Having an issue that we never heard of. The car is a 65 with an automatic transmission and air-conditioning when the air-conditioning is turned on its pulling refrigerant from the air conditioning system directly into the engine burning it off. I am assuming that the STV valve has an issue because that’s the only place where vacuum is connected, the valve was replaced with another, and it’s still doing the same thing? I believe the vacuum is on that valve at all times, why only when A/C is on????

 

Having a hard time remembering a vacuum connection on the STV. There is one on the hot water valve from the box inside. Getting ref. out of the STV would indicate a bad diaphragm and incorrect plumbing from my memory. There is a splitter in the underhood vacuum line. Can’t recall where both terminate. One is the heater HW valve. Sure sounds odd if hooked up correctly. Especially for two units.

 

Check the powerglide vacuum modulator valve, I remember (?) that it can do that.

 

only does the a/c. Pg disconnected.

 

Not likely.
The vacuum head on the STV used by Chevrolet is not connected to the refrigerant side of the system, it merely actuates a lever inside the valve. The valve can leak, but the engine will not ingest refrigerant from an STV valve faulty or otherwise.

Now the vacuum modulator on the rear of your Powerglide automatic has a diaphragm that can rupture. When this happens the engine will ingest transmission fluid and the car will smoke as if off to the crusher. The modulator is cheap and a 10 minute repair.

Dan

 

The weird part is the modulator is disconnected and plugged. The only thing connected to that vacuum line is the suction throttling valve. And only stinks horribly out the exhaust is when there’s still refrigerant in the AC system when it stopped smoking, the air conditioning no longer works and there’s nothing left in the system.

 

I'll agree its weird.
An engine can burn a quart of oil in 500 miles without visible smoke. If you saw smoke, it wasn't likely the 3.75 pounds of R12 or R134 that was burning.

Dan

 

OK, when there is no more refrigerant oil, and no more refrigerant in the air-conditioning system, and the modulator is hooked back up from the transmission. The car runs absolutely perfect without any smoke at all? This is why the problem is so weird. You fill the system back up with the proper amount of refrigerant R12 and refrigerant oil turn on the air conditioning and it smokes like a bandit again. Of course, only for a short time till there’s no more refrigerant and oil left?
The only connection between the air-conditioning system and the engine is the suction throttling valve? Could it be the diaphragm?

 

I doubt it but, hey, some stuff has gone down in the world over the past 20 years I'd never have guessed.

Refrigerant oil sucked into the intake from the STV? Most of the oil stays in the sump of an A6 compressor. Charge it up. Pull the vacuum line from the STV and plug that line. The AC should still work - if it works at all. The diaphram responds to vacuum from a vacuum switch attached to the temp control and is what modulates cooling level by throttling the suction side.

Come on, nobody else with AC knowledge want to weigh in here...Powershift?

Dan

 

I know, never say never,

Anyway, when I got my 63 with factory, I removed all the components and rebuilt them. I see no way any R12 oil or R12 Freon can get in the engine. I can not recall one connection.

Everything works on pressure and temperature. As mentioned, the only vacuum is on the H/W shutoff, but that is just to let H/W into the heater core.

 

You just listed my last guess - isolate the two and see?? I’ve done AC, but my experience started in 69, POA systems. Most of these STV systems didn’t have vacuum connections as I recall. I’ll check the SM

 

I'm not familiar with the car the OP is working on. However my 63 Impala with factory AC has an STV valve actuated by a vacuum can. Some GM divisions used direct linkage to the valve, others the vacuum can. I don't see how refrigerant gets past the actuator seal through a diaphragm in sufficient quantity to present what he describes. I am in this instance a know-it-all who evidently doesn't.

Dan

 

64 SM states the vacuum eliminates the affect of atmospheric changes from the adjustment chamber of the STV. Looking at a cut-away of the STV, a broken piston diaphragm could allow refrigerant and oil into the adjustment chamber and then be sucked into the vacuum source (engine). My understanding of the vacuum was the exact reverse function re: atmospheric pressure.

Odd that both STVs leak. If the above is true, the leak of refrigerant should be obvious if the two systems are separated. Should not even hold vacuum when separated.

 

I gotta study this some more..

 

64 SM is interesting. Lays out test procedures to isolate which parts of STV may be malfunctioning. The absolute pressure valve does not seem serviceable unfortunately.

I am still flummoxed as to how manifold vacuum is used to control suction pressure, but the hardware is there.

 

I am also with limited knowledge of the STV...............I know much more starting with the POA in 1967 model year. HOWEVER...........

The 1963-66 Corvette STV is made up of two components: the primary STV which is spring adjustable only, and the absolute vacuum regulator (AVR) which uses an intake manifold vacuum signal. The AVR acts as a trim adjuster to the STV to compensate for changes in altitude which could/would otherwise affect the STV operation and performance. The two components are joined together with a vacuum port. As the AVR senses altitude changes, it transmits a varying vacuum signal to the main STV valve which can slightly affect its operation.

The AVR is hooked to the engine vacuum source, but is only connected to the STV thru a common vacuum adjustment port. The only way refrigerant can enter the AVR (and hence the engine) is if the STV diaphram is leaking or ruptured. The GM Service Manual discusses how you can separate the two components and run on only the spring operated STV...... or simply check the AVR-STV connection port for refrigerant. Copy of CSM is attached. See Section 13 Pages 13- 14 in the manual for instructions regarding this testing.

I would recommend OP separate these two components and check the STV for diaphragm leaks. He can also operate his AC system without reconnecting the AVR, although a manual adjustment to the STV might be required to get the best/recommended suction pressure for sustained AC operation.

The engine vacuum connection for all this is on the intake manifold, and the vacuum hose is then split with one branch going to the AVR and the other to the vacuum switch atop the heater box that controls the heater hot water valve.

Larry

 

Absolutely agree with all you’ve written. I just don’t grasp the “how” for the bolded part. Seems engine vacuum changes with throttle position/rpm and greatly overrides atmospheric changes. Opening throttle at sea level would be much greater change than increasing elevation or weather atmospheric pressure by thousands of feet elevation or a few inches or Hg. .

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