Jaguar Enthusiast Thank You for Reading My Post. I am Fortunate to Have a Beautiful 1998 XK8 That is an Absolute Pleasure to Drive. I've Had About 5 Years Now and I Love It.
My Issues is with The Cruise Control. The System Will Initiates But Doesn't Set. The Red Light on Cruise Button Lights Fine. No Codes on The Dash Display. Please Let Me Explain Initiates But Doesn't Set. After Pushing the Cruise Button on The Shifter Bezel The Red Indicator Light Illuminates Fine. Then When I Push the Set Button on the Steering Wheel the Tachometer Will Indicate an Engine Acceleration of Approximately 500 rpm and Then it Settles Back to The rpm That Which it Was at Prior to The Attempt to Set.
I'm Curios to Hear From Ya'll if These Symptoms Can Lead to a Diagnosis to My Troubles. I Thank You For Your Time and Wisdom. Good Day to All!!

 

Vacuum leak?

 

I Don't Hear Anything Leaking and I've Read Some Threads Referring to a Vacuum Leak. Of Course The Lines Twist and Wind Around The Throttle Body So Hearing May Be Difficult. I'm Going to Put a Pump on it To See if it Holds Vacuum. Most Probably Will Start Drawing Vacuum From The Vacuum Canister Under The Wheel Well to Verify The Lines Completely. What are Your Thoughts on the The Actuator Up Top of Throttle Body Possibley Being Stuck? Thank You Again for Your Response.

 

Please When Previously Refer to Canister I'm Speaking of Vacuum Reservoir Under Right Front Fender Thank You.

 

Looks Like Your First Jag was an Early E-Type
Very Nice!!!

 

The AJ26 engines use vacuum for speed control but AJ27 does not.
Black plastic lines run into the wheel well area next to the coolant atmospheric catchment tank.
Also check for broken TEE connector at the fuel pressure regulator. (common fault)

 

Is The Vacuum at Fuel Regulator Constant? I've Looked at That Tee Connector and Once Again Don't Hear Anything Leaking. Is The Vacuum Constant at Idle at This Tee Connector? Thank You!!

 

The fuel pressure reg needs LOAD determined vacuum.
The speed control vacuum actuator has a vacuum reservoir in the wheel arch for steady source.

 

Check your brake pedal switch. There are two microswitches in it one for brake lights, one for cruise. Common outer lever. This is operated by a bar off the brake pedal. Mine was not closing enough (perhaps 1-2mm) and that created havoc. Brake lights mainly stuck on (disallowing cruise). When I could get cruise, it would disengage after doing over a bump in the road. Repeatedly engaging cruise and touching the brakes would eventually put the car into Engine Failure Protection mode. My solution was to glue a small piece of 5/16 I/D plastic tubing over the lever-bar. Others say you can detach the switch and reset it by separating the two pieces of plastic and turning one a little. Can't do that with mine as it was maxed out against the internal stops. The tubing worked great for me. I know have brake lights that go off and a cruise control that works. A 20 cent solution worth considering...?