I hate to say it: if the problem clears up or mostly clears up once the engine hits full normal operating temperature, it's likely the pre-cat oxygen sensor. By unplugging the IAC you prevent the idle mixture from being adjusted in a vain attempt to get the sensor to stop indicating an over-rich mixture.

We're having a run on failed oxygen sensors lately, and that's why I hate to say it. It's like I've become the alternate version of the guy who jumps up and down screaming "plenum gasket!" for every damned symptom that crops up, and I freakin' hate that. It makes me look like the hammer guy who thinks everything looks like a nail.

Eh, so it goes.

 

^ hammer any nail u see stickin up. im lookin for answers. so anything pointin me in the right direction is greatly appreciated. and honestly the 02 sensor was on my list. because from what i read it only starts workin when the truck gets to operating temp. i really think i should replace it and the iac at the same time and see what happens. anyone got any better ideas? and thanks for the help so far.

 

^ and it doesnt start actin up til it hits operating temp. just reread what u said. so i dont know if what u said still applys please chime in if u think it does. thanks

 

I've been chewing on this one for a little bit and going just a bit crazy because of it.

First off, the easy part: I'd rule out the IAC because it passed the test with the scan tool.

Now for the rest, which isn't necessarily so easy. The failure you're describing isn't symptomatic of the most common failure modes of an O2 sensor, which makes me back up and think a bit.

It really narrows things down knowing that unplugging the IAC takes the problem away. Something is causing the PCM to frob the IAC to the wrong position, so we can forget about most of the vehicle. I'd expect cam and crank position sensor problems to manifest themselves all the time, or if heat related then at higher RPMs as well as at idle, and to cause codes to be set. The IAT sensor shouldn't be able to cause the problem you've got, and neither should the coolant temp sensor. The field narrows...

I'd expect a failing MAP sensor to cause a code, but every now and then they'll punk out silently. Did you give a look at MAP sensor voltages with the scan tool? It's a long shot, but we're looking at an unusual failure so it's worth a peek.

If the MAP sensor is good (and I suspect it is), given that the IAC motor has tested good, I'm again suspicious of the O2 sensor but hesitant to point a finger at it. If it were my truck I'd replace the damned thing just to get it out of my mind, but I get to be a little generous in the time vs. money decisions.

While I'm hesitant to point a finger at the O2, I'm having a helluva time coming up with anything else at which a finger ought to be pointed. It's in the electronic engine controls somewhere, and most of the controls seem unlikely to cause this problem. Whatever it is, I'm really very curious to know what it turns out to be.

 

well here is another wrench in the gear. apparently im losing cam and crank sync. it threw a cam code a few months ago and it was replaced. the problem persisted b4 and after the replacement. however there is no code for either just says its losing sync

 

crank sensor?

 

i was checking the data with the solus scanner and it just said the signal was lost. however i was driving the truck today and the check engine light came on. it went off so i pulled into the nearest autozone cuz i didnt have the solus with me and it threw 2 codes
p0320
p1391
and the truckk has been runnin fine since the light went off. but thats only with 20 mins of driving

 

P0320 No Crank Ref Signal at PCM
P1391 Interm Loss of CMP or CKP

That would tend to confirm the crank sensor.

 

omfg so you mean i was right!? w00t w00t w0t w00t

 

And the Crew Chief wins the PRIZE!!! When we figger out what that is, we will let ya know.......