Have A 1998 f150 2WD 4.6 with 98000 miles. No check engine light yet, been doing this about a month. PROBLEM - idles fine in P & N, but when in Drive say stopped at a light, I get a slight rough idle and it stays there (does not fluctuate) when I drive off , no problems with accerlation. I can only detect it when I'm sitting in the seat stopped, I have checked everything I can touch and do not hear anything in the engine compartment out of the norm, engine sounds fine. It's had a full tune up (all plugs, wires, and both coil packs) including fuel filter and fuel system flush. I have also replaced the IAC, Still the idle problem, vaccum tested the ERG, when vac applied engine drops and stalls so I assumme there clear. Any suggestions? Should I pull the upper TB neck and clean? Where is the DPFE located and how do I check to see if it's good?

 

Try Sea Foam. Put it in through the vaccuum hose below the master cylinder. The one that connects to the T attached to the big round thing the master cylinder is attached to. It worked for my 97. I know I'm not explaining it very well, sorry. Sea Foam has been around a long time though. Ask around. It pulls in almost instantly. Let it sit over night if you can, if it's not too cold. Then start the truck up in the morning. Be aware though, it gets REALLY smokey. Its worth the price of the Sea Foam just to see that much smoke come out of the truck, fill the street and **** off the neighbors.

 

Check the pcv hose. Sometimes they split. If it feels like a miss get a scan of the ECM for codes. Most of the time Fords won't set a misfire code unless it occurs at idle. Even though a hard code hasn't been set, there may be a pending code.

 

I agree, check the PCV elbows on the hose, the one on the rear of the intake is prone to cracking and sucking air. You might also try cleaning the MAF.

 

I have a 1999 Ford F-150 4.6l v8. I got a code a week ago about p0401(insufficient egr flow), so I changed the egr valve which the code remained. So I took the throttle body off an the piece that the throttle body connects too an which connects to the top of the intake. I cleaned the carbon off of everything an out the two little egr passage holes. Put new gasket seals on everything I toolk off that needed it. Plugged an hooked everything back up properly. No check engine light an no codes are popping up anymore. But now when I’m in park or I get to a stop sign or stop light my idle gets really rough An bogs down until it sounds like it’s going to a stall but it doesn’t . I’ve replaced the idle air control valve and the pcv valve(which said could be why it’s doing that am popping zero codes.) I don’t have any vacuum leaks and my air filter isn’t clogged up or hardly dirty. My o2 sensors are good. Does anyone know what it could be I’m so over it an I can’t find anything that works.

 

This drivetrain is used in the Crown Vic as well.

Check the intake manifold after so many heat cycles the plastics in the manifold become brittle and develop cracks.

 

Thanks for the help but I figured it out . I did a cigar test for a vacuum leak an found where it was leaking from an fixed it now it runs like a Cadillac. Thanks y’all.