Temporary high sustained idle speed after pressing throttle

[johnsoncp]

I have a 1994 X300 Sovereign 4 litre auto with about 46,000 miles on the clock. Everything works OK and the car is perfectly usable but it has a habit of keeping the revs up at about 1500 to 2000 RPM after I have pressed the throttle. After a while this drops down to the usual 800 tickover. Is this correct? I'm new to Jaguar cars. It is quite annoying but being an automatic it just means pressing the brakes a bit harder. Any advice would be appreciated. Thank you, Chris

[davidr]

Hi there. Sounds to me like a sticky throttle caused by carbon deposits gumming up the works. I suggest you take the throttle body off and look inside at the butterfly valve. If it's covered in sticky, black tar then that's what's stopping the butterfly valve closing properly. Clean it up using something like brake cleaner or electrical contact cleaner that will completely evaporate. You'll probably find more detailed instructions elsewhere on the forum either under X300 or AJ16 engined XJS.
Hope that helps
Regards
David

[Brakebuster]

i agree , sticky TB butterfly

get a can of carb cleaner and an old tooth brush , and this can be done on the car by holding open the TB , and clean with a soft cotton cloth at wont leave any fluff behind,

( WD40 works almost as well, but will leave deposits , but not for long )

also check that the throttle cable is not stuck etc so get someone to press on the pedal without the car running, as you look at the operation of the TB

BB

[johnsoncp]

Thank you both. The cable, spring and externally moving items all seem to be operating properly. For me to minimise risk, would squirting one of these carb cleaner sprays through the air intake be any help or am I just wasting time avoiding the inevitable? Something like Wynns air intake and carb cleaner? Is it a tricky or risky process removing the TB and would I need to recalibrate something afterwards?

[davidr]

Not very hard to get the throttle body off and probably easiest to clean it on the bench than in the car. No need for much dismantling but make sure that you've cleaned thoroughly where the spindles go through the bearings. You can test it afterwards by making sure that it snaps sharply shut when you open it and let go.
Have fun!
Regards
David

[johnsoncp]

Thanks David. I will let you know what progress I make. Regards, Chris

[johnsoncp]

I started to look into the throttle linkage prior to removing the throttle body and I noticed that whilst the cable is slack, the actual activator was stopping about 2 or 3 mm from its stop. By physically pushing the link it returns to 800 rpm. I used some WD40 and gave it a bit of a clean and it 'better' but still not quite right. Does anyone have any recommendations? Thank you, Chris

[davidr]

Still sounds to me like the throttle body is gummed up and that's what stopping the butterfly closing cleanly. I'd still be inclined to take it off and clean it up!
Regards
David

[Brakebuster]

just bear in mind that the TB is NEVER fully shut , there will be about 2thou of a gap to allow idle without pressing the accelerator ,

if your unsure of what you are doing i would let a mechanic do the work , about an hours labour should cover it

otherwise follow the above advice

BB

[johnsoncp]

My official jaguar dealer quoted me £252. A local used jaguar dealer quoted £67. I've given the linkage a good clean and WD40 but havent driven it since. Will keep you posted. Once again thanks for the advice.

[Brakebuster]

it doesnt matter how shiny or lubricated the outside is, its the innards of the TB that need cleaning to determine if they are causing your issues

BB

[johnsoncp]

I think the external clean up might have cured it. I visited my local jag dealer today and they said first thing is to try a WD40 cleanup before removing the throttle. There is a recall for my car to increase the throttle spring strength but they couldn't tell if mine has been done. The revs definitely return to 800 after a blip now.

[whatagas]

I purchased a 1996 4ltr x300 sport 43,00 miles in May 2023. After 3 months it would tick over at 1500rpm and you could cruise at 70 without touching the pedal, NOT GOOD. at junctions I left the car in gear and held it on the brake pedal. Took the car to McMillan Jaguar in Motherwell, he diagnosed weak throttle return spring ( its a circular spring under the throttle plate). He fitted an additional external spring and its been fine ever since.

[Andy P]

Hi whatagas,
Thanks for this post,it will be helpful to other members I'm sure.
I would imagine that is quite scary.
Glad you got it sorted easily.
Enjoy your car.
Andy