I did a swap in my 1987 F150 a while ago from a blown 302 to a 351W. The 302 was EFI, and the 351W was a 1994 EFI. I switched everything over, and except for some problems that were fixed with the mate-up AOD, I've been happy with it. Except for one thing, whenever I first start it up, the engine holds what seems like an extremely high idle......(900-1200 RPMs?) I don't have a tach though, so i dont know for sure. When I put it in gear though, it comes down to what seems a reasonable idle (500-700 RPMs), and it's fine....however, when I put it back into park and/or neutral, it goes back to the high idle again. I want to get this fixed, so hopefully some of you guys can help me out. Thanks, and sorry for the long-ish post.

 

ok, forgot something, since I wasn't having problems with it running, I kept the original 302 EEC Computer, could this have anything/everything to do with the problem?

 

I was also wondering, could there be something in the throttle body or something that is holding it open? The throttle lever sits on its rest fine and everything, but I'm just looking for some kind of explanation for this problem i have...

 

i have this problem too. and haven't found a solution.

 

is your problem also because you did the engine swap, and did you swap the comps?

 

high idle in neutral or park is USALLY an indication of a malfuntioning IAC Valve. my experience, at least.

Mike

 

Just curious, how does the AOD like the 351? Any futuristic/current problems/issues or is it all good?

 

Oh, I did the same swap in an 87, except in was a four-speed, and I had the exact same problem, switched to a 351 computer it ran better, but same problem, put on new IAC, and TPS, no avail, turned out, it was the dang throttle cable sticking. Plastic was chipped on the inside and held it up a bit.

 

I am currently looking for the 351 computer, but i hope the throttle cable isnt the problem, as it is brand new, all the orange plastic on the old one was toast. Ok, another thing that seems to help and that might help us find out what this is, is when i start it, or take it out of gear and it makes the really high idle, if i take the gas pedal and give it a little push and then let up, it goes back to a lower idle, then the engine idles at its lower speed, but it fluctuates, it sounds around 500 or so, revvs up to about 650 or so, then goes back down to the 500 and keeps on doing that... Thats probably the computer isnt it?

All the tranny guys I have talked to tell me that the AOD is not going to handle the higher output of that 351 for long. As of now, I have no problems except, because of the high idle problem, every once in awhile it bangs into reverse or drive if i can't get the idle down. I had to get it rebuilt though when i first did the swap because i couldnt get the throttle pressure cable adjusted properly and it fried the tranny. The only futuristic problem is that everyone says that stock, it cant handle the 351, so i might need to swap in a 5 spd in the future, but for now, its all good and i can get about 18mpg with that OD.

 

18 mpg? Not bad for a 351W/automatic. Heck I've got a 351/E4OD and it'll only pull down 16.0 -16.3 mpg consistently.

If it were me, I'd try the computer first, even though I doubt it'll help the idle, but when I did change it, the motor ran like a new animal, I used one from an F250.

Check for any vacuum leaks first and test your MAP sensor.

If that didn't work, then check/clean the IAC valve, since this would be my next guess, then the TPS sensor if the IAC didn't help.

 

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The 302 speed density computer will run a 351 but not correctly and you should change it to a 351 computer.

 

My advice....

Get the correct computor FIRST. If that don't help...

Then block off (at the plenum) EVERYTHING that gets vacuum that you can...I believe that there are 2 that MUST be connected to the tree in order for the motor to start: MAP and I forget the other one. If it idles properly, you know that you have a vacuum leak somewhere.

 

the other one would be the fuel pressure regulator

 

ok, well right now im in the dealings of gaining a 351 computer, so after i get it, i will let you know if that fixed the problem, and see if there's anything else that i can look for/at. thanks everyone for all your help

 

You said your throttle cable was toast. Do you mean like it got really hot? If so, check your engine to chassis ground cable and connections.