dprantl

Yes, you need to let all the air out of the charging manifold lines. What I usually do is connect up everything, open the freon cylinder valve and all the other valves too, and crack a fitting for 3 seconds or so to make sure the freon displaces the air in there.

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

worf928

Dan, I did purge the manifold lines before connecting them to the ports. I didn't mention that above (along with the initial no-clutch-engaged fill) because you asked about the charging procedure so I concentrated on that specifically.

dprantl

Well, it's not often that an A/C problem makes me I've also converted many cars, including 928's. It sounds like you have the charging process down. Since your high-side is in spec and low side is high (with rear A/C off), it just can't be a blockage problem. The only thing I can come up with is that the evaporator internals have been coated with some crap or corroded from the inside by the weird mixture that was in there before. Were you able to get the temps of the pipes? Although I doubt it would help much since the temps should follow the pressure readings

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

WallyP

Dave,

Most info that I have seen says that at 1.6 Bar low-side, the evaporator should be at 26 degrees F.

You did say at one point that you had frost between the expansion valve and the fuel cooler. That sounds as if you have adequate refrigerant flow and low-enough pressure.

Perhaps the problem is with the flap system, not the refrigeration system...

SharkSkin

Hmmm... I have a similar problem with excessive low-side pressure that was diagnosed as an expansion valve. The fact that you had the same symptoms before & after expansion valve R&R is not encouraging -- mine won't be nearly as easy to change as yours was.

worf928

That's the rub. Obviously (to me for many, many reasons not on-topic and not due to A/C) this car has been mis-handled for years. I received the car for reconstitution more-or-less in pieces on a flat-bed; although it did move under it's own power. I have no idea what was previously done to the A/C. I did look up the various hard lines and the fuel cooler with a boroscope. Everything looked clean on the inside as far as I could tell - but the exam wasn't lengthy.

Since the evaporators (and hard lines) are the only things not replaced it is natural to cast them under suspicion. But, what's the failure mode? This is were I draw a blank. I'm not saying there isn't one, I'm just saying that I don't know what it is yet. So, thinking "aloud" here:

If an expansion valve is stuck closed the low-side pressure would be too low right? If the expansion valve is stuck open then the low-side pressure would be high. Zeus says stuck closed is the more-likely mode.

If an evaporator is blocked one would expect the low-side pressure to get lower right? If the evaporator is not absorbing any heat due to damage then what? I *think* that should cause the expansion valve to open more?

At 1100 rpm the temperature differential at the front expansion valve is about 15 degrees. Call it 95-100 input side to the expansion valve and 80 to 85 degrees output side of the expansion valve.

At 2000 rpm the temperature differential at the front expansion valve is about 20 degrees.

At 1100 rpm the condenser is at roughly 140 going in and roughly 95 at the input to the r/d.

I took two sets of vent/pressure data at 1100 rpm and 2000 rpm with rear a/c on and off.

The data says that the center vent temperature gets worse at the higher rpm.

Ambient temp is about 68 F.

At 1100 rpm rear a/c on:
LS: ~35 PSI
HS: ~140 PSI
FV: ~60 F
RV: ~50 F

At 1100 rpm rear a/c off:
LS: ~26 PSI
HS: ~150 PSI
FV: ~60 F
RV: XX

At 2000 rpm rear a/c on:
LS: ~30 PSI
HS: ~150 PSI
FV: ~70 F
RV: ~50 F

At 2000 rpm rear a/c off:
LS: ~25 PSI
HS: ~165 PSI
FV: ~70 F
RV: XX

The high-side pressure is at the bottom of Porsche's range. The low-side pressure is well-above Porsche's range.

The heater valve is fully closing. The center vent louvers are on the passenger seat and my temperature probe is touching the face of the evaporator. The heater valve's probably leaking and the dash pots are 'slow' but work - so there is a bit of a vac leak in the dash.

I could put in a brand-new heater valve and force it closed but I'm just not seeing a leaky valve as the primary issue hear. Although the higher vent temps at higher rpm seem consistent with more water flow through the heater core... might be worth a shot I 'spose. Might improve the center vent, but what's up with the rear?

worf928

Yup. The frost on the front went away after I flushed out the lines and evaps. Now it's cool to the touch but doesn't form frost and IIRC that's what I expect.

Ok. Let's assume I've never seen a correctly-functioning comb flab. That's what mixes the air from the core and the evap right? What should I see through the center vent with the louvers removed at the extreme temperature slider positions?

If there's a handy link I'll follow it.

dprantl

Zeus is 100% correct that a stuck open expansion valve would cause the low pressure to be higher. I wasn't even considering it though since you said both valves are new. New defective expansion valve is unlikely, but possible I guess. My thought was that if the evaporator is unable to absorb as much heat from the cabin, it will not transfer as much heat to the freon, which means that the freon will not evaporate as much leaving more of it condensed causing a higher pressure. Make sense? Also, is the 928 evaporator a single, double or multiple pass? If it's a double-pass and one of the passages is blocked, it could also cause a higher low pressure because again the ability of the evaporator to evaporate the freon is lessened. This is just grasping at straws though

Higher RPM making vent temps warmer could be explained by the above as well, as you are pushing the freon through the evaporator faster, making it evaporate even less.

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

worf928

Scratch that which I first wrote. I've been staring at 87-104 and need to double-check what's happening in the car.

worf928

Maybe. I've been pondering this question all afternoon instead of getting work done. I'm having trouble conceptualizing how higher-pressure would materialize at the low-side port if the refrigerant is 'cold.' PV=nRT and all that.

Another thought-experiment: let's assume the evaporators were just filled with oil and my flush didn't get enough of it out. Poor heat-transfer to the refrigerant. Besides the end result of the A/C not working, what would be the other measurable manifestations?

dr bob

Dave-- My two sense is that you still have air in the system.

The PV=nRT works with a "perfect" gas. R-134a doesn't quite qualify if it's changing states in your process. Hopefully it is.

The center vent temp with fan on one or two speed will be within 5 degrees of evap temp, ASSuming the heater valve is sealed completely.

The comb flap is visible in the center vent hole with the vent removed. Operation is guided by the setting motor and the position of the slider. Default position has the fingers up, partially blocking from from the airbox out the center vent. When you are selecting the [] position, the one that isn't footwell or defrost, the fingers will retarct down and allow full airfow through the center vent.

Having the rear air on at the same time will confuse your readings, so you should probably leave it off until you sort the front system by itself.

I hook up the can and the manifolds as you do, then use the vacuum pump attached to the port where the switch lives to get all the air out of the manifold and the lines. Disconnect at the switch port, and the schraeder there will protect you only to a certain extent; the DP will cause a weak schraeder spring to leak air in. So I valve at the pump suction, leaving the system sealed and evacuated until I see positive pressure in the system. Then it's OK to pull the vacuum hose from the switch port. No need to purge lines this way.

I also fill the system with liquid through the high side without starting the engine. Refrigerant bleeds into the receiver plumbing and the condenser, which usually have enough heat available to flash the liquid to vapor. Let the system stand a bit before you start the engine if you are unsure. You'll see system pressure rise to match ambient on the gauges and know it's all flashed to vapor. The liquid-fill technique is described well in the AC system training manual that's part of the Jim Morehouse CD manual set. I'll add that it helps to pre-warm the cylinder to ambient plus maybe 25ºF water before you liquid fill, especially on a hot day.

Air in the system is detected by looking at ambient temps and the static stsrem pressure. No engine running, just the at-rest pressure. Look at the low-side gauge and read the predicted temperature, see a difference? Or the reverse, look at the pressure for the temp you measure, and compare it with your actual gauge reading. Just a little air makes a huge difference, screwing up the condensing by hogging space as vapor, and screwing up the expansion valve action by not flashing to vapor as it passes through (it's already vapor) and also messing with the pressure/temmp curve programmed into the valve itself. The valve opens more trying to get colder temps but air goes through and doesn't do any cooling. You have higher suction pressures and higher vent temps too.

HTH!

ZEUS+

Check wsm 87-115 for temp door adjustment if needed.
Those pressures do seem low for the amount of charge. Too much oil will decrease cooling performance. The excess oil creates a formation of a film of oil internally on the heat exchangers. The heat exchangers are the condenser and the evaporator. The oil acts as an insulator and restricts heat transfer.

worf928

Agreed and human perception is biased by a lifetime of H20 phase change under relatively constant pressure.

Good, I like tests and measurements.

The upper engine bay temp is now - after the car's been sitting for several hours - at about 70 degrees, probably a few degrees higher deeper in the engine bay. Garage temp is about 67 degrees. The low-side gauge reads 70 PSI. High-side gauge is marked in 50 PSI increments but seems to be at 70 too. At 70 degrees r134 should be at 71.1 PSI (according to the MasterCool manual.)

So? Seems spot-on within measurement error?


I've been staring at 87-104 and this is what I see: The flaps that direct airflow through the heater core or around the heater core are controlled by the settings motor. The comb flap is controlled by one of the vacuum pots. I can hear the settings motor-controlled flaps operating and can hear and feel the resulting change in the airflow pattern. I think the comb flap pot is dead (need to check that again) and is likely the cause of the slow heater valve actuation. However, my temperature prob was stuck way past the comb flap, so the flap shouldn't have had a huge effect on the actual temp measurement.

worf928

It doesn't seem to be. To be sure though, I'll stick the boroscope in there.

Keep in mind that at ~67 degrees ambient it's right at the edge of being too cold to use anyone's charts. However, yes, the pressures seem right at the edge even given the low ambient. One other difference is that Porsche's spec'd pressures are at 2000 rpm. The Kuehl compressor's charts specify "idle" so one would expect pressure lower than Porsche's.

Yup. That much is understood. The question is: other than decreased performance as measured at the vents, are their any other measurements or tests than can be gathered or performed to support a conclusion of over-oiling?

I have some of these thingies:

http://autorefrigerants.com/oilphasetester.htm

that I'll use tomorrow assuming I can bring myself near the car for more testing.

worf928

I know what the problem is: