Dear Jag community,

I’m new in this forum and I certainly need your help… my father recently bought his dream car (XK8 NA coupe), the car is in very nice condition, 80.000km, but a month or two ago something odd happened when we were on vacation with the car.

The car, when he switched the ignition ON (car not running) the throttle body was making a ticking noise, like the valve was opening and closing too fast, ok it’s own.

When he started the car, it made full throttle on its own… of course we were a bit scared and put it in a mechanics garage. I know a thing or two about mechanics, though the mechanic that was supervising the car told my father he needed a full new or refurbished throttle body…
But that didnt convinced me… that must be a way to change some sensor that is faulty, im not sure if they diagnosed throughly the car.

Does anyone has any clue on what the car might have and the troubleshooting that we need to do?

Thank you very much for your help.

Kindly,
José Magalhães

 

Hi José
What year is it?

 

You could use OBD with ignition ON (but not run) and look at sensor values - especially throttle.

Ideally you'd check the pedal & throttle ones.

 

Does it start fast all the time? ...just once? ...occassionally? Is the Check Engine Light on? Throwing a new Throttle Body on without narrowing down the problem is like a doctor amputating your nose for sniffles.

I second what JagV* said. Grab a cheap OBD tester (bluetooth is handy let you drive with it while observing engine) and look for error codes first.

Bring up a display of the Throttle %. Before starting, step on the throttle to see how the tester follows your foot. There'll be a little lag but you'll get an appreciation for the feedback. Next, without starting the engine, floor the throttle and see if it says 100%. (make sure floor mat isn't holding the pedal up). It probably won't but it will give you an idea of how well the PEDAL Position Sensor is adjusted. Check out the Pedal Position Sensor for the model year of the XK8 to see how to adjust the slack out of the cable to where full throttle just achieves 100% throttle opening.

Butterfly makes about 3 movements when switched on. They don't really tell us what it is doing but my thoughts are it is checking the throttle motor movement and reading the THROTTLE Position Sensor (TPS) to make sure "fly by wire throttle" is working as expected. On the 2nd move it closes 100% which might inform the ECU where the 0% throttle opening is and then moves to 3rd position which is what it has memorized for the fast idle at start, around 5%, give or take a %. The TPS uses two potentiomers so they doublecheck each other to make sure the TPS is working correctly, or it throws an error code and likely drop to limp mode. It may be working perfectly well, but be out of adjustment, but it would have a bad start every time. If you feel confident, you can try adjusting the TPS and PPS via ODB.
https://www.jaguarforums.com/forum/x...ia-obd-264918/.

A word of advice: before starting the engine, turn the ignition on and wait a couple of seconds for the throttle to click away and most of the dash lights flick off. Give the ECU a chance to execute it's starting procedure and read the sensors. Your rip roaring start may have been from too quick of a flick of the wrist, creating a misread transient. The other night, we were running late and jumped into the Jag. In a rush, I turned the key, started the engine and put it in reverse in the blink of an eye, and was rewarded with a TRANS FAULT in the message box. OH NO!!! Shut it off, gently turned the key, waited for Mr. Ecu to do its thing and then carefully rolled the engine over- no more errors. Then hauled ***.

When I drive, I often set my phone on the console and observe instantaneous and trip MPG, or other times vacuum, volts, engine temp and a few other readouts, and sometimes Throttle %, and Short term Fuel Trims.
You may doublecheck Throttle% before engaging the starter. If it looks too high for some reason, turn the key off and then try it again.

 

OP appears not to be logging in.

 

I’m not sure but I think the jag is from 2000/2001

 

When the ignition is ON but the engine is off I hear a ticking sound as if the throttle valve was opening and closing instantly.

When we turn the engine ON the car puts itself on full throttle, constantly.

We tried all sorts of stuff, including letting the Mr. ECU rest in order to try to “erase” some error codes.

I don’t remember seeing the engine light on though.

 

Sorry about the ignorance but what does “OP” mean? 😅

 

Original poster

 

Stick an OBDII tester on and look to see what the ECU "says" the Throttle % is without starting it. Again, if the Throtttle Position Sensor fails to report the correct position, the ECU can mistakenly open it up to thin or too wide. There's no explanation why it all of a sudden goes haywire, but you see, it can. You can at least rea the Throttle % without starting it. A 10mm socket and screwdriver should answer that question quickly. If you take off the air inlet tube off the throttle body, you can see if it is maybe .020" barely cracked open or .250" or so.

If it is just barely cracked open, go look for a major vacuum leak. You can read the vacuum on the tester while it is running, but unfortunately, you don't have the time to wait for the reading. You may have to do detective work looking for a hose that became disconnected or broken.