Weird Idle issue...

[buildingdoc]

Okay... here is the patient 04 expy 4.6L

NO Codes.
when engine is warm the idle surges. Jumps from 700-900 and also makes a humming/fog horn noise between the MAF and the TB... seems like the IAC is constantly opening and closing raising and dropping the idle rapidly. I had changed the IAC twice with 2 different brands. But honestly I think its fine but just getting a different signal every second or so and responds appropriately. Before the IAC I also change the PCV and all major vac lines as the usual culprits needed attention as well as the cats on bank 2.

I almost need a little help understanding what sensors send info to the PCM so I can start testing them to see what signal is constantly swinging the IAC open and closed... or atleast that's my thinking.

Any ideas would be really helpful.

Matt

 

[buildingdoc]

I really appreciate ALL the help.

 

[FordandPolaris]

Is your air conditioning on when this is happening, and does the A/C work? My truck does this with Freon in the system because my condenser is locked up. Extra drag drops idle each time the idler disengages on the A/C pump.

Otherwise have you cleaned the MAF?

 

[buildingdoc]

Ac on or off. Only condition this happens in is when the engine is warm. Cleaned the maf with the spray designed for it and followed instructions as I understand it can be a $100+ mistake. Just trying to figure out which sensor is screwing with idle control so I can replace it without throwing money at it

 

[Msport]

You can try cleaning the throttle body. The first time I did this it was real dirty, also took over an hour. Re-check the vacuum lines with the car running, spraying the lines with soapy water, any leaks you will see bubbles.

 

[toms89]

Not a simple answer as far as what sensor controls it.

There are many sensors, tables and adders that the pcm uses to determine idle speed. The pcm targets a specific airflow based on rpm desired via the tables and adders but ultimately monitors actual rpm.

It may be on the control end of the pcm and not sensor related. The IAC airflow adjustments may not be as the pcm expects.

My guess would be the throttle body as Msport mentioned. There may be carbon buildup around the throttle blade altering the intended min idle airflow.

I would also suggest resetting the pcm after you make changes so the adaptive tables are cleared and it relearns from the factory default settings.

 

[toms89]

If you want to know the pcm logic for idle control here is some of the idle functions with a few tables opened in the software from my SCT advantage program. I can only open one table at a time so it would be tedious to show them all. This is my custom tune. parameters in red are larger than stock, blue lower, black stock.

You may think it would simply monitor rpm and adjust the IAC accordingly but unfortunately its not that simple.

Its my guess that its due to the lag time of the motor vs IAC opening or closing event. The motor simply does not respond quick enough to changes in the IAC for a simple feedback loop. So it "calculates" where the IAC should be based on the software. To confuse it even more the pcm also uses ignition timing to steady idle as the motor reacts quickly to changes there.

 

[Msport]

I agree with toms89 in trying the PCM reset, as well as taking a look at the IAC. I have a 2003 5.4, and i have cleaned my IAC twice, its still the original part, yours may be filthy. And if you decide on the reset, give it a quite a few miles to readjust.

 

[FordandPolaris]

I agree with toms89 in trying the PCM reset, as well as taking a look at the IAC. I have a 2003 5.4, and i have cleaned my IAC twice, its still the original part, yours may be filthy. And if you decide on the reset, give it a quite a few miles to readjust.

I believe he has already changed the IAC.

 

[buildingdoc]

Thanks for all the info. That's pretty much what I was looking for. I was thinking there were just a couple contributing sensors (MAF, IAT, and O2 sensors) and then the IAC was adjusted to satisfy those sensors. I did semi clean the TB right about when it started by spraying the TB cleaner past the butterfly, but not removing it. I will tomorrow.

As far as the IAC... Im on my third with the same result. Just change it out on warranty every 6 months and keep extending the warranty.

 

[buildingdoc]

also thinking of doing the intake silencer delete tomorrow as well. Wonder if that may help with my intake fog horn type noise.

 

[buildingdoc]

Pcm reset via battery disconnect. No change and added 1500 miles with no change. Guess I will take it in to get scanned and maybe something can be seen on the monitors. Did the gotts mod and saw a pretty good increase in milage on my trip. Over 400 miles a tank and one of them was 85 octane that I assumed was 87. Didnt look until full. Other than my compressor taking a crap everything is the same.

 

[buildingdoc]

It was a poorly made IAC. BWD sucks. Get a Dorman or Motorcraft

 

[bedrck46]

when I removed and cleaned the throttle body on my 99 with 4.6 eng the was about 2 shot glasses filled with carbon deposits that I had to remove from the exhaust ports. Cleaning them made a big difference You may also want to consider a
ford workshop manual on DVD can be obtained from seller "morpheousking" on e-bay cost about $10.00 much better than Clymer or Chilton and also includes wiring diagrams