Firstly, Thank you for adding me to the website. Now, I bought a 1994 F150 with the 4.9 EFI. The kid I bought it from apparently made some mods most importantly he converted it from a automatic to manual transmission and the wiring harness under the steering column was well lets say rigged to bypass the park/neutral position.

My question is: The truck idles high (almost every imaginable part has been replaced). It runs great until it gets up to temp then when you floor it the truck almost falls on its face if you back off the pedal a little it'll run fine. I can't for the life of me figure out what's causing it? A friend suggested I had too big of tires and when it gets warm the motor loses power and can't turn the tires over which i get but it's sporadic in which gear it will do it in for example its mostly 4th and 5th gear but sometimes it does it in 3rd. Another thing I noticed is the temp and alternator gauges are sporadic in operating and it seems (unless it's my wishful thinking) that when the gauges are working it doesn't fall on its face but i'm not a 100% certain? I know this is a very loaded post but I've read through prior post about the issue and tried everything suggested and still no luck? I'm tempted to just by a turn key crate motor complete and drop in but before I do figured I'd ask as the truck is only used for hunting and not a daily driver. Any questions to give a better answer are welcomed and any advice is welcomed. Thanks again for the add and opportunity to post my trucks issues.

 

The truck has EFI so it has a computer / ECU to make it run right.
Now I would say pull the codes but being its had a transmission change from auto to stick I think the computer could be confused as its fora auto not a stick.
Can you swap in a stick ECU and have it run right? I dont know and I dont know if I would spend the money to find out till I pulled codes just to see what the ECU has to say.

I also take it you have checked for any vacuum leaks and fix them.
I would also go through and check all the grounds and dont forget the ground for the ECU.
Dave ----

 

I did run the codes and the only thing I couldn't figure out was a KOER low idle? I'm still checking on that one. The other KOEO was truck was not in neutral. Unfortunately, I have no vacuum line diagram other than what I've found online and it wasn't complete. I do know that some were definitely in the wrong places. I'm going to go look at a fully running truck and copy the vacuum line locations. What's weird about this is it almost feels like it's completely losing power i.e. electrical power when you floor it and then have to ease out of the gas. I'll see if the boneyard here near me has an ECU for a manual. What the kid did and jumped the wires to the park/neutral switch or should I say spliced them together? I'll rerun the codes again now that I've fixed some of the vacuum lines. Thanks for the insight.

 

I would also check the fuel pressure at idle and when it does its thing.
If you pull the hose off the fuel pressure regulator dose the PSI change and was there gas there?

Thing is you dont know if this started before or after the trans swap?
Dave ----

 

I checked fuel pressure at idle and while driving and there is no fluctuation at either end meaning at idle it’s in range and under load it’s in range. I’m tempted to re wire the whole truck it acts a lot like an electrical issue. I can’t find the ground for the ECU. Any idea as to the location? Thanks

 

On the 80's EFI trucks I believe it is over by the battery and that is part of the problem as acid get to it and causes issues.

I dont know the pin out of the computer but if you can find what one it is you might be able to ohm it out to see if it is any good?
Sorry I cant be of more help
Dave ----