Strange issue with my truck. It's an 02 F-150, 4x4, 5.4L, 140k miles.

When idling and the A/C compressor kicks on the idle jumps from 700 to 900rpm for a second or two and then drops back down to 700. Normally wouldn't be an issue, but sitting at a stop light the truck will jump when the A/C compressor kicks on. Kind of unnerving.

Didn't used to do this. Not sure when it started. I fought a high idle issue for almost a year. I seem to have fixed that as the truck idles smoothly at 700rpm out of gear and 650 in gear. The only issue appears to be the surge when the A/C compressor kicks on. Since its winter the compressor cycles frequently with the defrost and low temps.

For kicks I disconnected the IAC valve to see where the truck would idle. Stays right at 500rpm and isn't happy. On my old 5.0 Mustang you would set your base idle by disconnecting the IAC and adjusting the base idle, but the Chilton's manual says there is no adjustment for these motors. I don't see any kind of adjustment on the throttle so I guess that is the case.

A/C compressor is 2-years old. Belt is in good shape and does not squeak when the compressor engages.

Have done the following:
-Replaced Fuel Vent hose from Purge solenoid on the firewall to intake manifold. Was falling apart and the biggest culprit for the high idle. Replaced with Ford part.
-Replaced PCV valve and hoses/elbow with new Ford kit. Elbow was falling apart. Also helped the idle.
-Replaced IAC valve. Did nothing.
-Inspected all vacuum hoses and other accessories that connect to the intake to check for leaks. Replaced a few questionable vacuum hoses.
-Cleaned MAF sensor. Had some dirt on it, runs a little smoother now.
-Cleaned throttle body, intake elbow, EGR valve, IAC, etc with throttle body cleaner.
-Plugs were changed out with Motorcraft units around 105k miles.
-Replaced fuel filter. Had only been about 25k miles but it was dirty! I used to run only Chevron gas, but with finances being tighter I go with who ever is cheapest.

Have the original coil packs. I've experienced issues with coils when the #4 was getting anti-freeze leaking on it from a bad intake gasket. It would miss under high-load, low rpms, aka 45mph in overdrive. Truck doesn't have any issues in that condition so I think the coil packs are good.

Also, the fuel mileage has recently dropped off. Over the past 6 years of ownership I get a consistent 15-16 mpg per tank with a mixture of around town & freeway. Lately I've been getting 13-14. Haven't done a freeway only tank to get a good benchmark as that used to get a consistent 17.

Anyone else experience this, or have any other ideas?

 

The only thing i would say is it is normal for it to jump,,, 200 isnt that much. When you turn on the a/c the engine needs more rpms. The other thing i would say is in the cold just go to vent and red in the temp gauge.

With all that youve done so far,,, id say dont worry to much. The cycling is normal for compressor.

 

200 is a pretty good jump. Mine cycles about every 3-5 seconds and rpms fluctuate a little bit, enough to notice but not 200rpms. Not sure what is considered normal. I think it is normal for there to be a slight delay in the idle compensation from the pcm when the a/c pump cycles.

 

The A/C circuit is supposed to signal the PCM of the loading.
If it does not, the result you now have is normal.
It's a delayed pickup when it should be a smooth contol.
Check out the harness for an open connection in the plugup..
Good luck.

 

If its never been done your due for cleaning the throttle body and the IAC.. It does make the engine run better weather it fixes the problem or not..

 

Looks like the IAC has been replaced and the TB has been cleaned, according to the original post.

 

Guess I did not read or comprehend!

 

So your comment makes a lot of sense and I spent a few hours dinking around checking wiring, connections etc.

I had a few things that were stumping me until I figured out that the Chilton's wiring diagram wasn't correct. I think the manual was written for the 97 models and they probably changed things by the time my 2002 was built. They show the (2) pressure switches on the A/C system hardwiring to the A/C compressor diode with a branch to the PCM. I wasn't getting any continuity from the cycling switch to the A/C diode which I couldn't explain.

I finally did a search online and found that Autozone provides wiring diagrams for free. Got that and it shows a different story that makes more sense. I can't attach it, so I'll describe it.

The A/C selector switch is in series with the (2) pressure switches and they provide an input signal to the PCM. The PCM has an output signal that fires a relay in the Power Distribution Panel (LH Engine Bay) which fires the A/C compressor. This makes sense so that the PCM has full control of the A/C compressor and can turn it off for other conditions like overheating, WOT, etc.

The fact that the A/C compressor fires and cycles normally tells me there aren't any wiring issues in the control circuits. That leaves an issue with the PCM or one of the devices between the PCM output and the A/C compressor. In this output are the relay & diode.

To eliminate the relay as a culprit I swapped one of the trailer lighting relays with the A/C relay. No difference. The diode measured the same resistance as the PCM diode so I assume that is in good shape.

So there is likely an issue in the PCM where it isn't adjusting the IAC valve correctly to compensate for the A/C compressor.

The PCM definitely had good control of the IAC as the truck idles up when cold and slowly decreases in RPM. Once warmed up in idles very well.

I did pull the PCM and all the pins were straight and clean so I don't think there are any wiring/connection issues with it.

Now if I could just find someone with a 2002 that will let me borrow their PCM...

 

Don't think that will work, I believe the programming is VIN-specific due to the PATS system.

 

I had a simular problem, ac compressor cycling and surging. In my case the freon was low, just enough pressure so the compressor would engage but after it ran for a few seconds the pressure would drop and the low pressure switch would shut it off then it would rise just enough to engage again and repeat causing an idle surge. Checked the ac system and confirmed it was low then recharged it and problem solved.
Mike

 

So your awhere, a Diode only passes voltage in one direction.
An ohm meter set to low resistance will measure a near short in one direction but very high or an open when the test leads are reversed.
An open or a short 'both' directions is a faulty diode.
Good luck.

 

Diodes should be good. They measure short in the direction of the arrow and 600ohms in the other direction.

 

600 ohms is not good!
Here is why:
A diode is a P/N junction.
It takes about .6 to.7 of a volt to force electrons to move from one side to the other as electron current flow in a Silcon P/N junction.
In the reverse, the electons are pulled back away from each side of the junction causing the very high resistance or no current flow.
A diode acts like a one way switch.
600 ohms amounts to just a low value resistor. This is not the action of a diode.
Sorry about that but this is how a diode works.
Good luck.