Hard starts when hot

The IAC is probably pulsed, so the ground is going on and off.  And your test light thingee is pulsing, but so fast you can't see the pulses.

Was KOEO as I've got nothing to backprobe it with at the moment without damaging the connector.

My '86 302 does exactly the same thing. I've been thinking IAC, but don't really know how to test one.
My guess is that they get sticky with age and the goo buildup that seems to happen with them. Is the problem usually electrical, or the valve itself?

If it makes a difference, my truck has been sitting the better part of the last 8 years. It's only accumulated about 6000 miles during that time.

I thought it was the IAC too and I fired the parts cannon at it (replaced without testing - mechnical anyway - basic electrical OK, ie power and ground), but it was not to be.  I cleaned the existing IAC extensively before replacing. Also sat for long periods as best as I know its history.  I've had it parked for our 6 months of winter so soon it will be back to the drawing board.

If you're still around, I just resolved it by adjusting the idle screw.  I was trying to do a KOER test and it would stall out.  However even after the stall and the engine type output, out came code 12 for idle RPM too high - tipped me off.

Just FYI - It is not the Idle Screw, it is the Throttle Valve Stop Screw.  The computer controls the Idle.  Adjusting this screw changes the electrical characteristics of the TPS.

The Stop screw adjustment is to keep the valves from sticking to the Throttle Body due to the engine vacuum.  Basically, it is adjusted at the factory barely against the Stop Arm connected to the throttle plate rod.

This screw should never be adjusted from the factory, unfortunately old school lads attempt to adjust an Idle by turning this screw in and out.

Thanks for the insights. I'm not sure what was done to it before I got it, but I'm slowly learning its quirks thanks to this forum.

Vinny, if it is a 1985.5 or 1986 the screw is adjusted for as close to 1.0 volts at closed throttle as possible, much more and it starts trying to open the EGR valve.

I don't believe the intent was to use that screw to adjust the TPS voltage, although you can.  
The early TPS had a slotted hole to adjust the TPS voltage.

This procedure can be used to fix a maladjusted Throttle Stop Screw.

But I would first back the screw out so that it is not putting pressure on the Stop Arm. Then adjust the screw inward so that it just makes contact, slight pressure with the Stop Arm.  This takes it back to close to the factory adjustment.

Then I would measure the voltage output of the TPS, adjust the TPS (if adjustable) to 0.6 to 0.9 V. closed throttle.

New TPSs don’t have the slots for adjustment, you can add slots by drilling/reaming the existing holes.

Check the idle rpm, If the idle is not as specified, I would try this procedure;
http://www.garysgaragemahal.com/efi-idle-speed-adjustment.html
Page F168

Re-check TPS voltage.

You may have to adjust this prosses a bit. I have used it and have been successful, others have as well, and some have not.

No tach unfortunately, but it idles a lot better than it did.  I may have gotten lucky and found the sweet spot.  Since the code 12 is no longer there, I won't mess with it any more for now.  

Yes, I was going to say you were lucky, and agree, move on to your other issues.

I wanted to point other readers to Gary's link.

It's tough to find a hand held tach that goes up to 2K rpm.
The newer Timing lights have a tach built in that will do the job.

I can tell you that, several years ago when my TFI module started to go bad, it would cause me problems exactly the same as the one you are experiencing.  I had no problems with cold starts, but once the engine was hot, starts were very difficult.  

Here's a guide as to how to test your TFI module, if you don't already know how to do that:

http://easyautodiagnostics.com/ford/4.9L-5.0L-5.8L/ignition-control-module-tests-1

Steve

Vinny, my son and I went through this issue with his 1986 F150, 5.0L EFI. He had purchased an aftermarket TPS and he was having idle surging, EGR solenoids chattering like magpies, two or three cold starts before it would stay running. What we found was, with the idle set to spec with the IAC unplugged we had 1.15 volts from the TPS at closed throttle. I had a 1990 Lincoln Town Car with the 5.0L and had problems with it.

There is a Ford TSB and modification kit and in it with the parts are instructions for setting the throttle for the 1.0V nominal and using the bypass screws in the kit to get the required 650 +/- 50 RPM with the IAC disconnected.

On the early systems, the EGR "open" point was at 1.10 V TPS signal so in my son's case the computer thought the throttle was opened enough for the EGR, EGR would open, RPM would drop, EGR vent would close the EGR, RPM would go up, EGR would re-open etc.

My EEC-V system and from what I have been told, later EEC-IV computers use the ign on TPS signal value as base for closed throttle unless it is way off.

Thank you, I do know about this site and have used it for other tests. I have not had any more issues after the throttle stop screw adjustment but I will keep it in mind for the future.