captainOCD

I was thinking the same thing at first until it clicked to me that this is only looking at the high side of the system, not the low.

Speedtoys

Ohhh..ya, fancy that.

captainOCD

Trust me, I had one of these moments.

dr bob

Your 'high-side pressures are OK until the engine warms up' symptom tells me that your coolant is heating your evaporator. Fix that.

Speedtoys

Weak clutch on the fan?

dr bob

No. Consider that there is no design condition where engine heat makes a difference in AC evaporator temperature. On cold start, the fan clutch is not spinning the fan very hard, since there isn't heat through the radiator until the engine coolant temp approaches normal operating temp and the thermostat opens. Prior to that, the only heat passing back to the thermostatic fan clutch is ambient plus the heat being shed by the AC condenser. In that colder state, AC heat is likely warming the radiator and coolant inside, so even less heat gets to the fan clutch. Meanwhile, prior to thermostat open, coolant circulates through the engine by bypassing the radiator, and a slipstream of that engine coolant is trying to flow through the heater core. The heater control valve is fully closed and seals perfectly of course, so no heat gets into the HVAC airbox, from which said heat bleeds into the evaporator and jacks the high-side temperatures/pressures. That added heat and the increased compressor outlet pressure indirectly cause the low-side pressure to go up some, so the evaporation temp goes up too.

Stop adding engine heat to the evaporator airbox. That way the AC compressor won't have to pump it to the condenser and try and get rid of it.

----

One of the items on the WSM "test conditions" list is that the vacuum system must be working correctly. It only stands to reason that this list item includes the devices that the vacuum actuators actually actuate. Add to that list the blend door that's operated by the blend door servomotor and the switches on it that control some of the vacuum actuators directly or indirectly. EVERYTHING IN THE SYSTEM MUST BE WORKING CORRECTLY BEFORE THE REFRIGERATION SIDE WILL BALANCE CORRECTLY. It's pretty easy to look at system pressures and see if they line up with the design numbers that Porsche so graciously shares with us in the manuals.

captainOCD

I'll test to see what the heater valve is doing again, but last fall when I replaced it all tested well. I'm getting vent temperatures identical to the temp of the evaporator itself too.

I'll measure the temp of the heater hose before and after the valve and see what it looks like. I can also try pinching it off too.

And interestingly enough, the clutch fan was pulling slightly more air when the motor was first started and cold compared to when warmed up. Maybe due to idle speed? It certainly wasn't pulling significantly more when warmed up. The engine temp was staying pretty stable at the lower white mark.

dr bob

There is definitely something adding heat to the evaporator. The refrigerant is evaporating at about 4ºC, indicated on the low-side gauge on the R134a temperature scale. Since you are seeing 70ºF as temp outside the evaporator, something is adding all that heat. Throw a thermometer in the passenger's footwell to see how much the cabin air temperature is dropping as it passes through the evaporator, as that will give us a better sense of the temp change air side of the evaporator. You should, by rule of thumb, see about 40ºF drop from passenger floor to center vent temp under WSM test conditions with the doors and windows closed.

Your reported high center vent temp and observed high hi-side pressure say you are adding extra heat to the evaporator. You can prove this by unplugging the compressor clutch wire, but leaving everything else the same. How's that center vent temp compared with passenger footwell temp? If it's higher, you have confirmed that you are adding heat to the evaporator from somewhere. Typical is the heater control valve, but it can be a combination of things that starts with a vacuum leak anywhere in the HVAC system. With limited vacuum available, the heater valve doesn't go all the way closed. The fresh air door doesn't go all the way closed. The footwell heater door isn't closed, nor is the defroster vent. With the "gets warmer as the engine warms up" symptom, the arrow points to a heater valve that isn't sealing. That can be caused by several things other than the valve itself, particularly a leak in ANY of the HVAC vacuum actuators. Get your MitiVac (or equivalent) out and get to work testing the vacuum side.

captainOCD

I just pulled the vacuum manifold out of the console and tested each line. All are holding vacuum and I can hear all of them working in the dash (apart from the heater valve). I'm not going to get time to get the car out and warmed up tonight so I'll have to check the temps tomorrow hopefully. I should add that the evaporator isn't getting more than a couple degrees warmer as the engine warms up. Even with the engine stone cold yesterday morning I was only getting 60 from the vents (was also the temp of the surface of the evaporator) and it was maybe 65 when warmed up.

Thanks again for the input, I genuinely am appreciating the help.

dr bob

check to see if the --whole system-- holds vacuum. Use the MitiVac at the line to that manifold. Engine running and AC engaged. You should be able to draw and hold vacuum.

While that's interesting do the temperature tests. Heat is getting in there from the engine coolant, based on indications and symptoms.

captainOCD

So take the manifold out again and apply vacuum from the mityvac to the solenoid manifold with everything running instead of using the vacuum supply from the engine?

dr bob

Go to the HVAC vacuum supply line that connects to the cross-connector by the brake booster. Disconnect it there, and use your MitiVac (or equivalent) there to test that the whole system holds vacuum from that point.. No console surgery needed for this basic test. Cycle the controls through every mode if you want to be thorough, but note that the AC position in the control head applies vacuum to the critical-for-AC actuators. If the system doesn't hold vacuum then you get to go find out which one has failed. WSM 87-49 has a picture of the test connection and the MitiVac tool connected. That section has a diagram of flap and actuator operation at each slider position. That's for an early car but the theory is consistent among all years.

captainOCD

I pulled the line off where the check valve is and tested both with and without the check valve and it held vacuum. I also turned the key on with the mityvac supplying the vacuum and was able to move the hvac controls and hear everything moving around in the dash and see what appears to be the comb flaps moving around as well.

When I started up the car for the first time today and turned on the ac the high side pressure climbed up pretty quickly to 250 and then rose up to 300 pretty quickly, within 1 or 2 minutes, well before the engine had time to warm up. Before the engine warmed up completely I shut everything off and swapped out the condenser fan with the new one. I turned on the ac with the fan disconnected and it went up to around 350psi on the high side within a couple minutes. I plugged in the fan and it came back down to around 315-325psi. After letting the car run for at least 45 minutes the high side pressure did slowly creep up to 350psi again, but it never went above that (it got as high as 400psi this weekend).

When first started up and even after the engine warmed up completely the vent temp was right at or below 60f. It was 95f outside, I forgot to measure in the passenger's side footwell. Neither of the heater hoses felt hot or even warm once the engine was fully warmed up.

Now this might be me completely mistaken here, but from looking at the hvac diagram in the wsm it appears that the evaporator sits ahead of the heater core and you can just see the top of the heater core sitting in front of the evaporator through the center vent. I stuck a temperature probe on this and shut off the car and immediately turned the key back on (but motor off) so that the heater valve would hopefully stay closed. The temp of what I believe was the heater core was initially that of the vent temp (60f) and very slowly over the course of 5 minutes or so came back up to about 85 (less than ambient). So if I'm not completely wrong here, it seems like the heater core isn't getting hot water to it.


Tomorrow my high/low shutoff switch is going to arrive, so hopefully either then or this weekend I can evacuate the system, install that switch, then vacuum for longer and recharge the system make sure to fully purge the charge lines.

captainOCD

Yesterday evening I changed the pressure sender out and vacuumed down the system. I let the vacuum pump run from then until this morning (about 12 hours). I charged it up making sure that I was getting all of the air out of the charge hose. I was aiming for 80% of the 37oz r12 capacity at 29oz, but got distracted for a moment and wound up at 33oz which is 90%. This may be a bit lower from purging.

It was only 80f this morning when I charged it vs 95f or 100f last weekend, so keep that in mind. At idle it was sitting at 30psi low and 275psi high. At around 2 grand it was 15-20psi low and right at 300psi high. The sight glass looks perfectly clear, no bubbles that I could see at all (there were last weekend since it wasn't fully charged).

When sitting in the driveway idling it was blowing 39f at the vents. I took it on a 10 min or so drive around the neighborhood at about 50mph. After driving around I was down to 25f at the center vent. I stopped for gas and the heater core getting coolant in it combined with moving slower (25mph) back in the residential area brought the vent temps back up to 39f or so again.

So seemingly vacuuming it for 12 hours vs 1 hour and purging the lines better made a big difference. That and it's 20f cooler outside today. I do believe the new condenser fan helps a lot when sitting still too. With the old fan I was seeing only about a 40f temp drop across the condenser. With the new fan I'm seeing a 75f temp drop across it when sitting still.

Some info about that switch. It does say the low pressure shutoff on it is 2 bar. It took about 3.5-4bar before it let the compressor come on (probably off at 2 and back on about there). Hopefully I don't have to test the 28bar/406psi shutoff.

It looks like I'll have to wait another week for the weather to get up to 100f again, but for now I have cold air.

j.kenzie@sbcglobal.net

Congrats Matt. Sounds like a significant improvement. It will get worse at 100 degrees, but still better than before. Great job.
Dave