OK history:
Truck(2000 5.4, 121k miles) started to have a low idle problem, generally at stop lights while in Drive. Idle jumped around 500-550 and sometimes would surge so bad the engine would die. While driving, basically anything past idle, it worked perfectly. The only mods to the truck are a CAI, Edge, and Magnaflow cat-back.

Currently what I have done:
-Changed all the plugs(motorcraft) and COP's
-Cleaned the MAF as best I could with CRC electronics cleaner
-Cleaned the TB and TB elbow
-Checked for vacuum leaks, even the 90 elbow behind the TB
-Replaced the IAC

What is happening:
So far the truck doesn't die, but the idle is never sitting in one place for long at stop lights. Sometimes it is at 700, sometimes it goes down to 500 and starts to run rough. If I put the truck in Park the idle goes up to 1000. This to me would indicate a vacuum leak, yet I have no idea where.

I am getting two codes P0171 Bank 1 lean & P0174 Bank 2 lean. I clear these codes out, and they come back within 20 miles. SO, I'm looking for ideas on where to go next. The stealership is last resort. I'm guessing this is something simple that I am over looking. I have not tried the propane trick for finding vaccum leaks, just a bit timid of exposing an explosive gas over a running engine...Any help is appreciated!:santa:

 

Intake manifold gaskets are gone or you seriously missing a few parts up top - take some pictures.. - Specially of the passenger side by your AC parts and back by the heater core hoses..

I don't know - it's that erratic??? Is it loopy or does it stay at a certain RPM for a while ? Did you relocate the wire harness or something?

Somethings goofed up.. - Check you hoses again - did you plug the IAC back in?

 

It stays at a goood idle for a while then drops out, but only when it is cold, generally on the way to work in the morning. Today on the way home from work, idle was smooth at about 670.

 

Ever changed the fuel filter? I had wack idle problems until I did.

 

Something wrong - With those codes , it's one of three things IMO -

1. MAF sensor - I would get a safty torque tool and fully expose the MAF to clean , just be careful. When you do this , you can see what your cleaning and your able to see if there's a problem with it as well. Plus , you can then use any cleaners on it - even carb cleaner on just the wires.. Windex works great for me.

2. A leak somewhere in the the vac system usually generates a high idle. A MAF problem usually generates a low or loopy idle.

3. Manifold gaskets - If they are leaking , the problem will most likley worsen when warm at the begining stages . They usually go bad towards the front of the motor by the water jacket - you also get pits in the head there that should get filled with aluminum filler..

 

That's a posibility as well !! Start with the easy stuff ..

 

Fuel filter was done about 10k miles ago, I'll try it again...

 

Ide check that maf

 

Did you change the pcv? When I was changing my stock one it seemed to be in good working order. But the hoses that ran past the pcv that go to the lower maifold were fraired. I would just change those too. At autozone you get the pcv for $4.00 and get a foot of 5/8 hose. Ben

 

Does the PCV hose need that foam surrounding it? The PCV passed the rattle can test when I checked it last week. It really only seems to have this problem in the cold mornings when the engine is cold. However when the truck is in park after being fully warmed and driven(i.e. when I pull into work in the morning) the idle sits at around 1k. At 121k miles should I just replace all the rubber vacuum hoses anyway??

 

I am having that exact same problem. I have a 99, 5.4 w/123k miles. The service engine soon light came on at the same time it started idling rough when cold. Too lean bank one and bank two. I have no idea why it does this but it seems worse if I'm on an incline with the front higher. It's not as bad if on level ground and definately better if the front is lower. I have no idea but I don't want to just throwing money at it.

 

That's an insulator - keeps the IAC from freezing up in cold climates. There's a TSB on that somewhere..

You know that white milky stuff that forms under the oil cap from condensation in the winter. That same stuff can turn to ice in the PCV line and that can stick the IAC momentarily. The IAC also can freeze from condensation. The insulator prevents that from happening.

 

RiJester did you figure out what the problem was? I'm still having it...

 

Well so far all I have done, is keep the stock program on the truck(have the Edge for gauges right now). I got codes a couple of times(0171 & 0174), cleared them out and just figured the truck needed to learn and adjust. So far, it's been over a week, and it idles in drive @620-650, in park it seems a little high around 900. I'm guessing a slight vacuum leak, but I do not have the hesitation anymore, or bad idle. It only happened first thing in the morning, when the truck was cold, and the outside temps were below freezing. So for right now I am leaving everything as is, just as a recap:
-Cleaned MAF
-Replaced all plugs and COP's
-Cleaned TB and elbow
-replaced IAC
-set truck back to stock program
-cleared memory(KAM)
-did a quick check of vacuum hoses, but never did see anything of concern

When I get some time, I am going to replace the PCV line, and some other rubber vacuum lines. Still a lot of cold weather months ahead so I will see what happens and update this if things change

 

Just want to throw this in there. My PCV line just past the PCV valve collapsed, and was barely noticable until i started moving the line around. The PCV was blocking, then unblocking creating idle problems- high idle for a few seconds, then back to normal (up to 1200rpm, then back to 750-800).
put a new section of hose on there, and the truck idles extremely smooth with no rpm jumpin around. so look at that. it is under the insulation.