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?

-Matt

PS: Sorry about the double post, but I can't seem to delete the other post, which was intended to be a response...

 

Guitar_Stitch:

Check this link out to see if you have a similar problem, we are thinking a possible cat. plugged on his truck.

1993 F150 ....

Another thing to do with the cat., is to replace is for a quick trip down the road with a straight piece of pipe. This would eliminate the cat from the system and any blockage that it is causing.

It could also be the Oxygen Sensor that is giving you problems as well. My '94 would bog while driving and then it would pick itself back up and go for a few miles and then start the process all over again. It turned out that once the truck was running for a little while the O2 Sensor would stop working correctly and start to dump major amounts of fuel into the system. Check the obvious stuff first to make sure that it isn't something crazy simple and get the codes checked. The check engine light will set codes that are stored for later analysis. Keep us posted.

Have Fun & Keep on Truckin'
Dean

 

Notice the second post says "This sounds similar to my truck". I posted that. I've noticed the ticking has gotten quite loud, but with the engine not smoking, and the ticking only happening under a Load, I'm siding with the cat. converter. Does anyone know if the thing is even neccesary in a state that doesn't do emissions checks? If not, I might just bypass it and save $150 bucks. If it is needed, then I'm gonna try to flush it out with some sort of brake cleaner until I can afford to replace it. I considered the o2 sensor, but the ECM doesn't throw any codes pointing to the sensor.

As of now, the misfiring has all but stopped, and the idle is under control again. Odd... It's also drinking less gas, and seems to have some power back. I haven't done anything to it to remedy any symptoms, except for the occasional 3K rev to "blow out the carbon." Maybe I did just that....

 

Update: I beleive it is infact the Cat converter. I had a really bad ticking develop but only under a load. At first glance, it sounded like valves, so I had someone load the engine while I listened around. It sounds like the exhaust manifold has blown a gasket.