I have a 06 Express with the 4.8 engine, and it runs okay when revved or on the highway, but as soon as I come to a stop it stumbles and dies. It will idle okay when the a/c is off, but as soon as it's turned on it stumbles and eventually dies. I removed the IAC and MAP today and cleaned them, but it's no better. I tried finding a new IAC valve, but I cannot find the part anywhere, apparently it's part of the whole throttle body which is crazy.

And I'm not sure if it's just a coincidence, but these problems didn't start until I had a dead battery one day.

 

Any trouble codes? Noises under the hood? What is your system voltage with engine on, and off?

Things to try:
- get battery tested @ a shop (usually free)
- Vacuum leak test.

 

The only trouble code is one for the cat converter, but it was already there before this started. The battery is new, but there is a drain on it coming from somewhere, so I started disconnecting it when I'm not using the van for more than a few days. I did replace the rubber vacuum hose that goes to that ball below the battery because it looked old and cracked, but it didn't help anything. But yes, I can hear a whistle sometimes when I rev it up, like there's a vacuum leak somewhere, but it has been like that.

The volt gauge on the dash will stay around 14 as long as the idle is normal, but when it starts stumbling like it's going to die the gauge will fall pretty low. Supposedly this is not the original engine, and the van has 230k miles on it.

 

A clogged cat can cause loss of power. Bang on it. Listen if there's any loose bits. It could have been gradually failing and now it's pooched.

If one vacuum hose is cracked, good chance others components are on their way out. Do a full test with a smoke machine.

When you say the voltage falls ... how low? Maybe slip the accessory belt off, and see if the alternator turns freely.

Rockauto.com should have your IAC valve part.

 

it'll occasionally dip to around 1/4 on the gauge, when the idle starts stumbling with the a/c on. As long as I hold my foot on the gas, it runs perfect.

RockAuto actually doesn't sell just the IAC valve, they want you to buy the whole throttle body assembly.

 

The voltage shouldn't drop below 12V. If it does then something is pulling it down. If it drops low enough , the coils can't make enough spark and engine dies.

 

I think I may pull the alternator tomorrow and have it tested.

 

Just an update. The alternator tested bad and I replaced it. So far so good.

 

My idle issues started with a dead start. I disconnected the battery over a day or two and subsequently when reconnected it started the van but the engine would not idle normally until warmed up.
I had the alternator and battery tested and both passed.
I pulled the IAC and cleaned it and noticed in parts illustrations I find and one video that the valve of new IACs is in the shortened position.
So I am assuming mine is seized in the extended position and in need of replacement.

I see you may have solved your problem with a new alternator BUT if you had anything to share on the availability of an IAC that would be great.

 

If the throttle body looks like this….


Cable actuated TB - uses externally mounted IAC valve - connected directly to the accelerator pedal via cable.

​​​​​​…..there should be a replaceable IAC on it.

BUT if the throttle body looks like this….


Electronic TB - uses throttle plate for idle control via PCM commands - AKA Electronic Throttle Control or Throttle by wire - no direct connection to the accelerator pedal.

….you do not have a 4.8 - you have a 6.0 which does not utilize a separate IAC valve.

They can be disassembled, cleaned, and repaired, but depending on the parts you need, good luck finding them.