I have a 1991 F150 XLT Lariat with 235K miles on it. It has the I6 engine, and a 5 speed transmission. The truck sat for four years, then I exhumed its remains, and brought it back to life. From the point that I first restarted it, it has had a slight misfire at idle, accompanied by a popping sound in the exhaust that made me think the exhaust was leaking. Recently, the truck has started really sucking Gas, and at the same time, that misfire got really bad. For the past two days, I have driven it to work, and a new problem has come up. The engine will dog down and misfire heavily, and doesn't have enough power to get above 2nd gear. It runs fine at 2K RPMs in second, but will not go above 800 RPMs in 3rd. This is also accompanied by a CEL. After doing this a couple minutes, it just seems to clear up and run fine. I get no warning as to when that's going to happen. The computer tells me that the Knock Sensor had little change during the Dynamic Response test, and that the IAC wasn't working properly. Someone else told me that it sounds like the Catalytic converter is plugged up, explaining the popping noise in the exhaust. I know for a fact that this isn't plugs/wires/dirty injectors. I don't hot-rod this engine, and I have kept very good care of it for the past 5 months, but I don't want to drop any more cash into it if the engine is about to blow. Any ideas?

 

I know an easy way to tell if it is your converter. Crawl underneath and beat on the bottom of it with your hand. If you hear a rattling sound then your converter is bad. You can also drill a small hole in front of the converter and put a pressure guage on it. Start the truck, and watch the guage. As you race up the moter if the needle moves very much at all, then your converter is plugged.