Rough idle ONLY in park.

[Dr Mordekaiser]

So I've done a few modifications to my 1999 mounty. Headers new intake etc..
And I've got a few problems. The truck idles roughly when in PARK only. When in drive it runs like a champ. I do have a couple of engine codes. P0171, p0174, and p0401. First two are lean banks. Not too big of a deal I don't notice dirvailitu problems. The last one is an isufficient EGR code. Could that possibly be making it run rough only in park?? I fail to see how.

ANY help would be greatly appreciated. The truck is the v8 if that matter and only 96.000 miles. Thanks!!
Darren.

 

[C420sailor]

Idle RPM is higher in park, I assume because the engine isn't under load. Not sure why it would idle rougher at a higher RPM though.

 

[08Exploder]

Lean codes make me worried could have a faulty o2 sensor! Or issues with fuel pressure and or delivery egr could cause a problem if it's stuck open!

Tom

 

[2000StreetRod]

lean codes

Vacuum leaks have the most effect at idle when there is the least airflow thru the engine. As engine airflow increases the vacuum leak stays constant so has less effect. Its not a drivability issue only because you don't drive in park or neutral.

There are idle speeds calibration constants in the PCM tune for park/neutral and drive up to a settable speed.

EGR flow is disabled by the PCM at engine idle and WOT.

 

[Faerco]

I had this issue with my explorer. I'm going to recommend two things: replace the PCV valve with the Motorcraft version and check the hoses to and from your DPFE (Delta Pressure Feedback EGR) sensor. If it is the original sensor (metal casing, like it is on most trucks) it will overheat and fry the circuitry inside. This issue affects Ford vehicles ranging from Focuses to F150s between the years of 1996 to ~2004. I wouldn't worry about getting the sensor from Ford, you can easily purchase it from any auto parts store and it will work just the same and cost significantly less.

The PCV valve should clear up codes p0171 and p0174 (had these pop up on me last week). What happens is that the PCV valve feeds from the valve covers directly into the intake manifold. If it gets clogged with carbon or oil residue over time it will trip the OBD2 saying that there is already gas in the manifold, causing a lean situation.

P0401 is also quite common, as I said before. The code itself points towards the EGR valve being defective, but it is almost ALWAYS the DPFE sensor. If you want to be certain, after checking the hoses you can clean the EGR valve if you want, but I'm fairly positive it is the DPFE sensor.