At this point I figured I'd put a wee bit R134 in to get the high side to spec. After this:
Engine Idling: 16, 160 (low and high side in spec now)
Engine @2K rpm: 2, 180
Engine Off: 70, 70

Feeling the lines, the high side is hot from the compressor to the condensor. From there, it all feels about the same - cool but not cold. No change in temp feel before/after the expansion valve.

What do ya guys think? I'm leaning towards (or is it hoping) Expansion valve clogged. But, I just don't know.

Would greatly appreciate any tips on isolating the problem.

[ToreB]

I can't comment on the pressures, but have you checked all ventilation servos?
Details here:
Porsche 964 / 993 CCU electrical repair
Cheers,
Tore

[warlock]

Thanks Tore, your postings and website have indeed been very helpful for various electrical questions. In this case, the electrics are fine (apart from the ballast resistor on the condenser) and the problem is not with the hot air servos being stuck

[bazar01]

It is best to totally evacuate the remaining R134A refrigerant with a vacuum pump since you already have the gauge manifold set. You need to evacuate at least 3 hours with the vacuum pump on. Once you reach 29-30 inches Hg, turn off the vacuum pump, close the gauge manifold and let it sit for a few hours and see if the vacuum holds. If on the other hand the vacuum pump keeps running but it cannot pump it down to at least 20 inches Hg, the system has a big leak.

Once the vacuum holds for a few hours, charge the system with the recommended amount. On the engine decal, it will state the R134A charging weight.

After charging, you have to monitor how long it will hold the correct charge. It can take days, weeks or months deoending on the leak. You will normally feel when it starts blowing warmer supply air at the vents. Then you need to start pressurizing the system again and use a sniffer to check for minute leaks and soap bubble test the fittings for big leaks.

It is not unusual not to find any leakage from external components and fittings. The weakest ac component is the evaporator which is always the culprit. And replacing it is not easy.

good luck.

[perelet]

Warlock. A/C on 964 is not so much different from any other car.

16, 160 that you are getting - 16 is low, should be 30-40 range.

Do you get any cold? Also, you indicated that pressure was low initially - most probably you have leak. It is good idea to find out where it leaks.

Bad evap valve usually blocks low compression side. Here's reading with bad evap valve and full charge:




Here's after replacing valve:




PS. It maybe me, but I removed (and weighted) all A/C components from my 964..

Oleg.

[warlock]

Thanks guys, let me see how the vacuum test goes. I've got a 1.5 cfm pump I can borrow, would that be sufficient? Also, when I switch the yellow hose from the R134 can to the pump (and then back again if I have to refill) how do I avoid getting air back in the yellow hose?

[bazar01]

Originally Posted by warlock

Thanks guys, let me see how the vacuum test goes. I've got a 1.5 cfm pump I can borrow, would that be sufficient? Also, when I switch the yellow hose from the R134 can to the pump (and then back again if I have to refill) how do I avoid getting air back in the yellow hose?

You can try that and see how low you can get on the low side gauge. If you can get it down to at least -29 in.Hg, it should do the job but the question is ho long it will take. You will need at least a 10 cfm vacuum pump.

1. Hook up the yellow hose to the vacuum pump.
2. Hook up the red hose on the high side port on the compressor.
3. Hook up the blue hose on the low side port.
4. Do not turn on the vacuum pump yet. Check vacuum pump oil level.
5. Read the system low and high pressure.
6. If standing pressures are still above 70 psi. You may need to recover. Or just release it to atmosphere but not advisable.
7. If pressures are below 5 psi, you can start evacuation
8. Turn On vacuum pump
9. Open low side knob on manifold slowly. (CCW)
10. Open high side knob on manifold slowly.
11. Let the pump run until deep vacuum is reached. Let it run another 1 hour.
12. Close the manifold low and high side knobs. (CW)
13. Turn off the pump
14. Let it sit for a few hours and observe the low side gauge for loss of vacuum
15. If system holds vacuum, remove yellow hose from vacuum pump
16. Hook the yellow hose to the R134A can.
17. Open the can charging valve.
18. Loosen the yellow hose fitting on the bottom of the manifold gauge slowly to purge the air inside the hose from the can to the gauges. tighten the fitting back. The hose is already purged and filled with R134A
19. You can start charging on the low side if you are a beginner.

[perelet]

Some Autozone stores have free loaner vacuum pumps. Store on McDonnald McKinney has pump for sure.

There are youtube movies that describe how to use gauges.

I'd be glad to offer help, but tomorrow movers will come to my place, pack all my suff, place cars on transport and move everything to San Diego. I'll miss DFW Porsche community and time trial events.

Oleg.

[warlock]

ok, back on this project now. Oleg and Bazar, I took your advise on trying to trace for leaks with close inspection and soap water before trying the vacuum test. Can't see any signs of leaks or bubbles, but when I peered down under the exp valve with a flashlight I could see some fluid trapped. It's right under the exp valve, behind the firewall. It feels just like motor oil and is yellow in color. I'm guessing this is PAG oil since I can't think of how motor oil would get in there. Yes?

So, given the low side pressure is too low, and the evidence of presumably PAG oil, I'm leaning towards replacing the EXP valve. Do I also need to replace the dryer while I'm at this?

[964TargaC2]

The guy who taught me AC stuff said that any time you open the system its a good idea to replace the dryer

[ToreB]

I would guess it's time for an evaporator change. Welcome to the club...
Here's how.
Cheers,
Tore

[warlock]

Heh, the dreaded gas tank and airbox removal. I'm going to replace just the EXP valve and see if it will hold a vacuum before I change out the evap.

[perelet]

Just expansion valve is maybe 30min job. According to your description - good chance that's your problem. After that you'll have better idea of what else is wrong. Also, make sure you buy new O-rings. it is good idea to replace as many of them as you can (not that hard on 964), cause they may cause slow leaks.

Oleg.

[bazar01]

Originally Posted by warlock

ok, back on this project now. Oleg and Bazar, I took your advise on trying to trace for leaks with close inspection and soap water before trying the vacuum test. Can't see any signs of leaks or bubbles, but when I peered down under the exp valve with a flashlight I could see some fluid trapped. It's right under the exp valve, behind the firewall. It feels just like motor oil and is yellow in color. I'm guessing this is PAG oil since I can't think of how motor oil would get in there. Yes?

So, given the low side pressure is too low, and the evidence of presumably PAG oil, I'm leaning towards replacing the EXP valve. Do I also need to replace the dryer while I'm at this?

It's possible somebody worked on the ac and put a dye in the system to trace leaks. It is also possible the the leakage is through the o-rings on the expansion valve fittings and connections.
Instead of pulling a vacuum to trouble shoot, why not pressurize with a little refrigerant and nitrogen to 200 psi and use an electronic leak detector?