Why does my car ac system keep blowing up?

Twice now my 92 Explorer has "blown" the compressor. In both cases it blew very shortly, within minutes, of starting the truck. It blew so shortly after starting the truck I have a hard time seeing how the pressure could be high enough to "blow" but in both cases it popped the high pressure relief valve and spewed oil and refrigerant out. The first time it did it I had started the truck, pulled out onto the street and immediately hit a red light. While at the light I put it in neutral and gave it enough gas to rev up the engine to 1500 rpm to run the compressor enough to get more cooling in the 100 degree heat. After less then a minute of waiting at the light there was a grunge/screech/smoke/smell and no more cooling. Seemed like the compressor had locked, oil blew, fan belt screeched over the locked pulley and the oil burned on the exhaust manifold. Then it unlocked but no more cooling. Upon tearing it down there was the expected "black death" look to the oil, it had about 100K miles on the system, and metal bits at the orifice tube screen. When I took the compressor apart there were broken bits and obvious new wear marks on the internals. I got a new compressor, accumulator, orifice tube and condenser and flushed the evap and all lines. Put it back together and it ran fine for 4 years.
Then last month I started it up after it sat all day, about a 95 degree day. Started it and just let it idle for no more then 1 to 2 minutes while I arranged my stuff before leaving. Then grunge/screech/smoke/smell and no more cooling - same thing again but I hadn't even sped up the engine. Immediately turned it off. Got out and looked, oil blown out all over the engine again. Turned it back on and the compressor spun freely, no funny noises, but no cooling. Unlike the first time, when I tore it down I didn't find anything much in the system, no little metal chunks, just a little bit of "blackness" to the oil, a small amount of sludge on the O-tube screen. The inside of the compressor did not seem to have any broken parts. From how everything looked I might have been able to fix it by simply recharging it but I had already ripped the compressor apart. So I bought all new parts again and redid it last week. Got a compressor, accumulator, o-tube and condenser from rockauto for $220. Now it's cooling fine.
Then about 3 days after fixing it I was at the SAME park where it had blown the second time. Started it up and heard what I thought sounded like the grudge noise - like the fan belt was just starting to slip and was fighting it. After having heard this twice before I immediately turned it off. Thought maybe, just maybe I smelled a tiny bit of burning oil but there was no smoke like the first two times. Drove about 3 miles to the store with the AC off and a frown on my face. Popped the hood and disconnected the compressor. Then started the engine and turned the ac on. Then plugged the compressor back in while I watched under the hood. Compressor came on, no funny noises, and it started to cool!! In fact, it seemed to be working perfectly normally. So I presume I caught it JUST in time before it blew out the high pressure blow-off again and spewed oil everywhere.
So at this point my thinking was that maybe I was putting too much oil in the system and slugging the compressor on start up. So I pulled the one week old compressor back off and checked the oil level. It was actually on the low side, certainly not at all overfilled. So put it back together and it's working just fine.
Sorry for the long post, ...
why is this system blowing up within 1 or 2 minutes after a "cold" start???? It's always in the 1 or 2 minutes when I would think the system isn't even working hard yet.
Every time I start the engine (and AC) now I have my finger ready on the AC on-off button ready to shut it off if I hear the slightest funny noise.
I put in 80% of the amount of R12 as I'm using 134a. Running it in the driveway at 2000 rpm I've seen the low side drop to 30 and the high side go to over 325. I don't see how they could possibly go even that high, much less to the 400 psi needed to pop the high pressure valve within 2 minutes after starting with the engine at or near idle speed.
Anyone got any theories? Too much oil? Too much refrigerant? Not enough virgins being sacrificed?
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Clogged expansion tube. Clogged evaporator. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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On 15 Oct 2015 20:59:46 -0400, snipped-for-privacy@panix.com (Scott Dorsey) wrote:

Evap was not clogged either time. Doesn't have an expansion tube, just an orifice tube. I guess that could clog but there is a filter screen before anything gets to it. Why would it do so immediately after starting and while idling yet never do so while running down the highway for hours on end?
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Ashton,
. Why would it do so immediately

You have very little data so concluding that it's a "starting" phenomenon may not be correct. You may be jumping to a conclusion.
Dave M.
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On Fri, 16 Oct 2015 08:51:21 -0400, "David L. Martel"

There is little data but all three times it was immediately after starting while idling. To me that would seem like the easiest part of the work day for the AC yet it overpressureized and "blew". I keep trying to figure out what there would be about the "refrigeration cycle" that would somehow create such a high pressure within a couple minutes of system startup. To blow off the high pressure valve it has to get to something like 400+ psi. Even with it sitting in the driveway as I finish putting 134a in it in 95 degree temps it only got to around 325 psi at 2500 rpm. At idle it's in the 200's. The only things I can see that would make it blow are a blockage or oil or freon slugging. I can see how a blockage could happen but if there is stuff in there that's going to block something it seems like it would have happened at high speed "the day before", not "today" when the system is just loafing along at low rpm's. Is there something about startup that would create an oil or refrigerant slug in the compressor?
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"Ashton Crusher" wrote in message wrote: >Ashton,

There is little data but all three times it was immediately after starting while idling. To me that would seem like the easiest part of the work day for the AC yet it overpressureized and "blew". I keep trying to figure out what there would be about the "refrigeration cycle" that would somehow create such a high pressure within a couple minutes of system startup. To blow off the high pressure valve it has to get to something like 400+ psi. Even with it sitting in the driveway as I finish putting 134a in it in 95 degree temps it only got to around 325 psi at 2500 rpm. At idle it's in the 200's. The only things I can see that would make it blow are a blockage or oil or freon slugging. I can see how a blockage could happen but if there is stuff in there that's going to block something it seems like it would have happened at high speed "the day before", not "today" when the system is just loafing along at low rpm's. Is there something about startup that would create an oil or refrigerant slug in the compressor?
What size of Vehicle or what make dose it utilize Expansion vale. dose it use electric fan motor, is it Mechanical directly hook up to motor. when Refrigerant was lost, did you find the leak is it on high side or low side. Have you Evacuated system before you added Refrigerant.. How much of refrigerant did you use, all systems are not same size. If your Vehicle is older then 1992 which means it is design for R-12 and not R-134, Sorry but that is the way it is!
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The system MUST be purged before recharging, partigularly when switching refrigerants. If it has an orifice it must be removed and checked - preferably replaced AFTER purging the system with nitrogen. If it is an expansion valve system I'd take a real good look at the valve and check function. If the system has been open any length of time , the receiver-drier should also be replaced. Then the system should be totally evacuated and held at a deep vaccuum for at least an hour, then the service valves should be closed and the system left for an hour or more- check for loss of vaccuum. If the system proves to be sound, install the r134 conversion valves on both high and low pressure sides, Charge to the low side with the specified amount of Ester oil. PAG is also used with 134, but on a conversion, in particular, Ester is better as it is les hygroscopic (it has less of an affinity for moisture). Then draw in the proper amount of R134A.. Proper fill is aproxemately 65% of the R12 charge.. A 75% charge is USUALLY an overcharge. On systems with an orifice tube, for conversion I REALLY prefer the automatic (AKA Smart VOV) tube. If not, the blue Ford tube works best for 134 in a conversion (even on a Chevy). I installed AC on my 1996 Ford Ranger, using the hoses, compressor, receiver and evaporator from a 1993 with the condenser for a '95. I put in the smart VOV and charged it to 1994 R134 specs (36 oz) - and promptly blew the overpressure plug. I then checked several other sources and found out the published spec was WAY too high., so I used the 1997 spec (22 oz), which ended up being just over 65% of the 1993-R12 spec. (32 oz) which would be 20.8 oz. Ford specifies PAG oil for R134A, but I use Ester because it is compatible with the old mineral oil - and on a conversion, there is ALWAYS a possibility there is some mineral oil remaining in the system - no matter how careful you are. It is also safe to mix with PAG.
PAG is NOT compatible with chlorinated refrigerants - like R12 or R22. It will turn to thick brown mud upon contact with R12..
Perhaps that is the OP's problem??? The "mud" will plug the expansion valve/orifice and cause the high side to go nuts. It might allow the system to work well for a day or so before plugging the orifice - and then blowing the system on restart.
It's been 3 summers now and it cools perfectly.

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On Sun, 18 Oct 2015 17:10:20 -0400, snipped-for-privacy@snyder.on.ca wrote:

There are almost no original parts left, the only significant one is the evaporator. Everything's been flushed twice now since it originally had the mineral oil. It's had PAG46 since the first redo 4 years ago.
What would nitrogen do that isn't taken care of by the flush and pulling a vacuum?
I would like to have tried a variable orifice tube but the local AC shop who is also a huge wholesale vender as well as custom maker of hoses and such told me they see a lot of people who try them pull them back out because they malfunction in some fashion. Mine is a real pain in the behind to get to so I stuck with a fixed one.
I almost put r12 in it but I see from your post that would have not worked at all well with the PAG. Good thing I went with the 134a.
You said the R12 to 134a conversion is 65%. Every other thing I've read has put it somewhere between 80% to 90%. I used 85%.
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Ashton Crusher wrote:

...
Black gunk may be contamination... or due to compressor failure. Will assume the oil was correct type. First occasion could be due to not enough cleaning and purging. 2nd and almost 3rd, not likely. Were the oem hoses changed out for 134a hoses?
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On Thu, 15 Oct 2015 21:36:46 -0500, Paul in Houston TX

I know one of them was as it had a muffler in it when it blew up 4 years ago and I had an AC shop redo the manifold with a new hose to replace the part with the muffler. It's possible the other hose on the manifold is an 18 year old one. From what I've read there is not supposed to be any problem with old hoses that have been well soaked with oil. But those two short sections are the only hose, the rest is pipe.
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Ashton Crusher wrote:

Might want to check out this a/c pdf. It's pretty good. Scroll down to ~ page 34. http://ecat.spectrapremium.com/bulletins/download/automotive_techtip_eng_2007.pdf
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On Sun, 18 Oct 2015 17:56:47 -0500, Paul in Houston TX

Thanks, that's a really good read.
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Ashton Crusher wrote:

SNIP

SNIP

SNIP
> So I presume I caught it JUST in time before it blew out

When did you do the 134A swap? After the first pop or the second? What oil are you using? Were ALL of the NEW parts shipped empty of oil?
If you are seeing 325 psi on the high side you have a problem. It sounds like a restriction on the high side. Possibly a chunk of crud that didn't get flushed out of the line.
--
Steve W.

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wrote:

The 134a is what I used this last time. Time before that I used a "drop-in", ES12a that I have used in two other systems with no problems. But to eliminate the ES12a as a potential cause I went with the 134a this time. Yes, everything was dry except for the compressor. It supposedly had 7 oz (the full amount for the whole system) in it. I drained it to check and only got 5 oz out. So that left me with the question, did they only really put 5 oz in or is there 2oz trapped in the compressor. I went with the idea that 2 oz were trapped for determining how much to put in the system.
The 325 reading was in 95 degree outside temperature with the vehicle not moving, no extra fans, just the engine driven radiator fan, and at 2000 rpm. At idle speed it dropped to around 225. Will a restriction in the high side also cause the low side readings to be high? They were at about 30 when the high was at 325.
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Serpentine belt. It's as tight as the belt tensioner makes it. So presumably no tighter then it should be but potentially loser due to the spring weakening. All the compressors were new. The only parts not new were the evaporator and two short pieces of hose. The hoses and evap were easily blown thru for flushing.
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