I purchased a 2006 Lincoln Navigator (5.4 Ford, with a separate front and rear A/C system) a couple of years ago and it seemed to be maintained very well, but the A/C was performing poorly in hot weather, especially at idle or in stop and go traffic. With the climate control modules on the coldest setting, with the front and rear fans on the highest setting, and with the front climate control module on recirc mode (the rear A/C can only do recirc)...... the outlet temperature wouldn't get below around 48 degrees Fahrenheit, and it would only muster that low of a temperature if I was barreling down the highway with the A/C on full blast for a while. Thinking the charge was low, I added refrigerant 2 summers ago and blew the front seal on the compressor. I took it upon myself to learn more about automotive A/C systems, and installed a brand new compressor, drier/accumulator, condenser, both evaporators, both expansion valves, schrader valves on the high/low ports new pressure switches, all new O-rings, new blend door actuators for the front and rear, new evaporator temperature sensor, etc. The only thing that wasn't replaced on the system was the climate control module in the dash (works just fine) and the hard lines that run to and from the rear evaporator under the car. Those lines were flushed properly with A/C flush solvent, and dried out with dry air.
The compressor was completely drained of PAG oil before installation, and I added 14 oz of PAG 46 oil, and used a Snap On A/C machine to fill the system with exactly 54 oz of Dupont R134A. This is exactly what the system calls for. Everything seemed to be operating fine, there seemed to be no blockages on the lines, compressor was cycling on and off at a normal frequency, but while idling the truck's outlet temperature wouldn't get below 60-some degrees on an 86 degree day. I ran it down the highway for a bit, and was able to get my usual 47-48 degree outlet temperature, but still couldn't get below that.
We tried adding a little more refrigerant, and a little less, but neither seemed to help. Last summer, we evacuated the system and then re-charged it with the same 54 oz, and replaced the amount of PAG oil that was recovered, but no improvements. I gave up on this A/C system and suffered slow-moving, 45 minute commutes home in 95+ degree weather, with an outlet temperature of around 65 degrees, until I could finally hit highway speeds.
I know that some will say that 65 degree outlet temp on a 95 degree day is normal, or that 48 degrees while barreling down the highway on an 84 degree day is pretty satisfactory-- but that's absolutely not the case. Every other vehicle I've owned, of about 14 vehicles, including an older '97 Expedition, has always produced outlet temps between 38 and 42 degrees, and they will do it while idling, let alone at highway speeds. And 65+ degree outlet temperature on a 95+ degree day, especially when the sun is beating down on the truck (I have tinted windows all the way around, which combats the sun some)... it just doesn't cool the cabin down worth a darn. Even in 55 degrees weather, I can't see better than around 48 degrees outlet temperature.
This truck only idles at around 550 RPM from the factory, and increasing the throttle to around 1,000 RPM while in park or neutral seemed to improved the outlet temps at idle by just a couple degrees, so I don't think it's all in the idle. But today, our first day in the 90's this year (91 degrees to be exact), I noticed that the A/C compressor clutch never cycled off (I use a Bluetooth OBD-II adapter and the FORscan app... and I've watched the PID for the A/C clutch cycle on and off with it plenty of times in the past and recently) while I was in bumper-to-bumper traffic and the outlet temps for 55-60 degrees, nor when I was doing around 55 MPH for around 10 minutes and the outlet temperature finally slowly dropped to around 48 degrees. I'd like to understand why the compressor did not cycle off in today's hotter weather, especially when I finally got the cabin cooled down a bit.
I've seen the F-150 guys do a mod where you add a resistor between the wires on the evaporator temperature sensor, which tricks the climate control module into thinking that the evaporator temperature is warmer than it actually is, therefor allowing the compressor clutch to remain engaged longer and help produce better cooling. I've tried that with several resistors, and no improvements. Plus with the compressor clutch only staying engaged the entire drive home today, clearly running the compressor longer (or 100% of the time) does not help. I've added two 12" cooling fans (puller fans) the back of the condenser, and they don't help with the outlet temperatures. I've reached my hand into the front and rear plenum and touched both evaporators, and they are never that cold at idle.
The blend doors are all working properly. The door that blocks off the outside air to the front plenum is sealing off perfectly, the blend doors responsible for mixing the evaporator and heater core air are sealing properly and the new actuators are working properly.... everything is working exactly as it should. I've even tried clamping off coolant flow to the heater cores and no improvements at idle nor at highway speeds... so hot air is absolutely not entering the front nor rear plenum.
What gives???? I've spent so much money in trying to get this system to perform better, but nothing has improved. I'm desperate at this point, and everyone I know that I've talked to about this has nothing to suggest after everything I've done and tried. Any solid help would be extremely appreciated!!
P.S. Here are some videos from last summer, of some manifold gauges I hooked up to car. It covers the static pressures, as well as the low side and high side pressures while the system is running:
The videos would have to be played in the above order to make sense. Warning: The videos are as long as my post lol.
EDIT 5/4/18: I failed to mention that I also have a brand new fan and fan clutch assembly that I installed last year, which did not improve anything unfortunately.