Hey everyone, looking for help with thoughts / ideas on possible fixes to a “stumbling idle at start up”, I’m having.

First is info on my rig. 2001 6.8L 2wd with 168k. Stock motor. Y pipe, k&n, no programmer, 456 gears.

The issue I’m having is a stumbling at start up and maybe a very slight poor idle. Start the girl up and the idle goes to about 1000. Holds there for just a few ( I think this is called “duty cycle”) and then starts to drop down towards 700 but will stumble and hits about 400 and truck nearly dies. Then I guess the computer sees this and signals something, so the girl chugs for just a moment and then gets back to say 700 and holds fine. Truck runs fine, no issues at speed. So I’m thinking it’s a bad IAC or throttle body setting.? I’ve already cleaned iac and the mass air but no change. I’ve been reading that if I unplug the IAC (all accessories off) and start the truck , it should start and idle. This indicates that the throttle body set screw is good and the throttle angle is fine. On my rig if I try to start with IAC unplugged, it will not start. If iac is plugged in and I start , then unplug as it’s running, it will die. If what I’m reading is correct, this means the engine is only running / idling through the IAC. So not sure if I have a bad iac or if throttle body setting is not correct? A note to this issue, stumble has just recently started and no work or parts have been recently replaced. TIA

 

Throttle body does not control idle on the 2V V10, the IAC does. The computer is likely shutting you down because it can see the component is missing when the IAC is unplugged.

IAC sounds like it is worn out, or sticky, they have a screw in them that opens and closes the valve, kinda like a jack screw/shaft. If the threads on the screw are worn, gunked up, rusted, etc. it will stick, then pop loose, and then stick again randomly, and produce intermittent idle issues.

IAC is not expensive, go grab a new one and swap it on, if the problem goes away, boom.

IACs across ford vehicles on similar year ranges are largely interchangeable with some exceptions, if you have another ford vehicle within a few years of that one, compare the IACs and if needed, use the IAC off another properly working vehicle to test with. 4.6, 5.4 V8s from 96-04 should largely be similar, 3.8, 3.9, 4.2 V6s should also be similar up to 2008. Not sure if the Various 4.0 V6s are, never owned one of them, and they are completely different than the Essex (3.8, 3.9, 4.2 V6s) and Modular(4.6, 5.4, 6.8 2V V8/V10s) motors. Anything 3V such as most but not all 05+ is likely completely different, as they mostly use Electronic Throttle by wire which is completely different in most respects.

 

If the IAC is unplugged, the engine won't get any air and either won't start or, if already running, will die unless you manually crack the throttle. This is normal. Whatever it is you read, it is wrong.

Your symptoms are of a worn out or sticking IAC valve. Replace it and see if the problem is resolved.

 

Thanks for the ideas/info. I have gone through and ordered a Motorcraft IAC and the gasket. If it helps anyone the gasket that goes on my throttlebody is not the flat paper style it is Ford part number CG765. I’ll update once the items installed and see how she’s doing. Thanks

 

update:.. Parts cane in and installed. everything went back together fine but a new IAC and gasket. Sadly it did not fully correct the issue. It did maybe helped 10%. So I guess my IAC was ok , just a bit tired.?
I reset the computer and before I did this I checked for any codes... none... and I do not have a check engine light. After resting I took her out on the highway for about a 60 mile run. She did just fine, no issues,
mpg was reading 11.7 and all gauges reading fine. Very smooth and no issue getting up to speed or throttle response.

The issues just seems to be at "first / cold start up", once the truck is at temp, it does not happen. I am going to say its not a fuel issue in that at speed and highway, where the fuel pump would be working the most.
Thanks for the help and ideas....