griffiths

Assuming sea level, water starts to 'boil off' at 80F ambient at 28.92 inHg or 25,400 microns.
You had more than enough vacuum, but the question is how long and what in between.
Water does not suddenly change its state, liquid to gas, and leaves the vehicle's system immediately when pressure drops to 25,400 microns.
It takes time, depending upon the mass of water. Think of an onion losing its skins, shearing off over time.
And, it is very easy if you have a sufficient mass of water to have the liquid start to change state to gas
and suddenly revert to a solid (ice) if you are pulling a deep vacuum quickly.
So part of the process in evacuation is time and the other part is purging.
Purging can be done with refrigerant gas or nitrogen (nitrogen requires more equipment; tank, regulator, adapters).
After pulling a vacuum from xx time down to xxx microns, the system pressure is elevated above ambient pressure (ie, above 0 psi, say 5 psi at least)
and held for xx time to allow any water that changes state from liquid to solid (ice) to revert back to liquid.
This process is repeated, vacuum, purge, vacuum, purge, vacuum, purge, until all the water has peeled itself and is removed.

Now, why go through all of that 'nonsense' procedure above? To ensure you not only removed all the water but all gases (air; nitrogen, argon, carbon dioxide, leftover refrigerant trapped in the refrigerant oil, out-gassed materials that degrade or are released from components; moisture trapped in oil, break down of rubber or polymers and so forth) in the system.

What symptoms would be observed when unexpected water is still in a vacuumed system? Signs of a TEV failure, such as fully closed, partially closed or fully open.
Moisture in the system freezes in the TEV and causes and produces an observation that looks like a failed TEV. The moisture can also 'freeze' or ice inside the liquid line usually
between the drier and the TEV.

Do you have those symptoms? Who knows. But you always want to prevent such symptoms from appearing and sending you down the wrong road
by vacuum and purging.

What 'sounds' odd to me, and since I don't really see a clear documented sequence of observations or data is this, and I'm assuming this question simply from what I'm reading:

1) at 80F ambient there was a low side of 38 psi and high side of 200 psi (whether that was at idle or 2k rpm is not that critical just yet), the system is not overcharged if all the system components
are operating correctly.
2) at 90F ambient, assuming the same weight of refrigerant is in the system, suddenly the system high side pressure is 310 psi? That is quite a jump in pressure for only 10 F increase to the condenser
and system overall. That is a head scratcher. One could assume the TEV aperture was much smaller however without a corresponding low side observation I can't say for sure. Did a condenser fan suddenly stop running? Who knows, no notes to say.
3) at 90F again (assume) with some removal of the refrigerant suddenly things are happy again?

Nah. Something is missing here

Rear Air in the Shark? It's normal to see a 10 F rise in front vent temps when you turn on the rear AC cause your front condenser is designed for one evap; the system needs more condenser function to properly support both coils. Hence, other than chilling a six pack of Becks on the rear vent, which is difficult to balance when taking turns, the rear air in my op-onion is senseless and useless unless you are in cool climates such as in Europe where it was designed (to be politically correct I did capitalize that section of a continent in that other hemisphere).

PAG or Ester or PAG & Ester is not causing your issue. Congealed mineral oil (snotty) from the old R12 could plug things up, but I would not focus on that yet.

merchauser

clearly I am only treading water, and somewhat lost. I have found many videos and posts about general AC vacuuming and charging and now I don't have a clue as to what I should be doing. most likely will start all over, but want to do it as "correctly" as possible. does anyone think I could have an expansion valve issue? if I have to evacuate again, will replace if necessary. can we agree my compressor is probably OK?

not clear on the vacuum/purge/repeat process?

system was vacuumed down for 90 minutes (long enough?) and gauges were reading at 390 microns (new measurement for me). my buddy said that's very good? most internet searches only vacuum for 30 minutes. aside from the lunacy of not knowing exact weight added, especially since we were using a scale (lol...not really), my friend was more intent on the sight glass. clearly overcharged the system. perhaps I will find time to play later today

FredR

Paul,

The whole point of pulling a deep vacuum is to ensure that most traces of anything in the system are sucked out of the volute and especially so the non condensable gases. As the vacuum deepens the boiling point of water lowers and at some point the boiling point of the water equals the ambient temperature and the water flashes off as steam. At your ambient temperatures this happens when the vacuum drops to around 30,000 microns, at 400 microns there will be sod all water molecules left in the system and the ones that are then taken out by the filter drier. Given there is not much water to start with and the depth of vacuum you pulled it is difficult to see how anything of significance would be left in there.

The only caveat I can think of is that some water molecules may bond to the POE but whether that is an issue remains to be seen. Contamination will impact the boiling point but again give the vacuum you pulled thermodynamics tell us that the water could not exist and would turn to steam to be sucked out by the pump. A good pump can pull well lower than the level you went to and the main difference between pumps is the time taken to get there. The main reason that most folks talk about time to leave the pump on is because they do not have a micron gauge [they cost more than the pump]. With a micron gauge you know what you have achieved thus that uncertainty has been removed. Thus my cryptic comment about the "microns" and my puzzlement as to how you knew that measurement- but then I remembered you comment about your friend and presumably he had such a gauge in his kit.

When testing for performance, close the rear unit down and measure the front temperature at 2k rpms. Whatever the low pressure is, look up the saturation temperature for that pressure and add about 5 degrees C to it to derive the logical air temperature coming out of the grill. For instance, if the low pressure side is 3 barg, the temperature in the evaporator should be around zero C and the air coming out of the vent should then be around 5C. Given the front and rear units come to equilibrium at the same pressure, if you see different temperatures for the air emanating from them that might suggest a problem with the expansion valve- as I am aware the two units have different expansion valve part numbers because the duties [flow through them] are different. .

merchauser

why would my friend tell me that 390 was terrific? perhaps there is a decimal movement that I missed? he seemed downright giddy.

fred, not sure if it matters, but the expansion valve for front and rear is same 928 573 123 05. does that change your position on different front and rear values and assessment of expansion valve health?

griffiths

Compressor seems okay.

What I noted quoting 3 different high side readings was the third reading suggests an issue alike a TEV problem. Cant say it is without the corresponding low side reading.

The vacuum procedure I suggested is to avoid moisture.

The level of micron you pulled down to was good.

Recover the refrigerant. Evacuate, charge and document everything. Take your time.

FredR

Paul,

Nothing wrong with a vacuum of 390 microns- the point was most DIY'ers have no real world clue as to what vacuum they are pulling and thus why the debates about how long to have the vac pump on.

I threw the expansion valves into the debate because I have seen both arguments made and although I have had both units in my hand and they looked identical that alone does not make that the case.

Whatever the situation, the pressures entering and leaving the expansion valves are the same and thus the temperature inside the evaporators must be the same. If they are identical items then the duty emanating from them both should be the same all things being equal. Unfortunately I do not know whether the rear fan and the front fan have equal air flows- I would think the main blower is more powerful but...? If the expansion valves are the same and the evaporator surface areas are the same, then for a given the airflow the outlet temperatures should be the same. If the air flows are different the air temperature logically should be different.

andy-gts

It seems to me that Greg at Precision motorwerks already has an additional condenser he adds to the system in the right from wheel well I think that is great, cant wait to get this done

griffiths

The front evaporator is much larger than the rear.
Vehicle interior air temperatures entering the evaporators can vary.
Air flow through the evaporators is different.

merchauser

^^^^thanks all the good info. unfortunately, I was not able to play with the car today.

this might be meaningless but I just checked my static readings and they are about 82 on the low side, and 88 on the high side, with current ambient at 86?? tells me what?

in general shouldn't all auto HVAC systems (R134a) have the same or similar high and low pressures? one internet search said proper was 30/150. another said 40/200.
WSM for GTS cars says around 14/250 for 85 degrees? seems all over the map, and 14 for low side running @2k rpm sounds very low??

j.kenzie@sbcglobal.net

Static pressure tells you that you have some refrigerant in the system, and corresponds to temperature. The pressures that Griff told you are what you should look for at their corresponding temps. 14 is too low unless the freeze switch has been bypassed.
Good luck,
Dave

Speedtoys

The internet likes to make things hard.

The proper fill by weight, is -the- proper fill.

j.kenzie@sbcglobal.net

This is absolutely true; for a stock system a stock charge. However, when converting to a different refrigerant, there is some variation. Not much though.
The pressure readings are also there to tell you if you have a different problem besides too much or too little refrigerant.
Good luck,
Dave

Speedtoys

Agreed, but the calculations will be within an ounce, and..wont affect pressures/temps. Yes, from there..observations tell a health story.

I will read from the Jenkins bible.. http://jenniskens.livedsl.nl/Technic.../MyTip1069.htm

Will suck at this point to find not replacing the $25 front expansion valve is blocked or borked.

dr bob

Paul --

Some experience with a converted car. Mine's an '89 but should be really close.

-- The factory-recommended charge of 85-90% of R12 by weight was a just a little light, based on observed sight-glass readings and temperatures. The final charge level was maybe an ounce above the calculated ideal.

-- The vacuum you pulled is excellent.

-- When I did mine I spent a bit of time making sure that there's no fugitive air in the charging manifold or hoses. I connected the R-134a bottle to the safety switch port, and left the vacuum pump on the yellow hose. I had to buy an extra hose. Without doing that, you want to push liquid refrigerant to the ports on the car and vent that so the lines have no air in them.

-- I jumper the freeze switch during maintenance efforts, and in desert-dry Los Angeles I usually just left it jumpered. Extended driving would freeze the evaporator so not at all a good idea if you live in a non-desert (>5% RH) climate. During maintenance and testing, I need to see the pressures consistently, not letting the compressor cycle on low evaporator temperature.

-- The rear system is shut off during charging and performance testing.

-- The gauges on your charging manifold have temperatures on the dials that correspond to the refrigerant temp at that pressure. Your goal is to have an evaporator temp/pressure that correspond to about 25ºF with engine at 1500+ RPM. I set a throttle-stop (handy tool...) at 2500 with cabin fan at position 2 for test readings. With a good charge, the fans will be running at close to full speed. I like to slightly under-charge looking at suction press/temp, and shoot for about 20 PSI suction side so long as ambient temp is low enough to maintain liquid in the sight glass at that pressure.

-- Vacuum system condition will affect what your gauges tell you. If you see heat and pressure balance readings that might otherwise tell you that something is plugged, check your flaps and mixing motor. Particularly, high condenser press with low evaporator pressure might tell you that an expansion valve is blocked or the condenser fan's aren't running. You can see if the fan is running, and whether there's frost on the expansion valve -- evidence that there is in fact condenser airflow or refrigerant flowing, respectively.

---

The refrigerant side of the system is absolutely basic simple and the same as other cars. The flaps and controls are, um, a Behr feature that's not quite exclusive to our cars, but it's darn close.

Speedtoys

When I hear my kid spending hours attempting to re-flash Panamera instrument clusters to make -a- single false ABS sensor light go away...

I sorta like the simple stone knives and bear skins engineering in the 928.

Such as..she both GETS and enjoys sunroof restoration for one..