My engine bucks intermittently when driving. My engine light recently indicated a bad catalytic converter and so I replaced one of the catalytic converters thinking that perhaps this was also the cause of the Bucking. Not so, the engine light did turn off but the slight bucking remains. It is not a constant bucking, it is intermittent. Thanks in advance

 

Can you provide some of the maintenance history and miles etc.
Reason I ask , sometimes a misfire can be described as bucking.
Misfires can have several causes plugs, coils fuel quality,quantity,alt, to name a few .
Misfires that are ignored can eventually take out a converter, so you can now see why past maint is helpful.

 

I have 309000 miles on the truck. I change the oil with synthetic every 4000 I use my OBD2 to monitor the engine and have probably replaced my coils at least three times all the way around. I have changed the spark plugs probably three times and my last coil swap outs were number four and 5 they both went within a month of each other prior to that I probably didn't change a coil for 6 months.

 

By intermittent,, is it engine load dependant as far as throttle angle/pedal position? Good, idle and or mid range drivability, and misfire/buck when your more deeper into the throttle? Fuel pressure/volume spec?

 

It is happening between forty and forty-five mph when I'm transioing form cruising to accelerating and again sometimes between 60 and 70 just when in accelerating.

 

If you can get the engine to do this when you want it to , I would suggest to have someone with a graphing tool to monitor certain functions at the time . There, the possible cause should become apparent and steps can be taken to go right to the problem. The care you are providing is commendable and does pay off in the long run.
Not knowing where the coils were purchased and if it is always the same cyls might provide a clue without testing, but there are a lot of possibles and just guessing gets expensive and sometimes disappointing too.
Here I am guessing the "bucking" you describe may be caused by an ignition misfire and the best way to confirm that is to observe the performance of the system. Something as simple as an ignition switch with intermittent contacts can be dropping out . Have even seen where the operator had a large key ring with heavy keys do this. Would like to have been more helpful, but you guessed it,,,, still guessing!

 

Thank you!

 

Sounds like a "DEAD" spot in the TPS which causes the CPU to shut down the injectors. Check the TPS with a DVM and Ohm for a direct short when sweeping thru the TPSs full motion.