Hi Guys,

Anyone have input on a "99 XJR high speed stumble?. It is intermittent, and happens at various road speeds, but always under full throttle.

It feels like a momentary lapse in fuel supply, if one can actually "feel" such a thing.

OBD scan shows no faults.

My first instinct is to replace the fuel filter, then work my way back to fuel pump. Then again, it could be some sort of high speed electrical gremlin.

Thoughts?

 

High rpm "misfires" are usually electrical, so the first thing I would do is change the spark plugs. It may or may not be the cause but it is a good place to start. It's easy to do and not too expensive. Removing the spark plugs also gives you a "window" into how the engine is burning, spark plug coloring/condition are a good indication of many things.

 

At full throttle, the second fuel pump is supposed to come in to cover the extra demand but it may be faulty.

 

Clean the throttle bore and plate; tighten all parts of the air tube between filter box and throttle body; remove any slack in the throttle cable; change the fuel filter; new plugs; clean the MAF sensor; oh well, it must be the secondary fuel pump . . . in that order.