I have a 91 F150 XLT Lariat, and recently I Have noticed the engine to be stumbling while at idle, soon after shifting gears. It is the 302 block. So far, I have put in a new IAC, new EGR, I have pulled and cleaned the throttle body, cleaned the upper intake, replaced some busted vacuum lines, and it still persists. Does anyone have any idea what could be causing the issue?

 

I'm a little confused when you say its "stumbling while at idle, soon after shifting gears." Can you be a little more descriptive? Manual or Automatic tranny?

I wouldn't throw any more parts at it until you diagnose it properly.

 

Whenever I shift into gear, especially reverse, the engine will drop to around 200 rpm and want to stall unless I work the gas. It's an auto trans, E4OD, with about 147k on both motor and trans.

I've done a smoke test and haven't found any additional vacuum leaks, other than the ones ive already corrected. Someone told me to check the plenum gasket, but I don't want to keep shooting the parts cannon at it blindly.

 

Fuel filter? Fuel Pump?

 

The pump and tank switch were both replaced less than a year ago. I did replace the fuel filter when I got it.

 

Ahhh ok that makes sense. I had not had enough coffee yet.

My knowledge with automatic transmissions is limited but I would first check the condition and level of your tranny fluid - Early E4OD's are really picky and they really didn't get super reliable until 1995. Stalling in reverse can also be caused by a dropped transmission filter....from what I have read on these forums.

 

I just did a trans flush with a BG synthetic trans fluid, but I haven't checked the filter. I'll have to check that out.