Hard starts when hot

[mp470]

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.

[vjsimone]

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.

[sgauvry]

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.

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

[85lebaront2]

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.

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.

[mp470]

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

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.