Spyerx

(posted on pelican too but expect less AC drama here - hopefully more facts)

Had car looked at today, at a shop that deals with R12, a generalist AC shop, not a specialist in the porsche system, my system functions, it's "cold as I would expect given the way I understand how poorly these systems perform" - meaning its pretty cool just not cold. On a 90 degree moderate humidity day its tolerable.

But this is what the shop found - Car is 89 all factory AC with R12:
--Evacuated R12, pulled about 1.5 lbs out (obviously low, but never probably serviced)
--Maintained vacuum and detected no leaks
--Charged up to spec - high side pressures were as high as 300 lbs @ 90+ ambient temps - front fan working
--Sprayed water on condensers to see if any adjustment happens but pressures stay high
--No change in pressures as they dropped R12 in system
--Adjusted R12 to what was in system before they refilled (back to previous state)

The shop is a general AC shop and not specialized in the porsche system - my original request was to just simply top up the coolant - but the high pressures had them concerned.

So, questions:
--Which components am I looking at here? Compressor issue?

Thanks

griffiths

To diagnose you need the following data:

1) Total R12 put in the system. Normally on an 89 about 47 oz R12.
2) You noted 300 psi. What is needed is the outside air temp around the car.
If the system had no issues, everything was running perfectly, 300 psi with 47 oz, at 'idle', would suggest the ambient temp around the car was 125F. So, as you can see logically there is an issue.. i 'll get to in a moment.
3) You need the low side pressure reported. This tells us whether there is an issue with the expansion valve (TEV, TXV), and relates to the evaporator core temperature.
4) Vent temp relates to evap core temp.
5) A simple observation of the evaporator outlet pipe at the evap tells you some things, normally it will be sweating with condensation and the the inlet connection to the TEV will be warm to hot.
6) You could attempt to observe the site glass on the drier however observations of temperatures and pressures tells you more.

In case they did do all things correctly, they would have the engine deck lid laying down, front condenser fan is working, windows up, fresh air and heater controls closed, thermostat set fully CW to max and evap fan speed on 2 or 3.

Assuming the expansion valve is not showing any issues, I'm going to suggest you have air in the system from an inadequate evacuation process. Most repair techs assume they just have have to pull a vacuum and things are good.
However the 911 and 930 have a very long or say large system (40 feet of hose, 2 condensers and the evap and drier). To do evacuate correctly you need to pull a few vacuums and purge the system with refrigerant gas.

Again, I'm sure you and your tech insured the normal check list (front cond fan working, deck lid down).

When they charge it, ask them to use 'virgin' refrigerant, not recycled or reclaimed. And, you will want a new receiver drier if you can.

Feel free to PM if you need further assistance.

And, good luck "posted on pelican too but expect less AC drama here - hopefully more facts" because you know what will happen to your thread.

wwest

As you see from Kuehl's post, more questions than answers.

But for instance.. Debris, or anything, blocking/restricting the flow of air through the evaporator CORE will have this effect, normal high/low pressures (yes, 300 PSI can be considered "normal")

John D.

You just quite can't help yourself can you?

As the OP stated - let's keep this a "drama free AC thread".

John D.

griffiths

Sprex,

Here are the clues:
1) You noted your system "worked" although typical of a stock 911, not cold enough with the stock system.
2) They pulled out an estimated 1.5 lbs or 24 oz, however I never count on their machines accuracy, but either way far from 47 oz.
3) Since we don't know the low side pressure at that time, but we can guess you felt the outside air temp was 90ish?, with properly operating system on a 90 degree day with R12 your high side bogey would be 216psi.
4) When they put in 47 oz (we assume) the high went up to 300psi, that means ... typically a TEV issue, not a sudden mechanical failure, but rather ice forming inside the TEV which raises the pressure do to the reduced aperture opening in the TEV.

You can pull a vacuum on a system down to 30 inHg (at sea level for example) and not get all the ambient gases and moisture out of the system and the moisture will freeze inside!

Many times the technician will blame the TEV and replace it, pull a vacuum and charge and everything is fine. The tech blames the TEV since that is what he replaced, however the TEV is stil OK. It was the procedure beforehand that was the issue.

I suggest this scenario to you because we see it happen all to often with a 911 or 930.

Sorry to see your thread get hit with drama, however there are quite a few 911/930 owners in this forum who have many years of practical experience; hopefully they will chime in and offer suggestions.

Regards,
Griff
Kuehl

wwest

No adversity intended, NONE!

CG: "To diagnose you need the following data: "

Or to paraphrase:

In order to be of real help "we" need more data:

Simply agreeing with CG that lots of questions need answering, answers.

Spyerx

Thanks @griff. Very helpful. Being a (former) engineer (now 'leadership' lol) these systems themselves are not terribly complex, but now I'm understanding how the parts work together.

So: Drier, possibly TEV, a few vacuum cycles + purge, refill and test... sounds straight forward.

Amber Gramps

Could there also be debris in the system?

My commercial fridge has a filter that has to be replaced every few years. Is there a bottleneck that can trap impurities coming out of the compressor?

Spyerx

My condensers are clean. Checked that.

Just ran in garage - over 90 in here ambient. Verified condenser fan is operational. At idle with deck closed fan med etc, 55 degree vent temp but no ball movement in the drier. It's low on freon for sure. So it is working, but need to chase those high pressures.

Spyerx

Brought car to another shop tech was very thorough and inspected whole system. Full Evac, etc. Charged to max cool as high side started to climb which was a little short of full spec. he said at idle low pressure spot on, high are a bit higher than he's like to see. Suspected drier or valve as described here. Was not concerned as when car is moving the high drops. I don't know exact # but not over 300.

This is at 70mph at 85-90 outside temp.

This is manageable. Definitely cooler. Will address components with some system upgrades at a later date.

wwest

Easier to read.

wwest

"low side pressure as expected.."

Sounds as if the TXV is doing exactly what it should.

The TXV is "throttling", restricting, the flow to attain the correct evaporator temperature.
But the compressor just keeps on pumping, and pumping, and..

Turn the thermostatic control CCW just enough that the compressor cycles on and off.

It appears that the thermostatic switch at full CW is "asking" for a lower evaporator
temperature than the TXV will "allow".

wwest

What was the max cool the tech attained?

Driving vent temperature?

And what was the "as expected" low side pressure?

griffiths

"This is at 70mph at 85-90 outside temp (strong sun)".
For a stock R12 system, 42F is pretty good; you will
have losses between the evap and the vent, just like
losses between the flywheel and tires.

You indicated you had 30 inHg on the low at idle.
So that tells you that you have a cold evap core.
You can't get much more heat transfer with the
stock evap so the thermostat will keep asking for
more. And, I'd suspect you still have some air
in the system (email to chat about that).

If you are happy with that result, I'd suggest
just a set of barrier hoses, orings, binary pressure switch,
drier, upgrade the evap, toss in a Kuehl vent, maybe
open up the compressor while the system is empty
and inspect the bores, pistons and wobble plates,
if all is well toss in a new seal and oring set.

If you want colder, feel free to drop an email.

wwest

Loss of OEM evaporator efficiency, TXV "satisfied", thermostatic switch (possibly) not.