Summer in Peru - need A/C troubleshooting help on my 996 cabriolet

[jmj996]

Hello all, looking for your help diagnosing A/C issues. It's summertime in Peru and temps are already in the 80's in Lima, so I really need my A/C on my 1999 996 cabriolet.  It hasn't worked properly for about three years.  It does make the air somewhat cooler when I turn it on, but nowhere near cold. 

 

The strange thing is that it's not necessarily low on R134.  The pressure gauges show that the low and high sides are both around 70psi when the car is off.  When the car is on and A/C is turned on, the low side drops to 30psi, but the high side never increases - it doesn't move at all.  When the car is running and the A/C compressor is switched off, the pressure jumps up to 55psi.  Then, after turning off the engine, it's back to 70-75psi.  According to the Porsche manual, the low-side pressure should between 20-24 psi (regardless of outdoor air temp), and when the car is running at 26 degrees celsius outdoor temps, (it's cooler this evening), the high-pressure side should be between 10-17 bar, or 145psi to 246psi.

 

The compressor is definitely running, and the radiator condenser fans are running.  The only other symptoms are:

1) I hear a slight hissing noise in the dash area when I turn on the A/C.  It starts out as a high-pitched hiss, and slowly goes away within about ten seconds.

2) I had the usual foam degradation on the air diverter doors and haven't really done anything about it - but from what I understand, it wouldn't have this serious of an impact, and from what I've read, the foam degradation usually affects the ability to heat the car.  I can't be 100% sure that the flaps are actually moving when they're supposed to, but I can confirm that when I select the recirculation button, it works correctly because I no longer.  Regardless, the main problem is evident in the fact that the high-side pressure doesn't move.

 

The last time I recharged the A/C (just a little low), in the summer of 2020 (in the northern hemisphere), it worked fine, then we moved to Peru and it continued working through the Peruvian summer from January through May of 2021.  It has not worked properly since.

 

I can only think of a few reasons that the high-side pressure won't increase:

1) The A/C compressor isn't coming on, or isn't working.  I know it's coming on - I can see it spinning, and the idle definitely changes exactly as expected when turning it on or off.

2) The expansion valve is (badly clogged). I'm not sure about this one, but the fact that it's hissing a little bit when the A/C is turned on, and the fact that the high-side pressure never changes, leads me to believe that this is possible.

 

Any ideas?  Has anyone seen this before?  Thanks in advance!

 

 

[jmj996]

Let's try a few simpler questions:

1) If you've ever had a clogged A/C system, what was the culprit?  I've seen an example on YouTube of somebody's A/C lines underneath the car getting crimped nearly shut, but haven't found anything else.  I've read anecdotal info about how leak sealer can cause a clog, but haven't seen any first-person explanations of something that was clogged because of it.

2) Have you ever needed to have an A/C expansion valve replaced?  If so, what were the symptoms that led to its replacement?  Obviously there a lot of these that need to be replaced, because they're available (in stock for this MY) on all of the major parts sites.

3) Have you ever had to replace anything in the A/C system (besides the compressor) due to something other than leaking or physical external damage?

[JFP in PA]

Typically, when an expansion valve fails, the high side pressure goes way too high and kills the compressor.

[jmj996]

Well, I decided to take a shot and simply add more refrigerant, and it worked.  So much for a methodical approach using gauges and Porsche A/C system guidance.  Porsche's charts indicating that it should have a low-side pressure of 20-24psi were just far too low for my car.  I was continuing my research this weekend and found a post from a mechanic saying that he usually charges the low side up to 40psi (referring specifically the 996 and 986), so I decided to add enough refrigerant to bring the pressure up from 30psi to 40psi, and surprisingly, it took an entire 940gram to do it, which is the specified amount of refrigerant required to fill an empty system.  Now it's blowing very cold!  And, the high-side pressure is around 240, which is within range according to the chart.

 

So, lesson learned, 30psi is practically nothing in these cars, and the Porsche chart for low and high-side pressure is way too low.  30psi was enough pressure for the compressor to kick and stay on, but it was not indicative of having enough refrigerant to cool the evaporator.

 

I suppose it's possible that the low-side gauge on my A/C hoses is not correct, but it's a nice set from Harbor Freight, so I doubt those are the problem.

Edited January 13 by jmj996