Firstly, I love the car but it is starting to wear thin . Cruise is fine until even a slight rise in the road when it drops out and the FAILSAFE ENGINE MODE amber warning appears. This goes away on re-starting. I have had the brake switch replaced , the vacuum tubes checked and a 3 sensor pack installed but problem persists. When cruise first failed many months ago the engine also badly misfired and the car was undriveable.
Finally a "loose" contact was found in the ECM and this fixed the engine misfire. Don't know if the ECM issue was implicated in the cruise problem or not and I'm not completely sure vacuum might still be the cause. If not a brake switch or vacuum problem what else might cause cruise to drop out ?
More background, 5 years ago soon after buying the car the throttle body sensor failed and the whole unit was replaced. The car was originally delivered new to an owner in South Africa in 1998, then privately imported to Australia in 2000.

 

Happened to several of us. Look at the vaccum line to the port on the throttle. This is exactly what happens when it falls off.

cruise control on. As soon as car heads up hill, 1998s use vaccum to increase throttle. Since hose is disconnected, throttle cannot respond, cc gets disabled, engine failsafe message appears.

What would cause it to fall off? First sign of worn engine mounts. Extended travel of engine moves the throttle vaccum port out of the hose's reach.

John

 

Hi John, the vac tube from the elbow on the throttle body goes down on the right side and securely attaches to a fine metal tube emerging from near the wheel arch. The metal tub must lead to a vacuum port somewhere ? Can't see where. Motor mounts seem firm. I think maybe on suspicion, I should replace bellows in throttle body if this is not too difficult for DIY. What do you advise ? Thanks, Alfaguar.

 

Stand by I will photograph it.

 

Hi Johnken, I thought you were going to post a photo regarding the location of the vac tube from the throttle body. Maybe I misunderstood and the photo was for something else, however I haven't found it so far. Cheers, Alfaguar

 

Toward the end of this thread are several pics of where the cruise vacuum hose occasionally pops off a T-fitting on my 97. In case that’s at all helpful...

https://www.jaguarforums.com/forum/x...97-xk8-171776/

 

Alfagar, apologies it slipped my mind. Here's where it disconnected on mine due to engine mounts.

John

 

Thanks John for the photo showing the vac tube elbow connection which identical to my car. (Nice clean engine bay, by the way)
The other end of this tube has to be attached to a vac source via the small metal pipe that disappears into the lower RH side near the wheel arch and must then attach to a vac source coming from the engine somewhere. The question is where? I need to find and check as this might be causing a leak.
I guess the black "tin can" in your photo houses the bellows and this might also be losing enough vacuum on hills to disengage cruise? Cheers, Alfaguar

 

The vacuum solenoids are located in the right side wheel well. Vacuum is supplied by either a tee or direct connection to the throttle body right side. I don't remember which.

 

A,
when Cc acceleration is required, like going up hill, this plastic vacuum pipe applies vacuum to the black can causing mechanical movement of the throttle. If/when the car applies vacuum and speed stays constan or decreases cc is disabled, yellow warning appears on dash. All of this resets after ignition cycle. You won't see a problem again until cc tries to increase throttle (so cc: on, engaged, car goes up hill).

Later cars use different tech. I've seen this a dozen times here on 98's.

John