Fluctuating Vent Temps Issue
 

Will probably hit the local Porsche dealer to get this issue resolved but I have a pretty frustrating situation at the moment and am a bit stuck.

I've got an 89 Carrera with stock a/c except for an R134 conversion. The a/c worked about as well as could be expected the past few years but required the occasional recharge. I purchased a manifold, read up on the process, and got pretty good at knocking out a recharge in a half hour or so.

Over the winter, the fittings on the compressor loosened to where I didn't feel comfortable just recharging. So, after reading up (again), I purchased a vac pump and hit the system this past weekend. On the first drive, I was getting 44 degree vent temps (75 ambient). When I got in the car again (10 minutes after arriving at my destination) the best I could get was 58 degrees, except for the last five minutes of the drive where it dropped to 44 again. This was all suburbia driving.

Drove it yesterday morning, 20 minutes each way; almost all highway. 58 degree vent temps except for when I jumped off the highway, where it dropped to 47. 60 degrees the whole way home.

I realized that I probably didn't charge the system completely so I verified and, sure enough, it was low. Added the rest of the recommended charge and took it for a drive last night; highway and suburbia. 60 degree vent temps the whole time.

Any ideas what the heck could be causing these issues? I've been verifying the compressor's running during each drive. I can hear it kick on and off, plus the vent temps climb back up when I kick it off. I know I have the right amount of R134 in it. The only thing I can think of is I only put one 'charge' of oil in when refilling.

Also, with regard to the vacuum process, I'm wondering whether it was successful. I started the pump per the instructions, then hooked it up to my manifold. Low pressure side said I had a vacuum going. I opened up both high and low sides and let the thing run for two hours. What has me puzzled is that I've heard about all the 'stuff' that vacuuming should get out of your system. That, and you need to change the pump oil each time. I didn't see any residue on the pump exhaust and the oil looks as clean as it did when it came out of the bottle.

Again, any guidance would be appreciated.

1) Most likely: The system was not properly evacuated, you still have residual
air or moisture, however that is not your major problem. You don't "have to" change your pump oil every time you use the vacuum pump, it all depends upon whether the oil became contaminated, if it lost viscosity, or its moisture bound, so in some cases pump mfg's will advise to simply replace the pump oil every time if they sense you can't see these issues.

2) The simple conversion process requires you to replace your drier, put on the charge port adapters, a conversion sticker and add ester oil. A better conversion includes replacing all the o-rings to insure you don't have any damaged ones, and add a low high pressure switch. Even better is to replace the stock hoses with barrier hoses. And, further you may need to liquid flush the system if there are contaminants. All this assumes you have a good compressor and everything is working properly. But for the minute you can play with what you have until you understand why its better to do the job right once.

3) Its normal for vent temps to rise when the compressor turns off, because the evaporator core is not getting a flow of refrigerant and the heat in the cockpit is raising the evaporator core temperature.

4) 58-60 vent temps (if you are using an accurate digital probe) at 75 F ambient outside air temp (again if you have an accurate method of noting that).... is not good. So you have a situation where it could be undercharged and or not having a good evacuation. Question is what was the low and high side pressures at idle when you charged it, with what was the ambient temperature outside (assuming you took the pressure measurements with the deck lid down resting on the hoses, your doors are closed and widows up and your front condenser blower motor ... near the battery, was blowing at the time), how much and what kind of refrigerant oil did you put in and how much refrigerant (in ounces) do you guess you put in?

Feel free to write:
sales@griffiths.com

So it sounds as though I need to try the process again?

I used a Robinair 2 stage 3CFM pump and let it run for two hours. Should I have let it run longer to insure no air or contamination was left in the system?

With regard to what's in the system now, I've got around 45oz of R134 and a 2 oz 'charge' can of oil. Can't recall the type and brand off the top of my head (I'm posting from the office).

I don't recall the exact pressures I was getting but they were within the 'norms' on the R134 chart (taking ambient into consideration). High was in the 250 neighborhood and low was around 30 as I recall. Vent temps were taken with a digital probe thermometer as you described except the windows were open in the car.

Also, the conversion was professionally done with as you described, including a new dryer.

Understanding that what's circulating in the system should be gas, it almost seems that the fluctuations (at least the first two times) were caused by 'sloshing' of material around the system. It seemed as though, as soon as I hit the brakes coming off the highway yesterday, the vent temps dropped.

Provided the front condenser motor is operating, deck lid was down, at idle,
250 psi high @ 75 ambient .. your overcharged or have air in the system.

Oh, probably also worth noting that I've strongly considered upgrading the system in the car; I absolutely see the value in doing so, particularly here in Richmond, VA. However, I'm probably going to sell the car later this year, once I find an appropriate 964 or 993 so not much point in dropping the money at this point.

I'm honestly not sure if the front motor was operating but will check it this afternoon. The ambient during the charge was probably a bit higher than 75 as I was in the garage (but did have an industrial grade air mover blowing on the front condenser).

Even if the front motor wasn't working, would that matter at highway speeds, assuming the system was properly charged?

So, it's sounding as though you'd recommend evacuating the system again (letting the pump run 4 hours? 6 hours? Recharging and adding more oil. Is the pressurized oil okay? Should it go in before, after, or between shots of R134? I just want to make sure I have all the nuances right. Also, high and low side valves open during evac?

Thanks again for all the advice!

Well, then clean up your AC equipment and post it here in the Pelican for sale forum
Miscellaneous Used Parts For Sale & Wanted - Pelican Parts Technical BBS

If I were going to keep the car, I'd be on the phone with you, having you send a new setup. But, again, I want to move to a 964 or 993. Not much point in dropping $$$$ to upgrade a car I'm going to sell in a few months. I just want to get the stock system sorted best I can both for myself and the next owner.

That is a NO NO! Falsifies the real world.
Real world is you come to a traffic light and sit there for 3 minutes, there is not industrial fan.

Yes. The front fan design pulls air in from behind the bumper and blows it down
over the condenser. Driving down the road does not have the same effect.

At this juncture, since you are selling the car it does not make sense to invest more time and money unless you can get it right; without getting into all the nuances of procedures, P&T relationships, how to use equipment, I will say simply do the best you can one more time. Since you are selling the car any savvy potential owner knows they will have to address the AC. And, be a "good guy" ... tell them everything you have done up to this point.

Taking your advice, I'll give it another shot this afternoon. If that doesn't yield the results that it should, I'll take it to a qualified shop. The car is a no issue vehicle and I want to be able to sell it as such, when the time comes in a few months.

Thanks again!