88,89 Ford truck EFI

I combined two old trucks ending up with a 89 300 engine and manual trans in a 88f250. I transfered wiring, computer and all. It starts but only runs if I pump the footfeed. Seems to run best when letting off the gas not while accelerating.

Any idea what is wrong?

What did you do computer-wise? Is it running the computer that belongs to the engine, or the one that belongs to the truck? Even if the '88 also had a 300 in it, the computers might not be exactly the same.

If they are the same, you could have damaged a sensor doing the swap. Getting the codes read might help, especially if the check engine light is on (if it works) and what would be really usefull would be having someone hook it up to a full-blown engine computer reader that can tell you what all the sensors are saying-- a mechanic who knows what to look for should be able to tell you what’s wrong in minutes.

Also, from your description, it could be something as mundane as a vacuum leak or a fuel pressure issue. It’s a little odd that you say it only runs if you pump the accellerator (I’m assuming that’s what “footfeed” means)-- on a fuel-injected vehicle that should have little effect.

I switched the computer too, so it is the same one as the 300 had. I had checked the fuel pressure a while back, it was ok. This is a spare time project so it has been sitting there for a couple of months. Check eninge light works, it comes on with the key but it is off when the engine is actually running.