worf928

Calling on the collective Rennlist experience, I'm looking for opinions on the most-likely cause of high suction-side pressure (and bad vents temps) on an an 89 S-4 with the 6 cylinder N-D compressor running r-134.

Previous State:
I won't go into the previous state of the A/C unless its needed. (Long story.)

Current state:

Exterior of the evaporator(s) clean.
Blower and Recirc airbox clean.
New "green" o-rings (_all_ of them.)
New expansion valves
Compressor body seals and manifold seals new.
Clutch gap set to tight-side of Porsche's spec. (Was just outside of wide-side of spec.)
While apart, hard lines, condenser, and evaporators flushed with the in-a-can type of flush, allowed to work, flushed with air, residual allowed to evaporate.
Spec amount of ester oil in system.
New receiver/dryer
New high-pressure line from compressor to condenser.
Vacuumed to <400 microns in a few hours, vacuumed over night. Held <800 microns vacuum for one hour the next morning.
Heater valve closes fully (might have a little leak, but that's not the biggie issue.)
~37 oz r-134a, slowly, carefully filled with attention paid to not allowing anything but 134 into the lines.
Clutch engaging. No evidence of slip.
Electric fans run on high.
Electronic leak detector gives clean bill of health.
No evidence of low side lines icing.

(See WSM 87-129 for r-134a working pressures.)
High-side pressure is in-spec at ~175 PSI (~12 BAR) at 2000 rpm and ~60 degree ambient.
Low-side pressure is ~23 (~1.6 BAR) and is outside (high) of Porsche's spec of ~1.25 BAR (~18 PSI).

I'm getting less than 20 degrees of temperature drop at the vents - 45 to 55 degrees (F) with 60 to 65 degree ambient temperature. Low-side pressure drops more-slowly than I would expect and randomly fluctuates higher then drops.

This A/C will not be cold in the summer.

Now, I know the specs are correct: a month ago I did Afsin's old car by the same procedure and its system easily hit the spec's dead center with the same calculated amount of r-134a and vent temps in the 30-40 (F) range with a drop of 40 degrees or more. And it hit those temps with only 24oz of r-134a, although the pressures were low until the last ~13 oz went in.

My leading theory is a week compressor. (In my experience - very limited on A/C - the compressors either work or are seized...)

My next theory is that there's a restriction on the low side even after the flush.

It's just about too cold to do A/C work here in Yankee Land. Any colder and the pressure charts drop off the edge of the world.

Any thoughts?

Thanks in advance!

Bill51sdr

Dave, in my experience it sounds like the compressor rebuild didn't go well. Either that or your new expansion valve is faulty or has debris blocking it.

dprantl

Based on all you have said, confirming that it must either be a blockage in a line/expansion valve or the evaporator/condenser, or the compressor is internally leaking and not generating enough flow. Do the vent temps improve when the engine is revved higher? Does the compressor make any noise? How much ester oil did you put into the system?

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

worf928

Bill, let me add some of "the rest of the story."

When I turned my attention to the A/C on this car, the pressures were out of specification. High side was low, low-side was in-spec, vent temps were poor. There was an r-134 fitting on the low side and an r-12 fitting on the high side. No sticker indicating conversion. I checked for leaks and found both valves leaking and the compressor leaking from the center body seal.

I had the refrigerant identified and it came up as 93% r-134, 5% r-12, 2% unknown. (IIRC). Blech. Just for experimental purposes I added a few ounces of r-134 and observed pressures. The high side came into spec, and the low side went high. I didn't think that was too surprising given that moisture or other unknown stuff in the system will likely cause pressures to be high.

I had the system discharged and then proceeded to take everything apart. The original "conversion" to r-134 involved, apparently, only the front expansion valve and its o-rings. All of the other o-rings were the old black r-12 o-rings.

I changed all the o-rings and valves. I took apart the compressor only as far as needed to replace the end-cap seals and the body seal. I put the clutch back on exactly as it came apart and checked the air cap at 0.8mm which was in-spec according to my Mastercool Manual for the N-D compressor.

Shortening the story... once back together the A/C clutch was slipping and there was ice forming on the low side line between the fuel cooler and the expansion valve. Great...

So, lather rinse repeat, I took everything apart again and flushed the lines. There was no particle debris from anything. But, the condenser acted like it was plugged until the flush sat for several minutes. Both expansion valves were clean as a whistle.

I also took the compressor apart again to see if I'd obviously messed anything up during the initial seal replacement. Nothing obvious, the seals were in place, everything turned smoothly. I drained and refilled the oil twice too. The only apparent problem was the clutch air gap.

My Mastercool manual lists a spec for the air gap that is larger than Porsche's spec. Mastercool indicates a maximum gap of 0.9mm, the WSM says 0.7mm. It was 0.8mm after the first re-seal. I took out one shim putting the gap at 0.5mm - just above Porsche's spec of 0.4mm minimum. Now, I didn't check the air gap before I took the compressor apart the first time. (DOH!). I also didn't observe the clutch for more than a few seconds during the initial testing before taking it apart. It might have been slipping the whole time. I don't know.

With everything back together the second time (and the oil in the system adjusted to the total system oil under the assumption that the flush removed all the old oil) it acted as described in the first post: the ice on the low-side line was gone. But the low-side pressure is still poor.

I can accept the possibility that I did something to the compressor when I replaced the seals. This, whatever it might be, would be evidenced by the possible change in the clutch air cap. On the other hand the low pressure behavior has been suspect from the beginning. I simply assumed that the poor pressure behavior was due to the condition of the refrigerant in the system.

True, the new front expansion valve could be faulty. If only the front expansion valve was faulty would that alone account for the poor low-side pressure and bad vent temps on both front and rear?

worf928

I do not know if the vent temps improve. However, the low-side pressure doesn't get low enough with revs higher than 2k.

See above. Now that the clutch gap is set within Porsche's spec, all is quiet. The compressor turns quietly and smoothly without undo resistance by hand. And I don't observe any weird noises from the compressor when it's turned by clutch and motor.

The spec is 260 to 300 CC. I put in about 280 total after the flush.

worf928

Oh - wait.

I did replace the shaft seals on the compressor. I didn't replace the reed valve gaskets. They seem to be the rubber-encased metal kind and looked like they'd be a stone cold bitch to remove without screwing up the faces. The main point was to get to the body seal in the middle.

Hmm....

Tails

Dave,
IIRC from my days as a refrig engineer I would suggest the following:

If there is a restriction in the expansion valve, then the high pressure side would be higher than spec with the low pressure that you have reported.

Possible cause of a blockage:
Foreign matter or ice due to moisture in system.

If there is moisture in the system, ice would form in the expansion valve due the flash off of the R134a liquid to gas as it passes through the expansion valve.

You stated that you pumped down the system to a low vacuum (please give a pressure reading of the vacuum as it should be as low a possible and maintained for a specific period to flash off any water and gas within the system), so at low pressure the water would turn to steam and be drawn out of the system when the system is evaquated.

You state that you replaced the liquid receiver/dryer, however, you did not state whether it was a R134a specific receiver/dryer. If it is a R12 receiver dryer the descant is not compatable with R134a and will break down and enter the system, which can cause blockage at the expansion valves. If the descant breaks down it will release the residual water absorbed by the descant back into the system. This means that you have double trouble in the system again, water and foreign matter.

You should have a set of pressure/ temperature graphs corrected to ambient air temperature for R134a which should show you the correct pressures and temperatures that you should have in the system.

Hope this helps.

Tails 1990 928S4 Auto

worf928

It is a r-134a-compatible receiver/dryer.

Bill51sdr

This would (unfortunately) be a good place to start. Could be the low side valve is leaking.

dprantl

The compressor really is kinda like a mini-engine. Think of those gaskets as head gaskets. If they are leaking, compression numbers will be way down. What that means in a compressor is a huge loss in pumping efficiency.

Also, from your description of the system before, just because you were successful in flushing out the condenser does not mean it is still not partially blocked with crap. Condensers have a multitude of tiny passages that go in parallel (even on the old serpentine 928 condensers). I would not trust one that I knew for a fact had a blockage in the past. Do you have an IR heat gun to measure different parts of the condenser?

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

Mrmerlin

you might consider doing a vacuum test on the console pods you have heated air mixing with the cold

worf928

Tails, see the body of the first post. The system vacuumed to <400 microns in a few hours, and was left vacuuming over night. It held <800 microns vacuum for one hour the next morning. This behavior was the same after both re-assemblies.

Already done. The accessory vacuum system is tight on all HVAC settings. The heater valve closes fully.

Sigh. Yup. I'm sick of removing and replacing this compressor. If at the end of this thread the weight of opinion is the compressor then I may just replace it with a rebuilt N-D.

That's just it though. I don't know for a fact that the condenser was blocked. However, I know that at the Porsche dealer, if they even suspect that a part is clogged they don't even bother to flush; they just replace. Can you imagine a Dealer bill for replacing o-rings, compressor, condenser, and both evaporators? It'd probably be more than $5k.

Yup. And that's a very good idea.

I assume that I should see a nice gradient from hot to cool from top to bottom? With no weird hot or cold spots?


The more I think about it - and from reading the responses - there are three potentials:

- compressor
- _both_ new expansion valves
- low-side line to the compressor is blocked

The compressor is an obvious possibility due to either internal wear or the valve plate seals.

A faulty new expansion valve that's stuck open is possible but seems unlikely. However, I do have another set of new valves. But, check me here: one faulty valve shouldn't cause both front and rear vent temps to be bad? If the rear valve is good I would expect the rear temps to be better than the front? Right?

Now, correct me if I'm wrong but it seems to me that:

A restriction in the low side between the low-side fitting and the expansion valve would result in LOW low-side pressure, not high. And any restriction on the low side might show icing at the restriction point.

A restriction in the low side between the fitting and the compressor WOULD result in high low-side pressure? If this makes sense, then the low-side hose to the compressor must be a suspect. And it wasn't replaced. It looks ok from the outside, but who knows. It might be the original r-12 hose from the factory. If this is true I should see a temperature difference or condensation on the low-side compressor hose?

Further thoughts? (And thanks for all the responses so far.)

Bill51sdr

Since you have equal temps at both evaps, I'd say the expansion valves are probably ok. If you still have debris floating around in your system from a previous compressor frag, all bets are off though.

I'll have to give some thought to your blockage scenarios...

One thing to consider, since your compressor was originally configured for R-12, are the rubber or other sealing components equipped to deal with the 134 pag oil, or whatever oil is in system?

worf928

This is one of the reasons I did the flush. I wanted to do my best to get all the old oil out after the first re-assembly didn't result in functioning A/C. The car was obviously barely converted so there was no telling what oil was put in the system. Although, I have to assume that if they used PAG oil and it wasn't the new-fangled double-endcapped stuff that the system would have suffered the black death.

On the other hand, maybe they did and the system is just beginning to show the signs of the PAG oil screwing things up. (I write this based upon previous research that indicated that the old-style PAG oil should never be used in a system that at one time contained mineral oil and r-12. Could be wrong though.)

Still the old oil looked fine. No particulates etc. There is, now,. ester oil in the system.

Note, that the other car I just last month did the the A/C on, had also been converted to r-134 using the factory-original N-D compressor. That work went by the numbers. Also, Porsche's guidelines for r-134 conversion don't specify a compressor rebuild. So, strictly by the book, replacing the seals and valve plate gaskets shouldn't be necessary. But, who knows what the hacks f'd up on this car before it was dragged to me in pieces on a flatbed. (Yes, long, long story on this car and not just about the A/C.)

The Porsche scorched-earth approach - just replacing all of it - is looking better and better. Blech. I don't want to go there, but I would be really, really pissed-off if after another disassembly and, perhaps, a new compressor didn't result in working A/C. And thus, why I'm looking for collective experience.

Bill51sdr

re: the compressor, can't say I blame you. I'd be pretty chapped if I made that purchase and nothing changed. Hopefully the braintrust here will help you get focused and a definitive problem shows itself.