Weird hesitation at idle...can't figure it out.

[mmorningstar]

Hi guys, new on the forum, not at all new to cars. Got a problem with the wife's 02 4.0. Started last October, I changed the plugs to autolite platinums at about 45k right before a big trip, later in the day I had a carload of people and at WOT merging onto the freeway I got a misfire, I checked the plug wires and none seemed off, I changed back to new OEM motorcraft plugs the next day. A few days later we got an EGR code for large leak, the dealer cleared the code, said it was probably the gas cap, and it has never come back, but I swear it does not idle right. I've done the fuel filter, the air filter, checked the MAF and IAC, everything is good. It's driving me nuts. I've even bought a new gas cap. I can't believe plug wires would cause it? It's very intermintant, no rhyme or reason, you'll just be at the store, throw it in reverse, and it will stumble, but not stall, then it will be okay for a few days, and do it again, it can be warm, cold, long triup, short trip, it just happens whenever, no rhyme or reason...MPG is still decent, much better since I did the fuel filter, and no CEL's or misfires since October, just the weird surge.....

 

[E.B. Cornburner]

I had a similar stumble with mine when I first got it. I cleaned the MAF sensor and the throttle body with Air Intake Cleaner, and it went away. I have no IAC motor on mine, as it's got the electronic (drive by wire) throttle body. I just made sure to clean both sides of the throttle blade and bore real well. I didn't give up till the whole thing was shiny again.

Re-reading your post, I'd start hunting around for vacuum leaks too. Listen around while the enigne is running, and if you can't find anything, listen while the enigne gets shut off. You'll need a friend to help with that. I've found some real tiny leaks that weren't audible with the engine running, but I could hear them for a second as the engine shut down. There might be a leak in the vacuum hose going to the canister purge valve, or such since you mentioned an EVAP code being set. But then again, I've seen gas caps do that too. EVAP codes suck...I hate trying to diagnose them.