Cruise control drops out on hills

With my 1998 XK8 cruise control works fine on the flat but drops out on hills giving the fail safe engine mode display. The brake switch has been replaced and my mechanic has checked that the vacuum tubes are ok but the problem persists. The issue seems to be to do with vacuum but the mechanic has admitted defeat. Suggestions please ?

Well the whole system depends on a vacuum generated by manifold depression which will be less when the throttle is open more for example when going up hills.

Our old Ford had vacuum operated windscreen wipers that always used to slow down and stop when going up hills.

I think you must have an air leak. I would look at the pipes and connectors. I think some of them are buried in the wing behind the liner.

Could be vacuum leak or anything else that the cruise control ‘doesn’t like’.

I had a Mazda that when it downshifted on a steep uphill, the cruise mistook that for transmission fault.

My experience with an x308: dropping cruse on a ‘noticeable’ or worse bump was a faulty brake switch.

Thanks Michael and Eric. The mechanic has been checking vacuum with gauges and so far not found anything much apart from slight reduction he thought due to normal wear and tear after 100,000 kms. So far as I know the mechanic has thoroughly checked for leaks in all likely places. If just "a slight reduction” in vacuum can cause the problem I give up with the car. The mechanic said he would keep looking but he’s already had the car for many months and I need it back, fixed or not so I’ll just drive without cruise I guess.

I have had similar problem in the past. Check the bellows are totally sealed.
I think it is less to do with vacuum supply and more to do with a leak which the vacuum can just manage to keep up with maintaining a steady speed but has no available reserve for when increased throttle is called for.
Should add my problem was on xjrs but assume early xk8 is similar.

Thanks for the suggestion baxtor. I’m still a bit ignorant regarding the intricacies of this Jag engine so could you please explain more on what and where the bellows are and how to check their function ? I hope this is a DIY job or something a typical non-Jag mechanic can do here in the wilds of Tamworth.

IIRC the CC systems on the XJ6 & X308 are completely different.
Have look on and see if you can find data there.

To a point, yes.

The X300 used a cable between the pedal and the throttle body. With a (cam?) assembly in between to de-power the engine during traction control events. (Loved it! A little slap on the bottom of the foot when one got naughty with the throttle). And a vacuum (I’m recalling electric?) bellows to pull the throttle for cruise control.

The X308 is drive by wire. The whole cruse control and traction control is implemented in software.

However, they both sense brake usage (disconnect) and transmission signals “did the driver just pop it into neutral?” (disconnect). Or traction control activated (disconnect).

I think the bellows is fitted to throttle body. Standing proud just beside air intake tube attachment centre rear of engine and has vacuum line fitted to top.
My xkr is 2000 model so different set up.

Thanks, if the bellows look like a black tin can about 80 mm diameter x 70 mm high with a 8 mm diameter tube attached to the centre top, then this must be it. How does one check if it’s working ‘ok , or maybe I just get a new one ?
Another symptom of cruise dropping out appeared today whilst on a test run. Instead of the amber Failsafe Engine Mode display I got a red Restricted Performance message. No idea if this is part of the cruise problem or something else. The car seems to run ‘ok without engaging cruise and the messages are gone after stopping and re-starting.
I like to find an independent expert electronic/mechanic capable of diagnosing and rectifying the issue. Any ideas who and where ?

We could tell you about the nearest independent Jag mechanic, if we knew where you were located.

Thanks Grahame; I’m in Tamworth NSW (not UK) and I’ve had work done by Thomas Jag & Mechanical in Islington , Newcastle NSW, three and a half hours away
They think it’s vacuum issue but have been unable to fix the cruise even after many attempts.
The Jag Dealers in Tamworth say the car is too old for their equipment/ etc and can’t fix it. The other “nearby" Dealer is at Bennets Green near Newcastle (4 hours away) but I fear they will be about as useful as the Tamworth one.

I would hope the mechanic has already done the simple checks but for bellows just apply a little vacuum and see if it holds. I have used a large syringe and short length of tubing for this purpose

I would suggest making inquiries about any independent Jag mechanics in your area. I have one in Calgary, Canada (a little too far away for you :wink:), who is factory trained, quite good and quite reasonable. Hopefully you could find someone like that in your area.

The Jag mechanic in Islington says he has already "checked all vacuum lines from the top of the throttles to the inner guards area”, so I assume that includes the bellows. If low or leaking vacuum is not the problem I need to look for any alternatives that could also cause cruise to drop out. What else could it be? Any ideas?

Don’t know if this is a factor or not,(?) but soon after the initial cruise problem first appeared, there was a severe mis-fire in the engine. It took the Islington mechanic a while to find and fix the cause of this problem which was a loose connection in the plug into the ECModule.

I suspect (but there’s no proof,) that the Dealer mechanic here in Tamworth might have caused the mis-fire whilst attempting to fix the cruise control.

If you rule out a problem with the vacuum side of the cruise function, according to JTIS, cruise is inhibited if the ECM senses a problem with the the throttle position sensors.

I think at this stage it would be helpful to get the car scanned for fault codes. These don’t always tell you exactly what’s wrong but can point you in the right direction.

If it were mine, I would scan the codes and note down what you have. Then I would clear the codes and drive the car until the fault condition occurs, then do a re-scan.

A switched on dealer, might actually do this with live data. Some systems allow you to freeze frame when the fault occurs. It’s a facility in the Jaguar IDS but I’ve never used it.

Thanks Eric, it’s a good lead that I will develop if possible. The fact that there was an issue with wiring into the ECM causing mis-fire (now cleared I hope) might be telling. I am tempted to try the Tamworth Dealer again to check and clear codes and test again after driving as you suggest but have reservations after they declined the car before as it was “too old”.

Some more background, initially, before he fixed the loose wire in the ECM and got the engine running, and over a period of some months, the Islington mechanic identified and supposedly cleared the numerous fault codes below:-

throttle fault engine codes PO121; P1224; P1229;

air flow, air temp codes PO102; PO112;

knock sensor codes PO328; PO333;

crankshaft position sensor code PO335;

vacuum switching valve codes P1235; P1236; P1237; P1238;

radiator fan codes P1475; P1476.

P0121 & P1224 are throttle sensor related but you have a mass of different codes there which could be more likely related to the loose connector or wiring problem. Once that has been properly rectified, if those two fault codes return after being cleared once the cruise has dropped out, it does direct your attention to the throttle body. A lot of faff going back the the dealer. Might be cheaper to buy a decent code reader

Thanks for the advice and sorry to be slow getting back on. I’ll try what you suggest and hope it’s not the throttle body sensor which failed several years ago and was expensively replaced after many months in the garage of my independent Jag mechanic.