A/C Pressure fluctuating?

2002 Mazda 626 2.0L 4cyl.

98,000 miles

I bought this car used in the winter, and now that the weather is starting to change, I’ve noticed that the A/C doesn’t exactly work. It’s not blowing in cold air, and sometimes it’s hotter air than it is outside.

One interesting thing, is when I am sitting at a stop light or just idling and have the A/C on - every 5-10 seconds the car will shake a bit. Like something it trying to engage and then doesn’t. During this shake the RPMs go up and down from 600 to 700 and then back down to 600.

I tried recharging the A/C today and found out another thing. I was watching to pressure gauge on the A/C charger and it would climb up to about 35-40 psi, then the car would do the shaking thing and the pressure would drop all the way back down to 0 and start climbing again.

Anyone have any idea what the might be? I’m completely stuck.

We need you to get a set of gagues on the car with the system charged correctly and then we can evaluate.

How are you going to do that? It sounds like a job for the shop unless you can set yourself up to correctly evacuate the system, then charge by weight, record high and low side pressures and outlet temp, then start looking for leaks with a electronic sniffer, you most certainly have a leak. Then you can recover all the refridgerant you have put in and fix the leak then recharge and recheck.

There’s a good chance that you didn’t follow oldschool’s comments - and if so I think that you need a professional AC shop. This stuff is not so easy to mess with.

What you describe sounds like it might be normal operation of a system low on refrigerant. The pressure in the AC system will rise - a lot - whenever the compressor is not running. When the compressor runs the pressure drops - a lot. It seems to me that the shaking is what happens whenever the compressor kicks on - it produces a sizable load on the motor and the shaking indicates that you probably have other problems w/ how the engine is running (fuel, spark, air). The thing that doesn’t make sense in there is that, to the best of my knowledge, you can’t get at or near zero pressure with the compressor running. Once the system is too low it should fail to kick on. You might be sitting at the borderline.

Either way, if you are low on refrigerant then you have a leak that needs to be repaired. I would stop messing with it now and take it to an AC specialist.

Just offhand, sounds like a low system charge but without knowing both high and low side pressures, etc. it’s hard to make a guess.
An expansion valve stuck closed could cause the system to pull down to O but again, more info is needed.

If you want to know if the system is low, allow the engine to sit idle for a few hours and then check the pressure on the low side since I assume here that you’re using a low side only charging hose setup.
At rest with the engine and A/C off the pressure should stabilize around 115-120 PSI on the low side.

I also agree with the others that you might consider getting an A/C shop to look at this as A/C work can be a bit dangerous on top of causing even more damage to the system.