I am having an odd intermittent high idle problem with the 351 swapped in to my CJ. For the backstory, I had a ‘84 351 HO block/heads with the EFI setup off of a ‘92 5.8 swapped in. Ran totally fine, no driveability issues with the stripped down harness. I found a clean ‘94 F4TE block and swapped that in using my existing harness & computer. No other changes.

Now, sometimes when driving, the engine speed will “hang” when shifting and when it does return to idle, it idles at 1200-ish RPMs. However if I give it a quick throttle stab, it’ll fall right back to a normal low idle. Same thing if I disconnect the IAC - normal low idle. Cold starts have it idling fine and if I shut it down & restart while it’s high-idling, it’s back to normal.

I double checked the wiring from the EFI connector, everything seems to test out fine that I can see with an ohmmeter. TPS voltage is about .9 volts, timing is 10 degrees BTDC, and I need to double check codes again but I don’t recall seeing anything aside from codes about non-existent equipment.

What factors would make the computer command a high idle like that? The only things I can think of would be if I somehow got the cam chain off a tooth (although it runs fine, so maybe not?) or are the operating parameters of the new roller cam engine throwing off the ‘92 computer? I don’t want to shotgun a fairly rare-looking and expensive ‘94 manual ECU at it as a test.

 

When you disconnect the IAC the idle RPM should be very low (400-500 RPM) or the engine may stall. If the idle RPM does not change much then someone has either jacked with the throttle stop screw or you have a vacuum leak.

A TPS with a bad spot, i.e. "noisey output", that can trigger high idle RPMs while driving. If you turn the key to Off then start the engine again and the idle RPM returns to normal that does point to a bad TPS or bad connection.

 

Well, as a follow-up, I tore the top end apart tonight, put new manifold / throttle body / IAC gaskets in, and slapped it all back together. I realized when putting the throttle cable back on that it's just a *little* bit tight. Loosened the adjustment nut on it a turn and it has just the slightest amount of slack now...idles 'normally' & drops down to a very low idle with the IAC disconnected.

It's always the simplest things. Ugh.

 

Indeed that is true. Thanks for the update and good news!