If it was like that on these cars:
1. there would be MANY reports (there aren't)
2. there would have been mandatory recalls (don't appear to be but have a look)

As I mentioned at least the S-Types definitely don't work that way. Any of the other jags with similar TB also don't. Some posts to the contrary are frankly wrong (driver error or whatever).

 

The throttle doesn't go wide open on shutdown because of the mechanical guard, which is positioned by the accelerator pedal. Perhaps with the accelerator pedal and input shaft at idle, the mechanical guard is in just the right position to allow the throttle to be in just the right position for normal engine starting. On page 28 the Jaguar Technical Guide says that there is a thermostatically controlled valve that allows some air to bypass the throttle because the mechanical guard keeps the throttle closed too much to allow starting in cold weather.

 

THANK YOU to all who have offered constructive opinions on the failure I experienced with my '99 XK-8 . I finally was able to check into the battery and alternator conditions. Testing battery voltage at the terminals while shut off showed 12.22 volts, and while idling showed 13.1 volts. I pulled the cables, which looked fine on the outside, and did a battery load test. Battery checked out fine. Close inspection of insides of both cable clamps showed pretty severe discoloration and deposits, which were not evident on the outside. After cleaning off all corrosion of clamps and battery posts, the voltages are now 13.25 (off) and 14.2 (idling). I also cleaned throttle body interior, which on test drive helped smooth out acceleration from a stop. Other systems on the car are now working more smoothly, such as window motors, auto. str'g. wheel adjustment, etc.
By the way, the car has the later all-electronic throttle body/cruise control. I'm now thinking that the lower voltage available to the computers and sensors may have caused the sudden-full-throttle glitch that I experienced the other day. In test driving after these fixes, the car is much smoother in performance. I made multiple test stops to replicate what had happened originally, and the car behaved perfectly each time. I'm going for a longer drive today to further test it, but I'll be ready to kill the ignition if necessary...
Again, THANK YOU to all who jumped in to try to help me and didn't accuse me of confusing the brake and accelerator pedals! If it happens again, I'll certainly let you all know (if I survive it...).

 

When I had a window dip problem, the good people on this forum pointed me to the battery, which solved the problem. I'll admit I was skeptical at first, but now I'm a believer. Sure, the battery was about 14 years old, but the car started and ran fine. Even if the car starts OK, an old battery can cause some pretty strange problems on these cars. And good battery connections are just as important. I'm not a fan of the crappy battery cable terminals most cars have. A couple months ago I replaced them on a Ford van with a nice set from Autozone for about 6 dollars. Money well spent.

 

To stu46h,
What prompted me to investigate the battery and charging circuitry was the fact that the car cranked a bit slower than it should. Having two other 4.0 v-8s, I'm familiar with the average cranking speed. My extended test drive yesterday was uneventful; no glitches showed themselves, so I'm feeling a bit more confident with the battery cable repair. Considering that the repair boosted the available system voltage by more than 1 volt, I imagine it was a major factor in that. Nonetheless, I'll be keeping my right hand within grabbing distance of the ignition key each time I approach a parking spot or intersection for the immediate future . When I was a mechanic professionally, I always looked at the simplest solutions first. I've got to remember to do that in the future, as well...

 

 

dielectric grease is one of the most misused products in the automotive world. As the name "dielectric" indicates, it does NOT conduct electricity. Which makes it perfect for spark plug rubber boots and rubber bulb covers. But not perfect at all for metal-to-metal connections. When dielectric grease is put on metal terminals, the current flow is happening in spite of the grease, not because it is helping.

A conductive grease such as a silver based grease is the best thing you can put on metal connections. Copper or Carbon based conductive grease are the next best solution.

I use the silver base grease on my battery terminals and immediately the under-hood voltage jumped 0.60 volts. It's expensive, but a small drop or two goes a long way.



https://www.merriam-webster.com/dictionary/dielectric
dielectric
noun
di·​elec·​tric | \ ˌdī-ə-ˈlek-trik \
Definition of dielectric : a nonconductor of direct electric current


Z.

 

I've never used any grease on battery connections but to each his own. I just want a good connection. Some people install spark plugs dry. I don't install spark plugs without FelPro C5A Antiseize Compound on the threads. This is the stuff I used to put on the threads of igniter plugs before I would install them in a Pratt Whitney JT8 engine on the 727s and 737s I used to work on. I have had wonderful luck with spark plugs doing it this way.

 

I've been getting 8+ years from my AGM batteries from NAPA. I read about others complaining about the Optima batteries not being as good as they used to be. However I been getting the same very good service from those also.

I may not be the normal user. I keep all my batteries connected to a batter tender plus when the vehicle is not being used overnight or longer. Just a simple step like that works wonders for battery longevity. Nevertheless, people never think about taking a small step in preventative maintenance like that, but are exasperated when a battery fails after 3 or 4 years.

Z

 

2004 Vanden Plas. Sudden acceleration is real.. Happened twice. Second time $3000 to repair.

A ticking time bomb. Any suggestions about how to drive when starting stopping? Put in neutral?

Thanks

 

Odd how it happens to you twice but essentially no-one else out of the thousands of cars.

The brakes (if properly maintained) will always stop the car. Just use them.
(In case they don't, you have your foot on the wrong pedal.)

Once stopped the rev limitter will prevent engine damage.

 

If a problem is intermittent it's probably electronic or related to a degrading electrical connection. Poor electrical connections can make a problem come and go. A computer can fail to receive or process a signal from a sensor intermittently and respond to this transient condition. Faulty wiring, or dirty or misaligned proximity sensors can have this effect as well as degrading electronics in the computer.
If sensor input as seen by a computer is within the normal range of operation, the computer should take appropriate action. If sensor input is not within the normal range of operation, the computer should give a fault message.
Lack of a fault message would indicate that the computer did not see anything it's not supposed to see, whether that input is accurate or not.
If something mechanical breaks, it can't fix itself but sometimes the symptoms can be slow to appear and they generally don't improve once they appear.
However, the problem can appear intermittent as it may only show itself under certain conditions.
This was my experience from decades of working on Boeing aircraft.
I always preferred problems that were hard faults.
Intermittent problems can be a bitch to solve.

 

I have two XJ-Rs, a 98' and a 99', and a 99' XK-8. The only one to experience the sudden acceleration is the XK-8. After cleaning the battery terminals I was able to boost the available voltage by at least 1.0 volts. That seems to have helped a great deal. I suggest starting there. Check voltage at the battery. Check the voltage you are getting under the hood, then work your way back to the battery if it is too low. Loosen and re-tighten every cable you find, then thoroughly clean the battery terminals and cable ends and re-tighten.
Also, ignore those who claim that you are just stepping on the gas pedal instead of the brake pedal. I think we all know how to drive...

 

Good stuff, then use the brake and the car will stop. No need to mess with N or any such.

Once enough people start reporting this problem, and as the cars are so old it must happen any day now, someone will figure it out.

 

This unwanted full throttle happened to me once in a low mileage & fairly new early 1990’s Mercury (Ford) minivan at highway speeds.

Very scary,

Braking did very little besides fade the brakes to uselessness. putting the car in neutral let me slow a little for a second then I had to go back into gear as the rpm was approaching grenade level. To finally come to a stop I had to turn the ignition off (no steering then) and use the parking brake.

Unless one has experienced this, it may seem like hard braking would slow the car. But it certainly didn’t work for me.

 

There is a 3,000 RPM rev limiter in Park and Neutral - does this not work when driving?

I think if I was worried my car was going to go into this full throttle mode on occasion, I would add a switch somewhere easy to get to, to turn off the fuel pump or the coils in an emergency. Maybe one of those big red emergency stop buttons.

 

No 3K limit when driving - instead it's (roughly) where the red line is.

Old car brakes may well not have stopped a car but properly maintained ones on these will always do so (unless badly applied).

BTW I suggest no-one holds their breath waiting for many people to report full throttle acceleration.

 

Fords and Mercury mini vans of early ‘90’s vintage definitely did not have any niceties like a 3K rev limiter in neutral. I can’t count the times I tried to haul that beast down from 80-90 mph by shifting into neutral and forcefully applying brakes. Brakes were not worn out, I hadn’t had the minivan for even a year on a FoMoCo new car lease , so it was essentially new vehicle. I was hitting the brake petal with all the force my adrenaline would allow, the brakes were no match for the full throttle acceleration, Believe it or not, I don’t really care. Thinking one knows everything is a fools paradise.


Z

 

Already fitted - just turn the ignition key back to position 1.

 

To test brakes will stop the car, stomp on accel pedal and after a while also the brake pedal. My car stops. Try it!


If your brakes don't stop the car:
1. let off the accel pedal
2. fix the brakes