Ive been having an issue recently with a fluctuating idle in my pickup. It was fairly intermittent but has begun to occur with greater frequency, the IAC has been replaced, all vacuum hoses around the throttle body have been replaced, and it still doesn't seem to want to work itself out. It most commonly occurs in drive at a stop light, with a steady rise and fall from about 500 to 750 RPMs, but those numbers have varied. At times it'll settle back down to a resting 500 rpm idle after fluctuating for a bit, other times it'll keep on going until the accelerator is pressed. Ive asked mechanic friends and been through all the forums so Im hoping a specific description may narrow it down a bit. Thanks.

 

Did you clean the throttle body?

 

Did not do that, but it didn't look all that dirty to me. After putting the new IAC in I realized that I failed to clean the actual port where the valve slips in to, and am having questions as if not cleaning that could cause these troubles. To try and paint a better picture, say coming off the freeway to a stop it rarely does it, but in stop and go traffic when the trans is shifting from first to second back to first its almost acting like it forgot where the idle mark is when coming to a stop.

 

The IAC (Idle Air Control) controls idle. The port it goes in to gets a lot of build up

 

Ill check that and clean it tomorrow, if that doesn't work are there other areas I should focus on?

 

Personally what I would do is take the whole throttle body off, then take the sensors out. And use throttle body cleaner to clean all of it. Now some will say use MOPAR sensors because these trucks don't like aftermarket sensors, but I never had a problem, it is hit and miss though.

 

If you do pull the throttle body but a new gasket for it!

 

If you own or have access to a basic engine analyzer, say one from walmart that displays current engine activity in real time. Plug 'er in n take 'er for a drive and monitor the fuel trims...especially the short term trims when the problem occurs. This will give you a big insight into the health of the engine parameter sensors (O2, intake air temp, coolant temp, fuel trims constantly varying at steady engine load could be a tell all about a hidden vacuum leak)...last but certainly not least is ye 'ol plenum leak.