On the way home I was sitting at a light and the truck started to idle roughly. I put in Neutral so not to stall. I took off from the light and the truck started a pretty violent shimmy at about 40 mph. When I gave it more gas it went away but if I got off the throttle the shimmy returned.

No CEL, plugs and wires are new and I don't hear any knocking when under the hood. Anybody got any ideas?

 

What gears, and what size tires? Does it get worse when the truck shifts to O/D?

 

Stock size gears and tires. I turned OD off and problem still occurred at 40 mph.

 

How many miles on the truck? Original oxygen sensors?

 

Right around 120K and I believe they are the originals. The guy I bought her from didn't necessarily keep up with the maintenance. I had a CEL light that came on yesterday and believe it or not it went off when this problem started. When I got home and put my code reader on it there was no codes.

I think i may have had an Oxygen Sensor code pop up at one point. Now that I think about it.

 

Try disconnecting the front O2 sensor, and drive it for a bit. Is the problem still there? (this WILL set a code.)

 

Where can I find that one? Would that be the upstream? Do I need to take the sensor out all together or just disconnect the electrical?

 

Yeah, screws right into the front of the cat converter. Connector is up on top of the trans. Just disconnect it. That will remove it from the equation, and the PCM will just run the engine by tables, no mixture correction from the O2. (if the O2 is giving false readings, the PCM has no real way of knowing.....)

 

Thanks again for the help. So if it runs fine unplugged then the sensor is failing and giving false readings. If it runs like crap it's the CAT?

 

if it still runs like crap, we need to look for a different cause. Bad cat would give different symptoms. (no power from x rpm and up, etc.)