I recently bought a 2001 dodge ram sport 4x4 with the 5.9 and I have a slight misfire at idle and at low throttle. This seems to get a lot worse when I am at a slight decline and has a tendency to jerk a little and backfire. If I let off the throttle or give it more throttle on a decline it goes away. I have replaced coil pack twice, pick-up coil, cap and rotor,champion copper plugs, premium wires. I have checked fuel pressure and it stays around 50 but when you shut off the truck it loses pressure quickly. I have inspected the plenum through the throttle body and it looks clean for the most part and I only noticed a little oil on the very outside. I did however spray seafoam through the throttle body so it may have cleaned it out a little. I have also ran sea foam and other injector cleaning system chemicals through it. The old coil and new coil both arc at the very top of the coil pack. It gets brighter at higher rpms. The only codes I get are random misfire and a couple cylinders when it does go off. Each time its different cylinders as if its jumping around. I have checked the distributor itself to, to make sure it didn't jump around. Some days its worse than others. The truck also doesn't lack in power at all.

 

How many miles on the truck? How old are your O2 sensors?

Might try unplugging the front O2 sensor, and see if behavior changes.

Welcome to DF!

 

185,000. I have not tried unplugging it no. are you think a clogged cat? its not lagging in power at all.

 

I am thinking the O2's are old, and lying to the PCM. What you may be seeing is a lean-burn miss. Disconnecting the 02 sensors take them out of the loop, and the PCM uses tables to determine how much fuel the engine gets, which tends to be a bit rich. If disconnecting the O2 sensor makes the problem go away, replacing them will solve your problems for you.

 

I will try that. Will it still have that slight misfire or will it just burn rich and hopefully run smooth when you pull the 02 sensor(being that is the problem)? Thanks for your help so far by the way!

 

If it's just running lean, disconnecting the O2 should clear up the miss.

 

I pulled the front O2 sensor and it didnt solve it. However it did make it not misfire as bad or as much. It was running at higher rpms than usual.

 

Wonder if your cat is going south.... Sure sounds like it's time for new O2 sensor though....

How's your plenum?

 

I have looked down the throttle body. It looks really cleanfor the most part. I did see a little oil line on the very outside drivers side. I did run seafoam through the throttle body though so it may have cleaned the center really well.

 

What really gets me is, when you are going downhill at very slight throttle is when its the worst. I beat on the truck the one night thinking maybe a possible catalytic problem but it didnt turn cherry red.