Hi,
My 99 XJ8, 150k miles, is surging under light throttle, low speed or freeway. I've cleaned the throttle body, changed the plugs, and couldn't find any blocks in the breather system. Rarely the idle dips also. It isn't showing any engine codes (only traction control system fault).

Any help with this would be appreciated before I start throwing parts at it. Thanks in advance.

 

Any luck with this?? I'm having the exact same thing...

 

Have a look at your air ducting from the air intake box to the throttle body. The smallest crack or pinhole anywhere will cause this symptom. I had it when my engine would rev up and down without any input from me. I found a very small crack in the air intake trunking, taped it up and job done !

 

Mine looks new, but I will take apart and really inspect it

 

In my case it turned out to be corrosion on the throttle body harness plug. You couldn't see it, but when I cleaned it (very thoroughly with electrical parts cleaner) and put dielectric grease on it it fixed the problem permanently. I also found a vacuum leak under the throttle body. There's a plastic hose that comes into the intake manifold on the passenger side that had cracked. I'm not exactly sure which thing fixed it, I can't remember. If you havn't already it's not a bad idea to go over your entire vacuum system, these cars are prone to leaks and are sensitive to them.

Hope that helps!