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A/C high side pressure spikes, then levels out

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Old 03-19-2017, 03:25 PM   #1
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Default A/C high side pressure spikes, then levels out

2000 XJ, 4.0, 138k miles.

Just replaced the compressor, liquid line (with fixed orifice), evap core and accumulator. Also replaced the low pressure (two pin on accumulator) switch. I ran A/C flush through the condenser and manifold lines before installing all the new parts. I drained the compressor of factory oil and filled with 4 oz of pag-100 and then filled the accumulator with 3 oz of pag-100.

Brought it to a shop for them to pull a vacuum and fill it back up. They filled it and mentioned that it was sputtering out of the pressure relief valve on the compressor (this is the same behavior that led to the old compressor kicking the bucket). I thought it was the high pressure switch so I got a new one and it seemed to engage okay for a bit, but then acted the same but only upon start up of the Jeep.

I finally got a good set of manifold gauges on there. High side pressure spikes up to 500+ (kicking off the pressure relief valve), then falls to 350 and the compressor kicks back on. Then it spikes to 500 (not enough for the pressure relief valve to kick on, but the high pressure switch turns off the compressor), then again drops to a point where the compressor kicks on. This happens twice more before the pressure stabilizes to 325 psi and the compressors stays on. Low side starts at 55 psi, drops to 40 over this time.

What could be causing my high side pressure to spike and then level out? Too much oil (that then redistributes)? Condenser is plugged (but some how it gets through later on)? Sanden A/C states high high side pressure and normal low side means air in the system, so maybe I need to drain it all and pull a better vacuum than the shop did. The fact that this happened with the old compressor as well makes me think the condenser is plugged up. I did run some a/c flush through it with compressed air, but it held a bit of pressure. I thought that might be normal, but maybe not.

TL;DR:
High side pressure spikes at 500+ psi before leveling out at 325.
Low side goes from 55 to 40 psi during that time.

Anyone experienced these symptoms? Condenser plugged?
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Old 03-20-2017, 06:13 PM   #2
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Well, that high a reading indicates either a refrigerant overcharge or a blockage somewhere. Depending on the ambient temperature even the 325 that it settles out to may be too high. Did the shop tell you how much refrigerant they installed, by weight? The amount of oil sounds about right assuming there was none in the system when you started. (Normally if there is too much you would get less cooling rather than a severe increase in pressure.)

The service manual for 2000 XJ shows the following for expected pressures at different temperatures:



This gives you an idea of what would be considered normal operating pressures depending on the outside temperature.

A sudden temperature change along the length of one of the hoses would indicate a blockage at that point. Also it is not possible to flush out the parallel-flow type condenser which is used in R134a systems. So the condenser needs to be replaced if there is a compressor failure or some other kind of debris trapped in it.

I have found that my '99 XJ is pretty sensitive to the amount of refrigerant installed in it. A little bit of overcharge and it makes groaning noises and spurts refrigerant out of the relief valve. The only way to be certain of how much is in there is to vacuum out the system and weigh the amount of refrigerant charged back in.
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