Ford 4.2 idle

graygun · Dec 14, 2018

2003 F-150,5-speed...got it last June...169K now...generally runs well

Some Info:

idles with IAC valve disconnected (shouldn't it stall?)
IAC reads 11 ohms which is supposed to be good
IAC drive voltage is 14.55 Vdc (one source said it should be 10.5 to 12.5)
I cleaned the throttle body plate and IAC valve with CRC TB cleaner.
It doesn't seem/sound like it has a vacuum leak.

I don't have any service manuals for this truck.

...any thoughts?

graygun · Dec 14, 2018

I forgot to say...the idle fluctuates from 800 to 1,000 rpm. I realize 800 is a little on the high side.

I was trying not to do any shotgunning of parts. I can research some more.

Dan in MI · Dec 14, 2018

They don't always stall when disconnected. 10.5-12.5 volts just means battery/system voltage.

Check the PCV line. (not 100% about 4.2) on other Ford's of that era the elbows for the PCV get soft/mushy and leak. Most of the tube is hard plastic.

MAF voltage? Stable?
TP voltage? Stable? Engine off - Check resistance and SLOWLY open the throttle. Is it smooth? TP's will get bad spots in high use positions. Often highway speed and idle.

Also, check the clean air tube to Throttle body for leaks (post MAF- unmeasured air)

graygun · Dec 15, 2018

I'll check all vac lines and TPS operation. It doesn't stumble when throttle is engaged. I'll have to find out what the MAF should be,good/bad.

Thanks for the information.

Dan in MI · Dec 15, 2018

If the TP has bad spots it will only have in issue in that spot. It's just a rheostat so the wiper can rubout out a tiny section.

MAF voltage is the same range as TP. ~ 0 - 5v (VREF)

Oh I forgot this too. Your idle bypass could be very dirty. This not generally recommended, but it works and helps with diagnosis. Pull off the assembly (normally only two screws, and the wire connector) and look inside. It will probably look like a dirty throttle plate. Hose it down good with choke cleaner, both ports, drain it and re-install when dry.