Help, Bucking after acceleration.

[mustangdave]

Has anyone else had this problem? My 2003 XLT with the 4.6L V8 has developed a wierd problem. After acceleration like getting out in traffic or getting on the highway, when I slow down to a steady speed, the truck starts mildly bucking like it is running out of gas. Other times I will get a rough idle at a stop and my gas mileage is down to 14.8mpg. When this happens, I can also smell the catalytic converter. From researching I thought it might be the COP's, but it accelerates great, I would think it would break down then if it was a spark problem. I have cleaned the IAC and MAF sensor, replaced the TPS, COP boots and plugs. It has had a fuel system cleaning from a local shop. I used a HEI spark tester on all the coil packs and they all sparked. Has anyone else heard of this problem and what might be causing it? Any help would be greatly appreciated.

 

[MrBean]

any codes? when was the last time you change your fuel filter? Im a firm believer in Seafoam, i dont trust any shop doing a fuel system cleaning for me. i would recommend checking your back pressure too

 

[mustangdave]

No codes, i have a scanner and have checked them twice. I have not done the fuel filter yet. I just bought the truck a couple of weeks ago and took it back to the used dealer I bought it from. Their idiot mechanics are the ones who did the fuel system cleaning, but obviously did not check anything else. I was going to replace the filter next weekend, just haven't had time yet.

 

[HCCA 250]

With the higer milage vehicles, there is always the possibility of a bad set of cats. It might not be, but bucking, can be due to excessive backpressure. As well as decreased milage.

 

[mustangdave]

Fixed.. Had to have a computer hooked up while driving to see it. I had an O2 sensor freaking out. After acceleration, it would go full rich flooding the engine, then try to correct itself by leaning out and was going full lean. After a few cycles, it would go back to normal. The shop said they have never seen one act like that and not throw a code. New O2 sensors and it is running like a top. Thanks for the suggestions.

 

[tburns61]

did you have problems when starting you truck when cold outside? Mine acts up in colder weather, when doing low speeds or stopped it started back firing as well.
Plugs and Wires *check
IAC valve *check
Throttle Air Control *check

 

[MrBean]

thanks for the update mustangdave, that is really weird that the ECU never detected it

 

[MrBean]

tburns61, have your O2 sensor checked out as well, I almost guarantee you have the same problem as mustangdave