99 ranger 3.0L automatic. I put a set of gauges on the ac system at 78 degrees static pressure around 82, which seems good.

I start the truck up and switch on the compressor within a second the high side gauge pegs out and the compressor dump valve opens.

Then the pressures fall to right where they should be, around 40 low and 165 high at idle. I can let it set there and run for 30 minutes no change in pressures and the vent temp is about 49 degrees. I can switch the compressor off wait a minute switch it back on no problem pressure right where they should be.

Shut the truck off come back 30 minutes later and it starts all over again with high pressure spiking and then normal.

Any ideas why it would spike high side on start up and then run normal?

 

When was the last time the AC system was serviced?

Is this spike momentary or does it slowly fade?

By compressor dump valve, do you mean the pressure relief valve on the compressor and that you are loosing refrigerant?

 

I don't when I just bought the truck. I know it had been setting a couple of years so it has not been done in quite a while.

The spike is momentary. Spikes the pressure relief valve opens big whoosh and then fall back to about 165.

 

The pressure readings you listed at idle really cant be used to diagnose the system. Re-check with the engine at normal operating temperature and set to 1500 RPM with the AC system set to max and blower set to hi.

Normal pressures at 1500 rpm with ambient temp @ 80 (depending on humidity)
Low side between 33 – 45 PSI
High side between 130 – 225 PSI
Vent outlet temp 35 – 45