I'm an admitted newbie to tune-ups so here goes...

Bought a '97 XLT a year ago with the 4.0L V6. Didn't need to do a whole lot of anything to it except filter changes when I bought it. Has run fine all this time and have been getting decent gas mileage (btwn 19-20mpg hwy).

Within the past two months, it's started to run rougher. The idle goes from around 2500rpm to nearly dying. Gas mileage is down below 17mpg hwy now.

Was going to change plugs, wires but is there something else I need to look at as well? O2 sensors? Air filter still looks real good, but should I think about changing it as well?

 

Check your IAC valve. 99% of the time, the idle fluctuation is due to a carbon covered spring assembly inside of it. It's very easy to replace and sometimes, you can take the old one off and spray some carb cleaner in the valve and bolt it right back up.

 

Removed and cleaned IAC valve. Seems to have helped a little.

 

It's more of a band aide for now, but it may just need to be replaced. You can also remove and clean out the throttle body with the carb cleaner. The ports that the air goes through and the butterfly get gunked up some and can cause the butterfly to stick and IAC to clog.

 

I've worked on several OHV 4.0s that needed intake gaskets. This has caused the fluctuating idle.

 

Been a couple of days now since I cleaned my IAC. Hasn't really helped the situation. Idle is still unsteady and gas mileage still stinks. Waiting for the weekend to kind of dive in and take apart everything for the air leading into the intake. Any suggestions what to look for, other than what's already been mentioned?