Mis-fire and bad gas mileage

[RedRocket]

Just like the title says. I have changed my cap, rotor, spark plugs, and wires, and it still has a miss fire. I also just changed my oil and fuel filter. The miss fire occurs through the mid range of the rpm, but only when you hold it steady. the misfire goes away under accelleration. My fuel psi gauge is not showing me any pressure drop, and the truck runs hard. does anyone know of or have had this problem? 97 4.3l sonoma oh yea, and it seems that when the engine gets to operating temperature, the miss goes away.

[Sparky2263]

If the truck has a mis-fire, the check engine light will be on. What code(s) is it setting?

[RedRocket]

I changed the gears a while back, and i have had my speedo turned off for a while. So, the check engine light has been on for a while, and i am sure that there is many different codes in the system.

[Sparky2263]

i am sure that there is many different codes in the system.

Okay. If you disconnected your speedo, the trans. is stuck in 2nd gear (default mode). So, that's not good. Unless there's more to the story. If there is, please spill it and we can move on with helping you.

Regardless, we can ignore the trans. codes (which will be many with the speedo disconnected) and still find out which cylinders are misfiring (or other problems) with the other codes.

So, what are the codes? All of 'em.

[RedRocket]

i did a 6-speed swap about a year and a half ago, and the speedo worked until i changed to the 4.10's. Im going to have to go to the store tomarow to have them read the codes for me. I also have gone through many caps/rotors that for some reason give me this same problem. but i just replaced the cap and rotor about a month ago.

[Sparky2263]

Has the distributor been removed? If so, was it lined up properly on install? There is a set procedure for doing that. A scanner will also show cam offset (read distributor) in degrees.

If it's not in properly, you will eat up caps and rotors.

[RedRocket]

that i know of, no. I myself have never messed with it. I bought the truck like 3 yrs ago, with 59,000 miles on it.

[RedRocket]

all the caps and rotors that i have replaced have no physical damage that you can see, but when i replace them, the mis fire went away.

[Rex]

Ok, I had the same exact problem with mine. I replaced everything cap, rotor, wires, plugs, all 3 O2's and nothing. Until I replaced the throtle positioning sensor, Bingo! That was the problem. hope this helps