Missing at idle - irregular idle RPM's
 

I have a '99 f-150 with the 4.6L Triton. Last fall I had some spark plugs foul out so I replaced all of them. About two weeks after that my truck started running wierd again, almost stalling when I would come to a stop, but then catching and reving back up to regular idle. I checked the codes back then and it said O2 sensor, and "running lean on bank 1". At the time I parked it and drove my other truck instead. Now I am back to the f150, and when I push the clutch in and let it idle down, it drops to as low as 400 rpms, then revs up as high as 1200 or so, before leveling back out around 7-800. But then at that idle speed I can feel the engine misfiring.
I just checked the codes again and all it said was "running to lean" again, no problems with O2 sensors.
I changed the fuel filter last fall and have only put maybe a couple thousand miles on it since then so I don't think it is that. Injectors are pretty pricy, so I'd rather not have to replace all of those to track this down.

Any advice would be greatly appreciated. All I know at this point are the IDI diesels, not much of a gas pickup man. Thanks

I'm going to guess, assuming the veh makes power on the road - you didn't mention. If so, you would worry about the injectors if you were getting rich codes, not lean.

You might have an air/vacuum leak significant enough to prevent combustion. You might even be able to hear the leak hissing.

I also guess that your IAC is probably working. It cannot get a good mix and an acceptable idle speed at the same time so the adjustments are fighting each other.

Oh yeah, it's mobile. I drive it to work everyday. I am just afraid of the day that it doesn't make it all the way. OK, so I could see where a vaccuum leak makes sense. Any tips to track it down? What is the IAC?

I oughta warn you I am still learning about Fords. But since nobody else has chimed in yet :)

IAC is a valve that is in a path that goes around the throttle body (to bypass it.) The throttle is closed at idle and the IAC adjusts air flow. Sometimes they stick in a place that lets too much or too little air in for idle. Sometimes they stick in a place that idles okay normally and then it can't move to compensate for a change.

Air leaks that matter are anywhere between the air flow meter and the cylinder intake ports. There are a bunch of things that come off the intake plenum to provide vacuum to other devices.

You can disconnect these items and plug the ports, generally without trouble:
Brake booster
4WD controls
Air conditioning controls (flap motors)

These items may cause fault codes to be set:
EGR control valve
EVAP control line
EVAP purge line

Unplugging the vacuum to the fuel pressure regulator will cause high fuel pressure. This will richen it up quite a bit.

A stuck-open pcv valve is a giant air leak. They are supposed to be free flowing at low pressure and restrict flow at high pressure.

You mentioned an oxygen sensor code in your first post. The front o2 sensors - before cats - regulate fuel mixture. The lifetime of an o2 sensor is about 100k miles. Then it starts to report to the lean side, causing the engine to actually run rich. However, they go bad in a way that gets worse over time. The exhaust usually plugs up (running rich clogs up the cat) before people notice idle issues. If an o2 sensor wire was broken, it would not be possible to warm up the truck without getting check engine light and fault codes about it.

From what I read, you need to go over things as a maintaince project, then work out any remaing issues.
You may have more than one issue causing confusion.
The IAC controls idle. The very low idle you indicated says the IAC may be involved.
There could be other issues as well.
Fuel flter, fuel pressure, water in fuel, IAC. vacuum leak etc.
Get these thing checked out to eliminate possibilities and keep from guessing the specific cause due to the age of the truck..

Alright, so this morning my ABS light came on. Does the ABS feed off vacuum as well?

The master cylinder booster works from vacuum off the intake through a check valve.
The ABS function is a seperate function and works by sensor wheel speed detection, and electric control, of system pressure.
The controller and hardware is just over the steering box/frame area and about under the fuse box area.

Thanks. I know the ABS does not directly run off vacuum. But as you say, the master cylinder boost does. So if I have a vacuum leak, that is probably why the ABS light came on as well.

So to find an air leak on a diesel I have heard you can spray ether on fuel fittings with the engine running and if idle increases then you have found it. Could I do something like that on my gas pickup? Has anyone ever tried that?

Ether is great, and it works the same way on a gas motor. It might not always pick up but you will hear the engine respond to the ether. I am hesitant to spray it on a plastic manifold (old plastic is old and I'm not a chemist) but that is my only reservation about using it.