Ok so this is the truck in my sig. 80k miles, no exhaust leaks, plugs, wires, cap and rotor and fuel filter in last 4k. I have a steadily worsening problem....under a load it started to buck....almost skip as it ran...just here and there. Now its to where it sputters heavily and missfires under medium to heavy load....empty truck too....faulty replacment part or missing something? Vaccuum lines replaced and timing is right....you name it, its probably been done....new 02 also.....any help?

 

Checked for codes?

 

I agree with Timbersteel.....good place to start is to check for codes, although it's a bit harder to find an OBD1 scanner to use these days. Another place to look are your fuel pump and ECM relays usually near each other by side of engine compartment. Even swap for A/C relay which I think is same part number to test out. Could be EGR issue too...sometimes clogged with carbon. If you are able to see if you haves codes or not would be nice in order to give a better idea where to look or what to rule out if no codes. BTW ~Even if check engine light is not on, there still may be "pending" or "historical" codes.

Also...have you rechecked the cap and rotor or a plug or two since the replacement? Don't rule out faulty parts. Even new from store there is always a possibility.

 

Swapped another cap and it didn't seem to help. EGR is new. Hasnt flashed any codes but I'll check.

 

Changed relays. No help there. Changed the rotor again....no help....its all low rpm....if its above 2k you don't notice it.

 

Have you checked to make sure all of the plug wires are in the correct sequence? Also check to make sure there is not a loose wire at the plugs, I bought a new set of wires for my 460 truck and had a similar problem, I actually found that one of the new plug wires was bad, the plug end was not attached correctly and it was causing a misfire situation, instead of firing the plug the spark was actually leaking out and firing on the head, very noticeable under load, but smoothed out at high RPM.

 

Well for craps and giggles I pulled the air cleaner and there I found a large mice nest. ..admittedly I don't drive it that much as its a leisure vehicle but it seemed to help. ...now to give the computer a bit of time to recalibrate and we'll see

 

It runs a bit better but still doing the same thing......

 

i just went through the same stuff... coil? Fixed me.

 

Its an msd coil....could be but when its doing this I can pull over and lay my hand on it and it isn't hot....so I'm not sure