Hi, I have a 1994 PU with 118K 4x4,this truck has been running extremly well, MPG is 18.5 on recent trip with plently of power from 2000 to 3500 RPM's. As soon as I got back from a road trip of 350 miles it started to run very bad. Idle has drop to 500 and a strange popping noise comeing from the tailpipe, and bottom end torque is gone if it goes below 2700 RPM's. Changed cap & rotor wires, plugs still no help, now it's a process of checking everthing EGR,PCV and vacuum anybody have any ideas let me know. Thanks

When was the timing belt done last? One of the cams could have jumped a tooth.

Fuel Filter?

The timing is spot-on, belt has less than 30k on it. Fuel Filter is brand new less than 6 months. Changed plugs and finally ENGINE light came on now. I might be getting closer to the fix. 2plugs had a oily deposit on them, 1 tech at my Toyota shop said he could smell Diesel fuel and just maybe I could have put in some gas that was contaminated with a small amount of Diesel, I have discounted that as being highly unlikely. But who knows, I would think that all plugs would have the show signs of bad fuel. Could this be a valve seal leak fouling the plugs just enough to cause incomplete combustion? I getting the diagnostic P-code and should get a little closer to figuring this out. Thanks to all