'96 4.0 v6 idle fluctuates

[ConnieGallas]

I just removed the IAC, throttle body and upper intake plenum and cleaned all the carbon build up off. That fixed the problem I was having with it taking 3-5minutes to idle down to 1100 rpm everytime I started it. Now it takes 1-2 minutes to find it's idle speed at 900-1000rpm but once it is put into drive or reverse the rooms go up to between 1600-2200rpms and stays there until I put it in park or neutral and then it takes another couple minutes to idle back down to 900-1000rpmsbut when I put it back in gear, there we go again racing down the hill. I'm not getting a check engine light.Two questions. #1 what could be causing this problem and #2 How many start/drive cycles does it take to trigger an OBD code? I reset the PCM after I cleaned my air intake components. Sorry for the lengthy post.

 

[Dirtman]

There is a idle relearn procedure that is often required after replacing the IAC but usually it figures itself out after a couple of drive cycles.

It's also possible your IAC is still sticking, they do wear out over time.

 

[ConnieGallas]

Actually I replaced it about a year ago.

 

[gaz]

Hello Connie,

Hopefully, recheck your vacuum hoses from the maintenance.

 

[Eddo Rogue]

I replaced my IAC with an autozone brand, it was bad out of the box. Replaced it again with a motorcraft unit and it solved the problem.

 

[Dirtman]

Unless you replace a part with Motorcraft you basically didn't replace the part...

 

[ConnieGallas]

Shouldn't the PCM put up a code?

 

[ConnieGallas]

Continuing from my, idle fluctuates... I replaced the IAC valve and the problem remains. While waiting for my new valve to arrive, I diconnected the wire harness from the current IAC valve. I have been happy running without it connected as I am not running or idling at such high rpms. Any suggestions of what I could try next?

 

[cbxer55]

Actually I replaced it about a year ago.

If you replaced it with a cheap $39.95 part from any of the auto stores, it's likely already bad. They usually go bad right about the time the one year warranty is expired. Auto Zone has a "Premium" part, made by Hitachi from Japan. I put one on my Lightning and Ranger in 2015. Both are still good to this day.

 

[Dirtman]

Continuing from my, idle fluctuates... I replaced the IAC valve and the problem remains. While waiting for my new valve to arrive, I diconnected the wire harness from the current IAC valve. I have been happy running without it connected as I am not running or idling at such high rpms. Any suggestions of what I could try next?

You have a vacuum leak. The engine should barely idle at all and or just stall out with the IAC valve unplugged unless air is getting in from somewhere else aka a vacuum leak.

 

[ConnieGallas]

You have a vacuum leak. The engine should barely idle at all and or just stall out with the IAC valve unplugged unless air is getting in from somewhere else aka a vacuum leak.

I have checked and double checked all my hoses and fittings everything appears good.Ive heard of a smoke trick for detecting leaks.I guess someone blows cigar or cigarette smoke into the vacuum hose that is connected to the brake boostér, then if there are any leaks, the smoke will escape from them.Do you know if this trick works?

 

[Broosedamoose]

I've never heard of that trick but a smoke test is what you need done.