I have a 2004 Nissan Xterra, the AC blows out cool air only when I accelerate. I already added more freon but the problem did not improve.

When you say you checked the pressure did you connect gauges to the high and low sides to get both pressures, this sounds like it may be low. also what is the temp coming out of the vents. note on really hot days and humid the ac system doesnt cool as good unless the truck is moving down the road or highway and does take a while to cool down. at a stop or sitting or in traffic the ac temp will rise as the condensor needs to be cooled like the engine does, if its not cooling then the temp will not be cold, check the front of condensor for blockage, also make sure the fan is running with ac on. this fan should run when ac compressor is on,maybe clean condensor with water hose blowing from radiator outwardto front bumper, then make sure the hot/cold blend door is closing all the way,a quick test is to turn a/c on, turn temp control to hot and see if it gets hotter then back to cold and it should get colder. this sounds like the system may just be a little low. have an a/c tech check it, you may find its low or nothing is wrong. good day.

http://www.denlorstools.com/autoblog/2009/04/adding-freon-to-car-ac-gauge-readings-explained/
You need a set of A/C gauges and this web site

Have the freon level checked. It might be a worn compressor that pumps insufficient freon at idle. Good luck!

you are supposed to hook up the guages then run the compressor, unless you have a different model than most. this method will give you the low pressure side reading as the compressor runs, this is the important one. as all systems are different there will be slightly different pressures at rest

Might need to add freon to your system.

As for the AC problem it has a leak somewhere if it worked good for a day.Have it pressure tested for a leak.

Where ever you take it they are going to do an AC service. That involves evacuating the system of any remaining freon, pulling a vacuum in the system, seeing if the system holds the vacuum, and then recharging the system with freon and adding some type of leak detector dye to the freon. That can run anywhere from $40 to $90, depending on where you live and the labor rates where you live. You can call around to the different shops in your area and ask how much they charge to do an AC service.

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your air con needs re gasing

are you sure the compressor comes on? if it is still blowing, it just means the blower motor still works, not that the compressor is ok. that stated, the most common cause is a small leak in the system that allows the freon like substance to leak out. You can recharge it yourself with kits available at walmart or pepboys (pretty inexpensive). These kits will stop some small leaks. If it is still a problem, get it serviced professionally.