My friend's 1999 XK8 hesitates on quick acceleration. Any suggestions as to the problem?

 

-Check for OBDII fault codes
-Check for correct adjustment of the throttle cable
-Check for correct operation and adjust of the kickdown switch below the throttle pedal
-Repost in the correct forum (this one is for the 2006 and later X150 model)

 

A '99 has a throttle cable?

I would check the fuel pressure!

 

Yes it is a hybrid system..the throttle is driven by a servo, but the cable is mechanical backup in case the electronics fail.

Good point on the fuel pressure, should be about 41 PSI if memory serves. There is a schrader valve on the fuel rail to test pressure.

 

Early Jaguar 4.0 V8's had a problem. I had first hand experience with the issue with my 2000 XJR. This problem was particularly acute in the Midwest. I lived in Chicago at the time. Winter gasoline in the Midwest was provided with an additive that ate through the coating put on the cylinders and pistons. The result could be felt when the car was put under full throttle; it would feel like the car was missing or pausing.

My Chicago dealer identified the problem which was widespread at the time. The result - Jaguar replaced the engine in 2003 under warranty. Also, know that BMWs suffered from the same problem at the same time. A friend of mine had a 740i which had the same issue and same result.

Your friend's car is obviously out of warranty; but it is worth a shot to speak to Jaguar NA about it. Good luck