99 5.0L V8 Won't Idle on cold start

[Zero Signal]

My 99 5.0L XLT has developed a strange idle problem. I first began to notice it this summer on rainy days... and now that the weather has gotten cold, it does it every time I start it up.

Symptoms: From a cold start, the engine won't idle at all. It may hover around 200-300 RPM and sort of lope and sputter for a minute before stalling out. If I hold the accelerator down, it will run fine, but as soon as I let off, the RPM will fall back down to 200 RPM or it will simply stall.

Here's the unusual part... every now and then when I'm having this problem, I'll hear a little "pop" like a subtle backfire, and then the idle will return to normal (around 800 RPM).

After I drive it for 10 or 15 minutes and it gets warmed up, it will idle. However, it's still around 300-400 RPM and seems like it's on the verge of stalling.

Here is what I've done so far:
Cleaned and oiled air filter (K&N)
Cleaned IAC
Cleaned MAF
New plugs and wires
Checked TPS
Replaced fuel filter

I'm at a loss right now, guys. I can hear the fuel pump prime. I've even looked under the hood after dark to see if I could spot any arcing from the plug wires. I'm stumped.

 

[bluesman2333]

I would readdress the IAC. It might be dead/sticking. When it is loping and you unplug a small vac line, does the RPM pick up?

 

[Zero Signal]

I would readdress the IAC. It might be dead/sticking. When it is loping and you unplug a small vac line, does the RPM pick up?

I haven't tried that yet. The next time it starts doing it, I'll give it a try. Is there a particular vacuum line that I should disconnect, or will any one suffice? If it begins idling with the vacuum line disconnected, does that point to a faulty IAC?

Also, Saturday morning when I was leaving the gym I started it, heard the backfire, and it began idling just fine. Who knows how long it will be before it starts acting up again....

 

[bluesman2333]

I suppose any small one should do. What you are doing is providing a vac bleed to mimic what the IAC might do, although unregulated.

 

[Zero Signal]

I suppose any small one should do. What you are doing is providing a vac bleed to mimic what the IAC might do, although unregulated.

No joy. It started acting up this morning, so while it was sputtering at roughly 100RPM I popped the hood and pulled the small vacuum line off the top of the EGR valve.... nothing.

 

[bluesman2333]

That one won't work. I meant one with live vacuum.

 

[Zero Signal]

I ended up pulling the IAC off again out of sheer frustration. I cleaned both sides with AD electronics cleaner and used a few cotton swabs to clean out the gunk. It really wasn't dirty at all.... Then I used my pocket knife to pick at the spring and plunger assembly. I gave it a shot of compressed air to blow anything out that might be stuck in there.

Once I re-installed the IAC, she fired right up and I haven't experienced the idle problem since. This is the 3rd or 4th time that I've cleaned the IAC. I think that next time I have this problem I'll just go ahead, bite the bullet, and pay $50 for a new IAC.