rpm's drop and sometimes stalls only at idle!

[mstrmp3]

99 4.0 ohv 4x4. 149K miles.

This is the symptom, sometimes only after stop and go traffic for about 30 minutes, at idle, the RPMs will suddlenly drop down to 200 and either come back to normal idle after a brief second, or just stall out.

Restarting is quick (not even a second) if it does stall. It is sporadic when it starts, but when it starts (like at a light) the rpms will drop, rebound, or sometime stall frequently.

Air temp is about 92+ when it happens.

There are NO codes! checked with scan tool, no problems found. even after disconnecting batt for 30 min, reconnecting, and running 4 drive cycles.

This is what has been replaced to resolve it.

spark plugs (motorcraft)
wires (duralast) however old wires were replaced at 80K and ohm'd out fine
new ignition coil pack
ECT sensor
IAC valve
PVC valve
new battery and connectors
upper intake gaskets
DPFE sensor
throttle body was cleaned and tps sensor checks ok
MAF cleaned (however it is stock to the truck)
Air filter
Fuel filter
CAM syncro sensor was replaced last year
Coolant flushed 6 months ago
all diffs fluid changed 3 months ago

If it was a fuel issue, wouldn't restarting take longer if the motor was starved?

Could it be an alternator/VR issue?

This happens in Park, Neu, drive, a/c on or off.

I'm about to go CRAZY!!!

***UPDATE***
I replaced the alternator after reading 13.2V at the battery. When I replaced it, the VDC was 14.4 at the battery. No more stalling at stops after 4 days of trying to get it to stall! Finally!

 

[joe doe]

sounds like the aic valve but i see you replaced it .was it replaced with new??i just had this problem and replacing the aic fixed it .

 

[mstrmp3]

replaced with new.

 

[joe doe]

i am still having this problem .i have now traced it to the fuel pump. today the truck ran normal all morning until the gas gauge reached less than a quarter tank ,then the bucking/stalling and loss of power started again .
i put my thinking cap on and said to the wife lets go put more gas in .

put 20 bucks in gas in ,bringing the level of gas up over a half tank .truck ran fine again after that .when i told my mechanic this he said fuel pump or fuel pump pickup has a hole in it .

sounds right to me .this is my next course of action .you can refer to my post about bogging and rough idle explorer for more info on my progress.

 

[mstrmp3]

I'll check that, but my tank is full.

However, I did check the volts on the battery while the engine was idling. I was getting 13.6vdc. When it stumbled at idle, the volts dropped to 13.3vdc. I'm thinking the alternator needs to replaced. (even if this doesn't fix it)

 

[Explorer Junky]

sounds like the aic valve but i see you replaced it .was it replaced with new??i just had this problem and replacing the aic fixed it .

sorry to be a dink, but, it's IAC not AIC

 

[joe doe]

your right dick....lol..j/k my bad.

 

[Explorer Junky]

your right dick....lol..j/k my bad.

HEY That hurt!! Bye the way I said DINK!! NOT DICK!! someone needs glasses!

 

[joe doe]

oh man my bad again......

sorry man...lol its been a long day.hey go check out my new fuel pump post and see if you can help.

 

[MakinJeepsWeep!]

Funniest post ive read in awhile haha