Al A. wrote:
Hi all,
Sort of OT, but my truck, being a 1990 is still made of metal. And
lacking a NG called rec.crafts.metalNOTworking, I thought I might try
here. I also posted to alt.autos.for.pickups, but I almost always get
most excellent advice from RCM.

Anyhow...

I have a 1990 F150 XLT, 2WD, auto trans, 5.0L EFI, A/C, dual fuel
tanks. About 90K miles on the (2nd) engine. In the last week or so, I
have had a really intermittent problem. When driving at a steady speed
maybe 40-50 mph, the engine would "stumble" almost like it was cutting
out for just a second. It would do this 2 or 3 times, then run fine.
Otherwise, it ran smooth as can be. No "check engine" light. A couple
of days ago, this seemed to progress to happening more often and also
sometimes running very rough at idle, almost like it was trying to
idle way too slowly. Yesterday it got worse, and now will barely idle.
If I do not constantly work the gas pedal, it will stall (in or out of
gear). Now I am getting a check engine light, which goes out once the
speed gets up high enough for it to keep running. Once you are moving,
it runs fine.

I checked the really simple stuff, air filters, plugs, cap, rotor,
ignition wires, no wires knocked off of sensors, etc. None of that
changed anything. This has the "feel" (whatever that means) of a fuel
delivery problem, but I have no solid reason for saying that. I am an
OK backyard mechanic, but a slightly stupid as to how to diagnose EFI
type problems, or maybe even how to narrow it down.

Any thoughts on anything else to try before I (GASP!) relent and drop
it with my mechanic?
Yes. You probably haven't changed the PCV filter, and oil muck has been
sucked into the idle air control valve. (This doesn't perfecly fit
with the stumble at constant speed, but there may be more than one
problem. That one could be a dirty plug or leaky spark cable).
We had a 1993 Taurus, I think, that had this slow idle problem.
The Ford procedure is to replace the idle air valve-motor assembly
at about $150. I'd take a crack at cleaning it. You might be able to
get the engine vacuum to suck some solvent through it. Also, you
would want to start it up, and then let it idle gradually slower until
it stalls, so the computer would have made the valve go to the full-open
position. (I'm sure Ford had made sure you can't open the valve for
cleaning.)

Jon