Kevin in Atlanta

For grins and giggles went back to my shop had them recover the 134a and pull vacuum again. Recovered 1.55 pounds of the 1.6 pounds they put in last Friday. After pulling vacuum we put in 2 pounds. High side is 250 and low side was 80 - both of these are too high. Air temp at the front and rear vents are 70ish. Small improvement, but those pressures are wacked.

Luckily for me the temps in ATL have been cooler than years past.

I think it is time to find a dedicated A/C shop near me. :-)

alabbasi

There was an A/C shop near where I lived in Smyrna on Concord Rd. They seemed to have a good reputation. I've not been back for a couple of years, but I believe that if you're heading from S Atlanta RD towards the covered bridge area, it would be on your left.

Kevin in Atlanta

I'll drop them a line tomorrow.

Bill Ball

Having both the high and low side pressure too high with ineffective cooling both front and rear is confusing to me (not the first time). The most direct explanation would be overcharging, which doesn't appear to be the case.

The basics: High pressure on the suction (low) side would be consistent with a failing compressor, but then the high side pressure readings should be low. High pressure on the high side (usually with very low suction side pressures) indicates a blocked expansion valve - in your case, it would be both valves, since front and rear cooling is deficient. Ineffective cooling of the condenser raises high side pressures. High ambient temperature raises both low and high side pressures. You'd have to have a bad compressor and both bad expansion valves and perhaps failing fans on the condenser to explain your results. Also, if the measurements were made at idle rather than 1500 RPM, that would raise the low side pressure reading quite a bit. I can't quite put it all together. There must be another explanation that my layman's understanding of AC operation is failing to see. Dan's good at this. He'll probably sort through it easily enough.

dr bob

Bill, you are on the correct path.

The most common cause of pressures like that is a failed condenser fan or airflow obstruction that limits the amount of heat carried away by the condenser. If the fan is working and you can see flow in the sight glass, my next concern would be air in the system.

The combination of excessive high- and low-side pressures tells us that the compressor is compressing, and that the expansion valve(s) are passing refrigerant to the evaporators OK.

ammonman

As I said in an earlier post, you either have air or other non-condensable vapor (water) in the system or there is excess oil in the system. I also think your expansion valves are bad.

280psig discharge pressure at 66% charge is not correct. Something is causing discharge pressure to be high at less than full charge. If airflow through the condenser coil is correct (fans running full speed, coil clean, no obstructions) then something else is causing a lack of condenser capacity. The two most likely causes are non-condensables in the system or excess oil built up in the condenser. lf the system is leak-free and has been properly evacuated (as verified with a micron vacuum gauge to ensure ALL moisture and air has been vaporized and removed) then something besides air is causing a reduction in condenser capacity. Excess oil in the system will reduce condenser capacity.

I suspect your high suction pressure is due to failed thermostatic expansion valves. Either they are stuck open or aren't responding to temperature changes. As suction temp drops they should start closing, then modulate liquid refrigerant flow in response to suction line temp. They seem stuck wide open allowing excess flow of refrigerant into the evaporator resulting in high suction pressure.

Evacuate the refrigerant, flush the condenser and evaporator with solvent to remove exces oil, install new expansion valves and a new drier, then evacuate and re-charge.

Kevin in Atlanta

Daily update. So I decided to see what is going on today. Started the car that had been sitting in the garage. Now the front vents are warm and the rear is blowing 62 dg. Felt the inlet of expansion valve at the firewall and it is too hot to hold for any period of time. While there was initial sweating on the outlet it was not that cold. I am beginning to think I need to replace the front expansion valve and while the system is evacuated I'll replace the dryer/receiver and clean/flush the evaporator and condenser. I have used brake clean in the past to clean the condenser, but how do I flush the evaporator core? I have heard folks also use mineral spirits in lieu of brake clean.

Any help is appreciated.

dprantl

I always use regular mineral spirits to flush the condenser, evaporator(s) and hard lines with great success; never used brake clean for this purpose before. You can get a flush gun that connects to an air compressor and has a convenient conical rubber attachment.

It does sound like your expansion valve is stuck open. I would also be wary of the A/C shop you are taking the car to. They seemed to not know the refrigerant capacity of the 928 (2lb is still not enough for a 928 with rear A/C), so that leaves me wondering what else they don't know. Also, if the system has been open to air for more than a day or so, the drier should always be replaced, otherwise it becomes completely ineffective in removing moisture. If you are going to flush, don't forget to drain all the old oil out of the compressor. You will have to remove it, turn it upside down and rotate the center portion of the clutch repeatedly to get it all out.

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

ammonman

Go to your local home store and get lengths of different diameter vinyl hose. Once the refrigerant is removed disconnect the lines from the condenser and evaporator. Slip the appropriate size hose over the pipe end and secure with a hose clamp (don't crush the pipe ends with the clamps.) The hoses allow the flush of your choice to be introduced and blown out without getting all over the car. Make sure your work area is well ventilated and there aren't any ignition sources as you'll be creating solvent vapors. I have used the chlorine free brake cleaner with good results.

Mike

Bill Ball

OK, this sounds like a good thread for some general AC education from Dan, Mike and Dr. Bob. I haven't done a conversion as the "flushing" process has always been unclear to me.

How much solvent (mineral spirits or brake cleaner) do you pump through the evaporator and condenser? Do you fill them until you get flow out the exit tubes and then some more? How do you pump mineral spirits? I would assume with brake cleaner you use aerosol cans. Do you just fill and blow out with an air compressor? What about the AC lines and hoses?

TIA

dprantl

Well, you can never flush too much. I usually use ~1 gallon or so of mineral spirits per component. If it comes out really clean on the other side, I may reuse it. Some components require more than others, i.e. I would use more for a parallel flow condenser/evaporator because there are multiple paths for the solvent to flow through. The later 928 evaporator is a multi-pass type. The hard lines themselves need very little. Sometimes I don't do the lines at all since very little oil rests in them. I never do hoses as there is always some possibility of the solvent reacting with the hose materials. I use a flush gun that I linked to in an earlier post. You fill the bottle with solvent, connect one side to an air compressor and the other side to the component, then flush away. All the solvent will end up coming out the other end, so be sure to have a bucket there to catch it. I then continue to push air through the component for a minute or so to make sure it's all out.

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

Kevin in Atlanta

It's been a year and I have not moved forward on this. But, I resurrecting this thread ask a question. The 91 GT I am parting out has what appears to be a Nippondenso A/C compressor, Griffiths aftermarket condenser coil and newer hoses. I don't think that it has the Kuehl compressor. See pictures.

Can I transplant this into my 87? And is it worth it? The 91 has always been 134a so the barrier hoses could e better.

alabbasi

I'm not sure why you would switch anything over. My 88 S4 running R12 will freeze me better then any other car that I own.

jcorenman

A '91 would have been R12. The change to 134a happened in model-year 93 according to the tech-spec book, but our (early) 93 is R12.

This ^^^^

Kevin in Atlanta

The S4 I own already has been converted. I am wondering if the nippondenso is a better unit and if so what will it take to swap it in. I have already decided to use the after market condenser. The markings on the A/C hoses do not appear to be Porsche, so I am guessing they were changed when the R12 was swapped to 134a.