So I been doing a metric buttload of work in trying to find the issue(s). 2003 f150 xlt 4.2l OVER 500K MILES. Multiple vacuum leaks it would seem (but that could be at fault of being deaf in 1 ear), as well as a/c fan clutch rapidly engaging and disengaging. With the ac fan clutch acting this way what has boggled me is that the roms actually go down when it engages when it never did that before, as well as a 100 rpm different higher than it ran prior to the replacement of the spark plug and IAC valve.

I have checked the intake mani with stethoscope, but can't hear any vacuum leak. Same goes for the isolator bolts. I know I have leaking hose for the ac causing a low pressure to kick the fan clutch on and off repeatedly. Just can't seem to find my issue.

Was originally throwing p0316 and p0301 codes (I believe those are the misfire codes) as well as p0171 and p0174, the misfire codes have since been cleared. It is now running either a p0511 or p1506 for Idle Air Control Circuit, as well as p0171 and p0174.

Upon acceleration, and only on acceleration, I hear a noise that sounds like using a air pressure sprayer coming from the engine bay or under the vehicle(unable to pinpoint).

Any advice would be greatly appreciated as myself, my grandpa, and my uncle, (all of us are experienced mechanics), have tried for about 2 months now to find the problem. It's becoming a stress factory for me as it is becoming expensive and it's really using a lot of fuel.

Also could it be the MAF sensor not picking up on a vacuum leak and causing a lean run error to the PCM, leading to an over-injection of fuel? Curious of this one because it's been on my mind. The only reason I haven't looked into it was because no indication of an intake manifold leak, upper or lower, and nothing on the isolator bolts either.

 

You might want to check the A/C cycling switch located on the accumulator.

 

Question is will that effect the idle at all or is it possible that would not need to be replaced until later. I don't seem to remember that ever effecting the rpms, but as always, could be mistaken.

 

Most of the time when the AC compressor is cycling it is caused by low refrigerant. A gauge set needs to be connected to determine the high and low pressure. Many just get a charging kit and read the low pressure and charge if needed. But, not knowing the high side pressure is a risk.
It seems from reading the post that you got the IAC codes after installing the new IAC. If that is true, get another IAC preferrably another brand and replace it again.
There is a Chris Fix video where he connects a shop vacuum to the exhaust pipe and reverses the flow and uses a bottle of soapy water to spray suspected vacuum leak areas in order to find the leak. A smoke test would be best.

 

We have replaced IAC valve 2 times 1 time motorcraft (current) , and the other was doorman I believe. No exhaust manifold/exhaust leak anywhere, have yet to do the propane/carb test on intake mani, and all A/C pressures are running beautifully on pressure. Hence my confusion as well as theirs.

While having/not having a/c does not necessarily concern me, the rpm increase upon shutoff does.

 

It might take a smoke test to find the vacuum leak if there is one.

The AC system has a high and low pressure cut out switch doesn't it. The high switch might be defective.

 

With that many miles I would:

1. run a vacuum test on the engine and see what you learn
2. run a fuel pressure test and see what you learn
3. test the MAF with a DVM and, if you have the tools, check its barometric signal
4.verify IAC is working by disconnecting the IAC connector for a couple seconds while your engine is idling. If your engine idle didn't change, you can suspect it to be the IAC. If it is working properly, unplugging it would cause your idle to freak out. Also check the wires going to the IAC connector.

 

MAF sensor is functioning properly, as well as IAC valve. Wiring going to/from IAC valve is 100%. Already ran leak down test to check for bad valve springs, as well as compression test to check HG. No issues on that front . *whew*. I, currently don't have the equipment to run a fuel pressure test but I have recently noticed a ticking noise, but not as deep as a lifter knock or a rod knock. Is it possible a fuel injector is giving me that noise from being bad or having a bad seal creating a vacuum leak?
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Vacuum leaks causing lean codes can occur in many possible places. Short of doing a smoke test you can try visual inspection or some form of spray to detect change in idle. Personally I would recommend a smoke test. If you still have the codes in your vehicle computer then you should also check the freeze frame data.

The ticking noise is probably a mechanical issue and can likely be traced with a stethoscope.

I would still run a vacuum test and a fuel pressure test.

 

Check the gap on the AC compressor clutch. If it's too wide it may not be holding. Chris Fix has a video showing hooking up a shop vacuum on reverse flow so it blows to the exhaust pipe and spraying suspected leak points with a soapy spray to find vacuum leaks.