01 4.6 searching idle tried everything thanks

[Sadler]

01 expy 4.6 After driving for a bit and having truck at operating temp the idle will start dipping bellow 500 then up to 700 then a few seconds later below 500 and up again. It only happens when I come to a stop at a light or sign. You can see and hear the rpms dip as soon as the truck is about to stop. Doesnt happen in park. Its very rhythmic meaning i can almost count to when its gona surge again. Truck runs great otherwise except for an occaisonal light tapping sound which i think is just sticky lifters and engine ping occasionaly under load which i swear i get only after using hess gasoline.

-I have replaced the oem pcv in it with motorcraft pcv
-replaced the pcv gromet and the assembly hose including upper and lower elbows
-replaced IAC with oem from pepboys, returned it after a few days because there was no difference. (maybe motorcraft iac woulda worked?)
-cleaned TB
-cleaned MAF
-cleaned air filter
-seafoam..very little smoke
-tried cleaning EGR but couldnt get it off, ran intake cleaner through the top of it.
-power steering flush with lucas fluid
-one way brake booster check valve replaced
-techron
-marvel mystery oil via tank
-lucas tune up crap via tank
-oem coil packs and oem plugs done by my mechanic x8 cylinders

Any help would be appreciated because its driving me nuts, and of course when I go to my mechanic it doesnt do it. I am going for back surgery soon and am trying to get everything perfect on this truck over winter when i cant touch it. Thanks guys.

 

[firefighter407]

i would check all your vaccumm for leaks I sprayed all my hoses with some carb cleaner ( or anything like that) and listen for a change in idle. if that dosen't work check your o rings on the injectors

 

[jayhawk2014]

I noticed one thing not on your list was resetting the ECM by disconnecting the battery. Maybe too simple a fix, but it relearns it's idle when you do that.

 

[slo-ryd]

I noticed one thing not on your list was resetting the ECM by disconnecting the battery. Maybe too simple a fix, but it relearns it's idle when you do that.

Bingo. Adaptive learning needs to be reset even after cleaning the MAF
I don't remember the exact procedure, but I always remove both clamps from the battery posts and hold them together for a minute and let it sit unhooked for 15 min or so.

 

[Sadler]

Itl be hard to check for a leak because it idles fine in park. In park just the fan gets loud every few seconds like irs trying to cool off but the rpms r steady... as far as resetting the ecm. I have done it a bunch times after any change.. i didnt do it this time after new pcv assembly.. i will try it again though now that that 90 degree elbow is replaced and may have been the problem.. thanks for the reminder on the reset totally forgot

 

[Sadler]

Thanks

Thank you for your help, sorry for the delay just had back surgery.

Before surgery my mechanic took my truck home for a day and drove it about 60 miles. He noticed the idle too and said everything was perfect on the computer and readings and said he suspects a small intake manifold leak and not to worry about it till it gets really bad. I will be doing some more work and replacements soon when i am better to get the truck top notch since I am looking to move from NY to TX and want no surprises on the 1700 mile journey and would like the best MPG too. Thanks all again for your help.

 

[Sadler]

just wanted to update for anyone with a simillar problem.

My tranny just went on the truck at 135k. Had it replaced with a new jasper tranny warranty and all.

The surging idle is no more, it does not happen anymore so it must have been the tranny going that caused the surge in idle. hope this helps someone else out.