I have a really weird thing happening on my XJ8 4.0, it's a model year 2000.

1. Start the engine and let it run for 10 minutes.
2. Suddenly the engine surges without any input from me and goes up to 2000 revs, when I hit the accelerator the revs rise but then fall back to about 2000.
3. Switch the engine off. It then cranks but will not start accompanied by the warning "engine failsafe mode".

I had this happen a few weeks ago just as I got home and haven't used it since as I have been waiting for a break in the lousy weather as the car is stuck on my driveway.

Any ideas ?? Thanks

 

Throttle body.

Perhaps cleaning it would make a difference.

 

I would like to see if you have any OBD codes. Do you have any way to check them?

 

Codes are P0507 and P1229.

 

P0507 shows higher than expected rpm, but you knew that.

P1229 is 'Throttle motor failure'

See the attachment for a few items to check before removing the throttle body.

 

A simple test: Try cleaning the TPS and Throttle motor connectors with contact cleaner/lubricant like that from Radio Shack. Both connectors are readily available on the right side of the throttle body. Cleaning takes 5 minutes and got rid of similar symptoms that I was seeing.

 

Thanks for the replies, I'll let you know how I get on.

 

The saga continues. Cleaned the connectors on the throttle body, the throttle body itself is totally clean inside. Put everything back together and the car started straight away with no problems.
Let it run on the driveway up to operating temperature, shifted through the gears, revved the engine, the usual stuff. Did not go out onto the roads. Switched the engine off and then attempted to restart but got "Engine Failsafe Mode". Did a soft reboot, still the same. Did a hard reboot, still the same. Any ideas where I go from here please ?

 

You need to check for codes and see what they are telling you. Then be sure you are getting fuel pressure and spark, if all is good it could be the throttle body. I would like to see if you get any other suggestions.

 

Did you get any of the 'surging' as previously described after about 10 minutes?

If not, perhaps you had more than one issue and the first has been resolved with the cleaning.

 

Thanks QuadManiac, I think you are right and I have multiple issues.

So this morning I went out to the car, gear shift in P and turned the key..."Engine failsafe mode". Put the gear shift in N and turned the key....the engine started.

Maybe I just need to pump the tires up or change the wiper blade or clean the carpets ..........or jump off a bridge !!

This is just so infuriating it makes me want to start wearing a trilby hat with a feather in it and smoke a pipe.

 

This is only the second car I know of that seems to have a problem with the shifter switch. Could this be a new problem on the horizon?

 

Where is that bad boy shifter switch located please. It's bound to be really easy to get at, like inside the gearbox or maybe on top of one of the pistons.

 

From what I understand it is mounted above and outside the trans. I will look it up and let you know.

 

I think the switch is under the j-gate module

 

I think item #27 is the switch
https://www.jaguarforums.com/forum/a...1&d=1267914556

 

Thanks Gus, I think you are spot on. I contacted an Ebay Jag breaker and they won't sell the switch alone because he said EVERYONE wants just the switch so they sell the J gate complete with the switch for big bucks. Sounds like you are right when you say that it is a problem waiting to happen on the older XJ8's. New switches cost a fortune so if you come across a J gate or three, get your hands on them.
This switch could be the cause of so many gearbox warning messages as it slowly breaks down but is only an intermittent fault until you get what I have which is no start in P but will start in N.

 

Nothing is officially broken until you take it apart and cannot get it to work.

 

I would first see if it is broken. I like Tom firmly believe that it is not broken until I had ha chance to look at it. What do you have to lose?

 

I wouldn't know where to start to find out if it is broken. I have a few spanners and sockets but that's about it. Oh, and a crowbar and a club hammer.