bad gas mileage, roush idle

[bre's dad]

2000 4.0 4x4, auto tranny w/ 75k miles. In the last 8 months or so, gas mileage has dropped considerably from about 19/15 (highway/city) to around 10-13 regardless if in town or ont he road (sometimes worse on the highway than in town.) Its developed a very rough idle, almost a miss (bobbing as one mechanic put it) sitting at a light, parked or at crusing speed, sometimes more noticiable when the A/C is on. I'm on the verge of getting rid of the truck but want to make one last attempt to get this thing back where it should be. I've had 3 mechanics look at it several times and no one can find anything noticiable wrong. Had the fuel pressure regulator changed, but made no difference. Here's what all has been look at or changed:

• new plugs (OEM) old ones weren't burnt or in bad shape
• new wires (Bosch, will be replaced w/ OEM)
• new PCV
• new air filter (needed replaced)
• new fuel pressure regulator
• cleaned the MAF

All of that has been done in the last 6 months. Tires are stock 235/75/15 with no other mods, other than a K&N filter. Truck posts no codes and CEL doesn't come on (airbad light flashes and stays on, but that's new and another issue). Truck isn't driven hard, no jackrabbit starts, etc. The thing is killing me (yeah I know, if gas mileage is THAT much of an issue get a Honda. I need a truck.) In light of many other things I've fixed or replaced on it, we are considering getting rid of it but I'd like to make a lasat ditch effort on it.

I'm planning on putting OEM wires back on it to see if that helps on the rough idle, but I don't think it will. It tends to idle rough when its warm and runs great when its cold. I'm considering replacing the pre cat O2's, but am wondering if it would be worth the $100 and looks like a bitch to change (I can reach the sensors but not the connectors... any hints on hw to get to them?)

 

[BlackBII]

If you cannot pull codes, it's hard to say whats wrong, there are so many possibilites, you took it to three mechanics and the rough idle did not phase them? That's odd. I suggest pulling the codes again, autozone will do it for free, throwing money and parts at it is usually not the best idea, as it will cost you a lot of cash.
If it is missing or 'bobbing' it could be a timing issue, also if it runs great when cold and not when warm, it could point to a sensor of some sort, engine coolant temp. sensor, mass air flow, there are just so many things it could be. I do not know a whole lot about the newer rangers, as i work mainly with older vehicles. Someone with more tech knowledge will chime in.

How did this all start?