AC overpressure release

[JHZR2]

Hi,

I converted my 91 BMW 318i from R12 to 134a back around 2004. It has been tight and has worked great since.

The other day I was in slow moving traffic on the interstate, in direct sun and well over 100F. The AC just wasnt keeping up, the air was lukewarm. Twice I smelled what seemed to me to be refrigerant, 134a.

Now, the AC still works fine and blows cold when ambient is a bit lower than 100F in direct sunlight. The compressor cycles fine, etc.

Is there some kind of a release valve that opens and reseats, so it isnt a rupture disc? Does refrigerant seep by weak points in o-rings when the pressure gets too high? Is what I explained a sign of pending issues?

Thanks!

 

[Chris142]

Most cars do have a high pressure release valve that is supposed to reseal after the pressure goes down. Usually they pop off @ 500+ psi.

I'd look at your fan clutch to find out why the pressures got out of control.

 

[EricF]

+1

Usually if the freon exits via the blowout valve you'll see a cloud of freon which'll look like steam and probably a fairly loud wooshing sound.

check the fan clutch for sure, and make sure the condensor isn't too plugged up with debris.
a puller fan might be in order too..

 

[JHZR2]

Im pretty sure my fan clutch is OK, as I replaced it a few years back. There is no verification that I DID reach high pressures. No woosh, no steam, no noise. Just a smell that seemed to me to be 134a.

Maybe it wasnt. I find it hard to believe that I reached 500psi.

 

[chrome]

Sounds like a momentary overpressure in the system, and tripped off the overpressure switch which will shut off the compressor until pressure drops.

Overpressure can be caused by a build-up of moisture in the system, which points to a weak receiver drier. Have a set of gauges hooked up and see what the high pressure side is doing on a warm day. You might need to evacuate and replace the drier.

 

[Colt45ws]

A plugged oriface can cause an over pressure as well. Seen that in an F150 that had it installed backwards during a previous compressor replacement.
I'd look for any evidence of oil near the lines and compressor, just to be sure. If you can get a set of gauges on it, that would be even better.

 

[JHZR2]

I have gauges and can do that. I know that last time I looked at the system on gauges the pressures were perfectly inline with what would be expected (static and operating ratio/range-wise). No 500psi type situation in my system then, nothing added since.

Still Id imagine the compressor could overpressurize and the lack of heat shedding could be troublesome.

I think I have an aux fan that I should check.

It isnt a matter of an overpressure trip-off. The AC system was actually working slightly the whole time it seemed. It was just the smell that was odd. That's all the basis that I have. Nothing else. No oil around the compressor or lines either. If this did happen, it was at/near the evaporator or orifice, which I believe is right under the p/s dash.

 

[mechanicx]

Exactly R134a doesn't cool as well as R12 to begin with and especially when a conversion is done. A/C systems vary on how effective they work after conversion. European cars aren't generally known for the most effective A/C systems vs American or even Japanese. R134a works best at a slightly higher high-side pressure and lower low side pressure. R134a has a tendency to leak more, and some R12 systems don't have effective enough hoses to prevent leakage.

When you convert to R134a you should generally use 80% of the R12 charge or you can get over-pressure and poor performance. Plus you have to add a full charge of R134a compatible refrigerant oil (draining the R12 oil out is helpful but not a necessity). But even if you used an 80% charge of R134a, maybe this converted system just hit the wall.

Someone mentioned the expansion valve might have been sticky and that is a possibility. As I mentioned an R134a's orifice valve is set a few psi lower than R12 system's. Sometimes the dessicant in the an R12 systems receiver-drier is not effective with R134a.

I wouldn't change the expansion valve unless it was bad as that would defeat any cost savings of the conversion, but a new drier that's r134a compatible if available, and getting a charge of not geater than about 80% of R134a is important.

 

[mechtech2]

That release is usually pretty loud and unnerving.

 

[JHZR2]

Im not arguing the efficacy of R134a vs 12, particularly in older systems made for 12. I am very aware of the correct changes necessary to operate, and know that all were done properly.

My main question was if there is a pressure relief valve of some sort that can release and then reseat versus something that is a blowoff or rupture protection device.

Hearing that it is loud and unnerving means that this was not the case. Maybe some refrigerant slipped past an o-ring due to an elevated pressure or something like that, Im doubting that there is anything more to it than that.

The system has cooled adequately and held pressure without need to top it off since it was done. When it was done, I replaced the compressor, R/D, all the seals, flushed the whole system, metered the right amount of refrigerant, etc. It was a high quality long-term job.

But I dont believe that what I was considering, i.e. some overpressure relief being tripped, occurred.

 

[onion]

Originally Posted By: mechtech2
That release is usually pretty loud and unnerving.


Yep, I've seen it happen a few times in the shop when guys over-charged or otherwise didn't know what they were doing. It makes a pretty good noise and releases a big cloud of oil mist. If the pop-off valve popped off, then at a minimum you would have an oil mess around the compressor.

 

[mechtech2]

Also, we don't know what you smelled.
I wasn't aware that r134a had a smell.

 

[artificialist]

I had one overpressure valve open up on a Volvo. The car had some weird setup that didn't allow high and low side manifold gauges to be installed. I had to trust my refrigerant machine, and NOBODY TOLD ME THE SCALE WAS INACCURATE. When the overpressure valve opened up, it sounded like a jackhammer, and a cloud of gray smoke blew out from under the hood.

I have been told that in order to get a good R-134a retrofit, you have to replace the condenser and evaporator with more efficient modern designs to offset the weakness of R-134a versus R-12. In addition, the expansion valve or orifice tube is different as well.

R-134a has a smell, it smells just like certain aerosol cans being sprayed. That is because the two things are similar.