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Ok I need some help obviously...


2005 Express van with a 4.8L v8. This is not a factory optioned vehicle, I.E. it does not have the electronic throttle body system that is so common with these engines, it still has an old school throttle cable and idle air control motor.

Low idle, constant stalling. IAC ports and TB are cleaned.... did not help.

I decided to replace the IAC motor at my wits end, I somehow found a Standard brand IAC listing for a 2005 4.8 van on summit racing, ordered that part number locally, installed it, worked good for a few minutes, then same problem.

Here is where it gets odd, using Snap-on scanner you can go and change the requested idle speed, with the idle air control motor unplugged from the harness, the idle speed changes perfectly. What the hell is changing the idle speed? The TPS pid is still showing 0%, its not like something is pulling on the throttle cable and cracking open the blade.... Any ideas?

I'm getting ready to manually adjust the IAC motor, leave it unplugged so the vehicle will always idle around 600rpm, and ship it out. The CE light is always on for cat problems and this is a fleet customer known for a long time, so I know he would be ok with the light still on as long as the van can be put back in service. I really don't want to do that though, I want to fix this! Any help is appreciated.....

Let's start by identifying the vehicle. Can you provide the vin and date of manufacture? Are there any codes stored?

I believe that in 05 you could have either system. Perhaps the computer knows the iac is unplugged and is modifying other actuators to compensate.

What are the iac counts when the idle is too low?

Have you seen the diagnostic routines for DTC PO506 (low idle)?

I've had issues where other shops have been hacking away before it was brought to my shop. Look and see if the idle screw on the throttle body where the cable loop bottoms. If the loop stop looks bent or the cover is knocked out of the screw adjustment hole it's a sign it's been tampered with. There is a procedure to open the iac all the way closing off all air flow bottoming our idle and it should be a certain rpm. If it's low you need to adjust the screw. An improper pcv will cause a rough idle, low fuel pump pressure will do it as well. Fuel trim may be a tip off.

1GCGG25V751206542

02/05

p0506 was the code stored and reoccuring

Awesome, thanks.

Now how about those IAC counts? And can you command the idle with the IAC plugged in? Have you scoped the two drive circuits for the IAC. I suspect that one of the drivers may be bad (PCM). A scope test would surely show that.

You don't happen to have one of those IAC tools, eh? They were popular when IAC's were more abundant.

I do have the IAC tool, the motor is working properly when I command it to.

when it is working right/cold, I'm at 144 for IAC count, idle is 500 right now It'll drop down to about 410 when it warms up, and it is still requesting a 550 or 500 and will not adjust to it. If you rev it etc and let the idle drop down, it'll hit 200 or 300 and usually stall if your in gear...

Cool.

Those counts sound awful high. Typical count warm at idle should be around 30 ish. No issues with the throttle body or the base adjustment?

Assuming the wiring is ok from IAC to PCM and the TB is ok, you likely have a bad driver in the PCM

I'm starting to think PCM too, but its so odd that you can command a higher or lower idle with the scanner, WITH the IAC UNPLUGGED, I can't seem to grasp that concept.. maybe it is changing the timing on me?

Timing and possibly fuel? Not sure on that one and it is weird. But surely the PCM has the ability to know that the IAC is unplugged. Strange indeed.

If one of my techs told me they could command higher and lower idle through the scanner with significant changes like from 800 rpm up to 2 thousand with the IAC unplugged I would automatically think they had unplugged the incorrect sensor. Does this have a governor on it or add on cruise control? Pull the oil fill cap off and see how much suction there is. Plug off the PCV and see how much then. If there is a significant amount of vacuum there may be an internal intake leak.

Fixed! Map sensor was giving intermittent faulty readings

still have no idea how the idle was changed with the IAC unplugged, and yes it was most certainly the IAC unplugged and not another sensor...

It does have an aftermarket throttle body/cable/pedal setup... no cruise control. Glad it is gone!

Congrats on the fix. The map sensor will adjust the fuel to move the idle.