I have a 99 expedition with a 5.4L w/ 99K. I bought it from a small independent dealer 600 miles from my house. Before doing the trip to get the truck, I had the dealer take it to the local Ford garage. It checked out good. I went to pick it up and I test drove it and it ran excellent. I wanted to throw a bottle of Sea Foam into the tank considering I had a 600 mile drive back home from where I bought it. Good opportunity to burn some crap out. I went to Walmart to buy a can and the service manager talked me into a Gumout fuel system cleaning done. It was only $20. The sea foam was $9. So I let them do it. It idles like crap now. I checked for trouble codes and I dont see any. My buddy at Autozone checked the O2's and they're checking out ok according to his meter. It doesn't almost die which would suggest an IAC issue. It will just idle and I'll feel the truck shake from the engine. I replaced the plugs with E3's, fuel filter, installed new K&N intake system (Not for this issue, but just for the extra HP). The coils over the plugs looked good. One had a micro hairline crack in the white plastic filler on the top, but it wasn't open to let moisture in. I removed & cleaned the throttle body. Haynes said not to use throttle body cleaner, but it was pretty damn dirty so I did it anyways. I'll keep an eye on it to make sure it's always clean. I replaced the throttle body gasket. I Cleaned the MAF with MAF cleaner. I also replaced the PCV valve for the hell of it. I will mention that a lot of the rubber that the vacuum tubes fit into appears old and kind of brittle. Today, I washed the wheel wells of my truck. Oddly, after washing it and waxing it, I started it up and it didnt idle rough! After warming up, it started idling rough again. It usually always idles kinda rough whether it's cold or hot. What else could I try? Any ideas?

 

The Gumout could've loosened up some junk within the fuel system and now you have a partially clogged fuel injector.

 

Where did the Walmart connect to do the Gumout cleaning? They may have bumped a vacuum line loose somewhere or one of those brittle lines you mentioned may have a small crack in it, allowing a vacuum leak.

 

I replaced all the rubber elbows and straight runs in the vacuum system. I basically looked for all old dry rotted rubber and went at it. Oddly, 2000 RPM used to get me 65MPH... now Im at 70MPH. But still rough idle.

 

Thanks for the response! I bought the truck with 98k miles on it. I run shell 87 gas and sea foam in all my vehicles. So, if the previous owner used junk gas, I am sure to loosen things up. I've put on about 4k miles since buying it. It might be a good idea just to bit the bullet and replace the injectors. Alloro, how could I go about checking the injectors? There's a seller on ebay that has a fuel rail and 8 injectors from an 07 expy for $89. Mine's a 99, so they probably wont work. I have no idea on whether they are similar. I never ventured into replacing injectors. Closest I ever came was removing a fuel rail to access a hard to get a spark plug. I've even seen rebuilt injectors on the web for around $120. I'd spend it if I knew it would do me some good. Any ideas? Thanks!

 

With the engine running, disconnect the injectors one at a time and listen for any changes in the way the engine runs. If you find one that makes little or no difference, then you've found the problematic cylinder. You can test the COPs in this same manner. You will get a CEL on the dash during the test but it will go away once everything is reconnected.