A/C not as effective when stopped

Question

I purchased a 2010 Ford F-150 with the 4.6L motor. The AC seems to work fine, but only when I am moving. When the truck is moving above 30 mph it produces a relatively cool stream of air even if the cabin interior is very hot.

Once I slow down, the AC becomes ineffective. While I'm sure the air is still cooler than the air going in, it barely has any temperature difference. Is this normal for this vehicle? It has a two electric engine fans which run whenever the AC is turned on. The condenser under the hood appears to be clean and free of damage or obstruction.

The previous F-150 I had used an engine driven fan which moved a huge volume of air. So much it would actually blow leaves and grass on the ground around. I am wondering if this newer vehicle just has less effective AC as a consequence of the electric engine fans.

Answer 1

Could the compressor be weak? The compressor drives the coolant into the condenser, so that becomes a liquid that can evaporate under the dash in the evaporator and cool the air coming out the vents. If you're moving fast, air flow past the condenser (which looks like another radiator, and is found in front of the engine radiator) would help cool the refrigerant. If you're stopped, maybe a weak compressor can't fully liquefy the refrigerant.

Answer 2

This may be related to engine RPM. When you slow down, the RPMs will be lower and therefore, the AC compressor is operating at low RPMs as well. These old-fashioned mechanical AC compressors operate at an RPM proportional to engine RPM.

I used to have a 2011 Toyota Yaris that had electric cooling fans for the engine and an AC that worked quite well even at idling RPMs. So, your guess is incorrect: electric cooling fans for the engine do nothing to reduce (or improve) the AC system performance. In fact, my current 2016 Toyota RAV4 hybrid also has electric cooling fans for the engine, but the AC compressor is also electric, so this isn't a fair comparison.

I would start by checking if your system is low on refrigerant.