OK my 99 4.0 has been idling rough lately. When i first crank it up it acts like it wants to shut off and sometimes does. But if i turn the key off and crank it again it fires and idles fine until the rpm's drop back down to around 700 then the idle goes to around 500rpm and it tries to die again. When accellerating it acts a little sluggish but otherwise no problems there. The other problem is when cruising at say 60 mph i can feel like a little bump or misfire every now and then. But after the truck has warmed up it idles fine and doesnt try to die but you can tell something is wrong. Within the last 10-15k miles I have changed plugs, wires, all O2 sensors, fuel filter and the IAC valve. There are no mods to the motor other than a K&N drop in filter. Any ideas???

 

Update: It's throwing codes P0171 and P0174 Systems too lean (Banks 1 & 2) Any ideas as to what this could be?

 

good luck finding out, im curious too, as i have been hunting this problem down since i got the truck. too lean means too much air not enough fuel so look for gasket leaks and that kind of thing, or not enough fuel pressure.

 

to lean most likely start with O2 sensors that should fix the lean, as for idle I would go with IAC.

it still misfires you might have the common cyl #4 and cyl #3 cracked head problem.

 

Changed the upper intake manifold gaskets this morning and she's idling like a champ! No more lean codes! Although it does still feel like it has a little "miss" in it i'm thinking it may be a bad plug due to it running lean for an extended period of time.

 

good 4 u, least it fixed ur prob. i did that too and lowers and didnt help

 

My truck has just become sick with this issue, still researching but here is ford's first round of checks.

 

Swapped the brake booster no CEL for 2 days now after the reset.

The booster was about half working, first half of travel had boost second half massive vacuum leak.