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AC blowing hot when idling

I have a 2002 Ford Windstar and the A/C has started blowing hot when i idle but it blows cold when I am driving. I had the compressor replaced back in February and the pressure is normal so it doesn’t look like it has a system leak. If I am at a traffic light, I can put the van in neutral and press the gas and it will start blowing cold again. Since I just spent $1300 a few months ago when my compressor seized, I really hope it is not an expensive fix.

Are your fans working?? If your fans are not working then that could be your issue. However since it gets better if you hit the gas, I am going to guess that your AC charge is a little low… Which means you may have a very small leak.

The problem might be with the expansion valve at the evaporator. The expansion valve is what controls the pressure of the refrigerant that enters the evaporator. When the compressor is rotating faster the expansion valve opens more. When idling the compressor is rotating slower and the expansion valve closes slightly to keep the pressure constant at the evaporator. If the expansion fails to close slightly when idling the pressure falls off at the evaporator and the AC stops cooling.

Tester

The fans are working. The only time you can tell something is wrong, if you stop and it starts to blow warm. The engine temp is also normal. We checked the pressure with one of those do it yourself refill kits and it is in the normal range so we were afraid to add any.

You need a set of manifold gauges so the high side pressure can be monitored to see if the expansion valve is working. This can’t be determined by just monitoring the low side pressure that comes with the kit.

Tester

@jeshest Long shot here but I have seen it once. Is your charging electrical system up to the task in hot weather? I had a 2001 Galant that would not charge the battery only in drive and at idle by design. Never knew that until the battery started to fail load test. Only symptom was the AC would start blowing warm air like yours. I picked up a cigarette lighter volt meter from Walmart in the automotive area and put that in the car. I saw battery voltages as low as 9.5V while sitting in gear. Cooling fans would slow and not draw enough air across the condenser and therefore the cooling inside the car would suffer. As soon as you bump our car out of gear at stop lights the AC would start blowing cold again as the alternator would return to charging at around 13.5V.

Thanks everyone for their comments. Guess I need to take it to my mechanic and have him run some tests. Ugh. Hope it doesn’t cost a fortune.