AC blowing off at compressor?

[midnightblue]

Has anyone experienced freon/oil blowing out from around the back of the compressor? So much vapor that when it happened I thought the truck was on fire. 1984 K5. I have owned it 6 yrs and the AC has always worked. The prior owner converted it to 134a. I was pulling it out of the garage last week and heard a loud hissing sound and smoke (vapor) coming from the engine compartment. After closer inspection I saw freon and oil blowing / leaking from behind the compressor. If I turned off the ac it would quit. wait a few seconds and turn the ac back on and it would blow off again after a minute or two. I could not tell where exactly it was leaking and not knowing the history of what was replaced 6 yrs ago I replaced the compressor, condenser, drier(?), all orings and orifice. Flushed both hoses and evaporator core, all seemed to flow well and the cleaning fluid was coming out clear. Then had an ac shop evacuate and charge the system and said there were no leaks. All was well for one day. Today it did the exact same thing. I cannot tell where it is leaking other than the back of the compressor area (too much vapor I cannot get close to see). There is freon oil all over my valve cover but I do not see oil on the hoses or the back of the compressor.
Can anyone tell me what is happening? Is it possible for a hose or the hose manifold or seals to leak and then stop? Its like it is building too much pressure. Any help is appreciated.

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[bucket]

It's blowing off excessive pressure from here:

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^^^That’s the high pressure blow-off valve. It's happening because you are building too much pressure on the high side of the system. How old is your fan clutch?

It really needs to have a set of gauges put on it to properly diagnose it.

Also, that other port with the blockoff plug in it, that's where there could/should be a high-pressure cutoff switch to prevent this from happening. I don't recall if all years had a high side cutuff.

 

[bucket]

Nevermind about the fan clutch. I just noticed your electric fans. Do they come on right after the ac compressor has energized?

 

[midnightblue]

It's blowing off excessive pressure from here:

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^^^That’s the high pressure blow-off valve. It's happening because you are building too much pressure on the high side of the system. How old is your fan clutch?

It really needs to have a set of gauges put on it to properly diagnose it.

Also, that other port with the blockoff plug in it, that's where there could/should be a high-pressure cutoff switch to prevent this from happening. I don't recall if all years had a high side cutuff.

Thanks Bucket, I have been reading as much as I can about AC systems to get educated. Yes the fans are wired to come on, I had to go Elect fans to keep the engine cool when running AC. One thing I never paid attention to for the last few years is the compressor never cycled when running, it just runs continuously. SO, maybe that explains why the engine temp runs so high if the compressor never cycled?
But why did it work fine and all of a sudden start blowing off?
I do not know if this year ever had a high pressure switch, there wasn't one when I got it but who knows if some one removed it? I think its time to put a high pressure switch in the current compressor's plugged port.
I read some where (i cant find it now) to use a "blue" high pressure switch, does that sound right?
and cut one of the green wires on the low pressure switch, and connect them to the high pressure switch?
Thanks again

 

[YakkoWarner]

That looks like the same type of compressor I have, and mine also does not have a high pressure cutoff switch, nor can I find any reference to one in the wiring diagram for my year truck. If it is something that can be added I'd really consider doing it because I experienced a blowoff event once.

If it just started after working fine, my 1st expectation is a blockage somewhere in the system. A weak/out of calibration blowoff valve could also be an explaination but I'd get some gauges on that thing and see what it really happening both high and low. If your high side is way high and low side is overly low, than you likely have something clogging up the works (debris in orifice tube possible?).

 

[bucket]

Yep, if the fans are coming on, you really need to get some gauges on it. An infrared temp gun can be handy too.

Yes, the high pressure switch can just be wired inline with the clutch circuit. There's several different colors of pressure switches, but I do not recall the specs of each. Also, the connectors for each are keyed differently.

 

[Hunter79764]

Agreed to all above. Dumping charge there is a feature, not a bug, but the high pressure switch should help. I think they mostly used them after the 134 switch, and I'd guess that F bodies might be a good option (since they would likely experience higher engine speeds that the factory would plan on?). How hot was it outside when it vented?

Overcharge makes for high pressure, so does an obstruction, so does an electric fan that isn't actually working like you think it is etc. Gauges will help diagnose, if both pressures are high, you have too much charge or too little airflow on the condenser. If high is high and low is low, you have an obstruction. Since you replaced it all and had it again, obstruction seems less likely. Since it sounds like it was fine last year, overcharge doesn't sound as likely. Possible you have an intermittent feed to your fans, so they sometimes aren't working right?
And if the compressor never cycled off, then you need to make sure your low pressure switch is working correctly. The compressor should cycle in normal operation. If it doesn't, it can either be the chicken or the egg that causes the pressure to go too high. Constant compressor can build too much pressure, or too much charge can make it where the low side never gets low enough and high side goes crazy.