ZEUS+

That little bastard !

WallyP

You can do quite a bit of useful testing by just feeling of the system as it runs. These are all guesses, based upon past experience.

The high side between the compressor and the condenser should be hot - at least 125* F, as high as 140* - at an ambient of 70*.

The high side between the condenser and the expansion valve should be warm - perhaps 80* - 90* - at 70* ambient.

Any cool spot anywhere in the high side indicates a partial blockage.

The expansion valve block should be very cool. The evaporator should be below freezing if the low side pressure is low. At 1.6 bar the temp should be 26* F. BTW, the anti-freeze switch is set for 32*-33* F, so it should be cutting the compressor off.

At low (by A/C standards, 70* is low) temps, the refrigerant isn't likely to be fully boiled away in the evaporator, so I would expect the line from the expansion valve (outlet side) to the fuel cooler should be very cold. The line may stay cold all the way to the compressor, or it may only be very cool.

If the low side pressure is 1.6 bar, and the high side is within spec, and there are no cool spots or warn spots, you have to assume that refrigerant is flowing thru the system, and that the refrigeration system is working as designed.

At this point, either the incoming air isn't able to transfer its heat to the refrigerant, or it is.

If the heat is transferred, but the air coming out of the vents is warm, then heat is being added by the heater, or the incoming air is being really heated by the engine.

The comb flap is designed to mix the air when both heated and cooled air are being produced, so as to blend the temps to a uniform mixture. The flap opens when only cooling is called for, so as to produce the max cooling effect.

It might be time to pull the rubber boot on the plenum and feel the evaporator.

I still suspect the flap system...

I also suspect that the condenser is coated internally with old coked oil. I once cleaned my Jag XJS condenser by hooking it to the parts cleaning pump and circulating mineral spirits thru it for about ten hours. There was a big difference in flow before and after, and a big difference in performance of the system.

The evaporator shouldn't be as liable to coke or coat internally, as the temps are much lower, but I suppose that anything could happen.

If you set the front system for minimum cooling (low fan speed) and the rear for max cooling, you should get cold air if the problem is the flap system on the front.

If you have 1.6 bar low side pressure, and the evaporator isn't cold, perhaps you don't have R134a in the system. One of our A/C experts bought a sealed thirty pound bottle of refrigerant that turned out to be water...

worf928

Yup. I have felt-up the system previously, but not since this latest fill.

I'm going to poke at the flaps tomorrow with the boroscope, but then I have other work to do; (and you know what work that is ) So, I won't come back to this beast until likely Sunday at the earliest.

Condenser is new so if it's coated with coked (or cooked) oil someone's in big big trouble.

Yup. I know of whom you speak.

I'm reasonably certain that this is real brand-name DuPont r-134a.

dprantl

Any update on this?

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

worf928

Hey Dan, nope. No update yet. I should be getting back to it by Friday.

worf928

Diagnosis of last issue: over-oiled

Summary of all the issues with this A/C system:
1) weak compressor
2) contaminated refrigerant
3) over-oiled initially

Minor Issues:
1) slightly leaking heater valve
2) haphazard replacement of o-rings during previous r-134 conversion
3) leaky and/or in-op expansion valves

When I last left this car, all measurements indicated that the a/c should be ice cold. I talked on the phone with ZEUS+ (thanks Paul) and he outlined a procedure for gently forcing out oil from the system. I did this and got about 12oz of oil out of the system. (Note, that at each of the previous stages I'd carefully calculated how much oil to add based upon the WSM/SB charts etc.) My conclusion is that this A/C was, initially, very, very over-oiled. So much that even a flush only got about 1/2 of the oil out.

I dropped and drained the compressor - just to make sure - and added 4oz of oil to the compressor, replaced the r/d (again) with the required < 1oz of oil. Vacuum, etc. The heater valve looked very new, but I pulled it anyway and it was leaking slightly. Replaced with new.

Result: sub-30F° center vent temperatures when cruising on the highway with or without the rear A/C on.

Next: test the anti-freeze switch

dprantl

Good deal. I knew it had to be something like that causing poor heat exchange in the evaporator/condenser. Although I find it hard to imagine how all the oil was not flushed out. Most of the A/C flushing agents emulsify the oil and using compressed air, everything usually gets blown out.

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

ZEUS+

Glad to hear it's fixed.

worf928

Rule #1 - do not buy your flushing agent at Autozone

Ordering the good stuff presently.

worf928

You're glad? $h+t man... I'm ecstatic.

Thanks for all the help btw.

dprantl

I've had great success with just regular mineral spirits over the years. Some say it leaves a deposit, but I always blow it out with compressed air anyway and have never had any problems.

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

Bill51sdr

Good to hear it Dave. Enjoy it in the months to come!

worf928

Thanks for the sentiments Bill. But, I won't be enjoying this car as it will be going back to its owner (finally).

Tampa 928s

While waiting for my replacement condenser wondering if I should to flush it. You said you had a way to remove all the oil how did you do that? It has a new expansion valve new HP hose, seals and receiver dryer. Compressor is easy to get at now front of the motor is off. The system worked till used condenser sprung two leaks should I leave well enough alone or try to flush it.

Giovanni

I charged the a/c today and I get about 150 PSI (high-side) at idle and about 200 psi at 2000 rmp. Low-side is around 45 PSI at idle. When I turn the engine off the high-side is 95 psi and low-side jump to 100 psi. Is that normal?