Hi y'all, new to the forum. I recently bought a 96 f150XLT i6 5spd. When I bought it it had a bit of a high idle issue and I figured it was a good deal. Anyways, one very expensive head replacement and about 70 hours of resealing later, I'm back to tracing the odd throttle characteristics. I'm idling at around 1000-1100 rpm consistently after warm-up, and getting some throttle stick (generally holds rpm for 3-5 seconds. Already replaced the IAC and connections, and get this, if I unplug the IAC, the sticking goes away but the idle does not drop! My mind goes to vacuum leak, but the IAC having something to do with it is odd especially given that it is practically brand new. Curious if anyone can help me spin up a theory.

 

When you say throttle sticking, what do you mean exactly? Pedal slow to return? Binding and then release? If so, did you clean the throttle body before install? Inspect throttle cable?

 

I did, and the throttle is actuating smoothly. Basically, if I tap the accelerator the rpms increase and then hang there for a few seconds. If I push up to a higher rpm I get an an immediate drop to around 1500-2000 and then a few seconds later a return to around 1000.

 

If you don’t do the second tap, does it always stay there or will it drop back down to 1000 after a few seconds? Basically, do you have to get the second tap before it will drop?

Have you tested the TPS?

 

I'm not sure what you mean by second tap. I'm talking 2 separate events. It doesn't really matter what I do with the throttle though, it's always slow on the return. And not gradual, it holds, then it comes down. Seems typical of a stuck IAC but again, brand new motor and connections. I'm also some other intermittent weird stuff like rough patches with misses and high fuel trim. Other times it runs nearly perfect but the idle won't come down below 1000. Funny enough I had it running around 600-800 before I put the exhaust and O2 back on but I never made any adjustments. Just a lot of weird, intermittent bugs that all seem to suggest different issues.

 

Any codes?

 

Bank 1 lean code. Same bank that occasionally runs high trim. O2 sensor voltages seem normal and issues are not happening concurrently.

 

If you think the O2 itself is good, check out injectors?

 

All new injectors with rebuilt harness!

 

Sounds like an intake leak to me
Have you checked it for leaking with brakleen et al.?
Have you checked the torque on all the intake bolts and verified none have fallen out?
On a 1996 F150 with a 4.9 you can check the fuel trims and a bunch of pids with a NGS