ok about four years ago my truck started doing this odd idle at startup. it fires up and goes toaround 1500rpm then drops to 500rpm somtimes 400rpm almost kinda stals out and then jumps back to 1000rpm and is fine, then it slowly drops back to 700-650rpm. now i say four years not ever day but maybe once a month or so and at times it wouldnt do it for months at a time. Well after being on here ive learned what to check and clean etc. Well imat the point that im tired of this happeneing and want it fixed! So far this is whative done

within last 12 months- OEM fuel pump, OEM spark plugs, OEM COP boots, none of this fixed the issue
past month- all parts are OEM, idle air control valve, fuel pressure regulator, mass air sensor, all new rubber line on the vent lines from the gas tank, fuel filter, air filter, i forget the name but the plastic fitting on the valve cover for pollution?, tube from IAC to intake (dont know what this is for). Also did a vac check and fuel pressure check all are normal


all i can say is when i start my truck about three seconds after when the idle starts to drop you can hear this SOUND from the drivers side front fender. Its hard to explain but it sounds like a loud click sound kinda like a pump trying to start but it doesnt? When i hear this sound thats when the truck drops below 600rpm and usually stumbles some. I can try to take a video of the sound but with the motor running its kinda hard too.....

 

also it does it running stock tune or my SCT tunes. Ive also raised my sct to 800rpm at idle and in gear just to see and even raised that high it still drops below and does it?

 

my new truck does the same thing, thought it might be normal

 

That's we're the vacuum pump is, it cycles a few seconds after the truck is started. when mine cycles it dims the interior lights if they are on just for a split second but has no effect on the idle speed. I've noticed that if you have your foot on the brake pedal when you start it doesn't cycle till I let off the brake. not that has anything to do with your problem but if you notice it doesn't make the noise till you let off after starting maybe you can narrow it down to the vacuum pump. Oh and mine is a 04 F-250 5.4.

 

Wow-I would have bet money on IAC. But you said you replaced or cleaned right? Not sure what it could be, but good luck.

 

Does that engine have an egr valve? If they open when not supposed to the idle will fail..
I would hang out under the hood and have someone start the truck to see if you can peg the noise to a part. The engine normally will try to maintain a certain idle even if something is amuck..As long as the iac is working.. There may be a sensor or sensor voltage which is being affected causing the computer to react to the input...The tps tells when the throttle is closed which signals the iac to control idle...Timing if retarded for some reason will kill idle. A massive amp draw on the alt could drag the idle down as could the power steering pump if something is up with it...
Try to find the noise!
Thats all I have for now..Good luck

Dick

 

i have an F350 so i have no egr as far as i know. there is nothing on the headers i had to plug mine with a stainless plug

 

my vacuum pump is on the passenger side not the drivers said. the only thing on the drivers side front fender is the canister for the vent from the gas tank, my fuse block, and a black box that has wires in it i think its the cruise control box?

 

yea i cleaned the old one with no luck then i said screw it and bought all the new parts from the Ford dealer even the big hose with the canister that hooks to the IAC i replaced! And it still does it ever once in a while......

 

i have five trucks to go and look at, my old work van 2000 E350 with 5.4, uncles 2001 F150 with a 5.4, girlfriends dads 2002 F150 with 5.4, brothers 2006 F150 with 5.4 and my 2010 E350 work van with 5.4.
All i know is that all these trucks start up normal and then SLOWLY idle down to 650rpm. My uncles started acting up last year but i told him to remover the IAC and clean it and its been fine ever since.....

 

Thanks, I will try cleaning this, this is my first gas pig in years, good luck finding out why your truck does it

Btw uncle si rules lol

 

yea duck dynasty is a OK show, i just wished they would actually HUNT on the show but o well. I still laugh watching ever episode!

 

OK this is my drivers side front fender nothing at all that looks like it makes noise but i defiantly hear that thudding sound i guess it could be the canister for the gas tank vent building pressure? Or maybe the brake booster building pressure?

 

this is the vacuum pump on the passenger fender




and i have no idea what this thing is, but it has a 1/2" tube hooked to the back of it and the one vacuum line off the bottom and then a two wire plug, is this a EGR on a gas powered F350?

 

now this is the BIG ? i found out last week. I was checking over the throttle body again and i noticed that the stop screw even though it has blue thread locker was LOOSE. i can turn it with my fingers in and out and then i opened the blade and it looks like the blade is digging into the body you can see the dark line where i have it opened up?? Could the blade be out of adjustment and is closed to much causing my problem??


throttle closed

opened up to show GROVE?