Hiya everyone, i have a issue with a 1999 xk8 x100 4.0 na. Car drives fine but when the car is in neutral or park, if i rev the engine it misfires and hesitates. Its got new spark plugs speced to owners manual. New fuel filter. Ill pop a vid on here so u can hear for yourself. Any thoughts or ideas welcome Rob

 

Since you’re asking, I do have a thought, which does fall short of an idea or “fix” :

you say the car drives fine, so by my loose standards, I say you just drive it and be happy.

If the car starts to not idle at all, then yes, action is called for. But occasional “hiccups” ? No, not a necessary actionable problem.

****How high rpm are you revving in Park or Neutral ? The engine is limited to 3,000 rpm when it’s in Park or Neutral.****

Z

PS / I’m just not pulling this answer out of thin air. My 2002 XKR has had occasional idle hiccups for the entire 8.5 years / 95,000 miles of my ownership. But I can observe no problems with the actual driving. That’s just my experience,

 

Hello,
For a classic car with a distributor, I would have recommended checking the mechanical ignition timing in the distributor to ensure it is running smoothly. But with the XK8, I don't know which values/sensors influence the ignition timing.

Best regards, Willi

 

It's always worth adding a can of injector cleaner such as BG44K to a full tank of petrol (assuming you can still afford a full tank) and see if it helps.

If you don't already have a scanner, a cheap OBD dongle and a copy of the Torque app is all you need to see the live data - that will give you a much better idea of what is going on in the engine and certainly better than randomly replacing anything else.

 

Be aware that the engine is electronically limited to 3000 rpm in neutral or park (as noted by zray) to protect it under no-load conditions. You may be reaching that limit.

If the car drives normally, then you can probably ignore it. Better still, buy a code reader - even a cheap ELM327 will read powertrain (Pnnnn) DTCs.
That will allow you to check if the car is throwing any codes & is a useful tool in your toolbox for these cars.

 

+1 on not trying to rev so high in park or neutral. Potentially damaging which is why the car cuts power.

 

Ah thanks for the replys, yeah i scanned the car and it has no faults detected. i also have put a bottle of injector cleaner through it. Like i say car drives fine. I mostly pootle everywhere but just seems weird it doesnt rev clean. I understand the 3000rpm limit but it feels different to that. Ill book it in for a check up and see if anything is found. Again thanks everyone

 

After listening to his video, I would say that it is def the rev limiter hitting to keep the revs below 3K. Not to be disrespectful, but why do you want to abuse the engine by revving it in neutral? You're not Parnelli Jones, are you?

 

Parnelli Jones knew better.

Z

 

A leaky/tired injector? When revving the injectors has to "open up" so to speak, maybe one or a few has issues closing tightly... leaving fuel residues to burn of in the exhaust.. RPM limiting are done with software, so i would put my money on something injector related... to do the simple thing first ie injector cleaning maybe a few times, is the way to fo first..

/E