Vehicle is 1997 Jaguar XK8 RHD Auto 4.0 Convertible.
Acceleration 'slowly' through to 50-60-70 mph etc presents no problems.
If I accelerate 'quickly' pulling away or overtaking etc then once I get to around 50 mph I get an Amber Warning. With 'Engine Fault'
If I continue to accelerate then I get a RED warning with 'Reduced Performance'

The engine then sounds sluggish with occasional missing....

Once I slow down again then the RED warning goes off and then followed by the Amber and all returns to normal.

It seems to be an acceleration problem....

Engine Temp is normal... Fuel use is normal... Spark Plugs recently changed... Performance Air Filter fitted...

It only occurs when I put my foot down..... What's the likely problem...?

 

Have you read the OBDII codes? Best place to start....

 

When is the last time you replaced the fuel filter and cleaned the MAF Sensor with CRC MAF Sensor cleaner? Either and definitely both will cause this as well.

 

I had the same recently caused by water in one of the plug hoes ...that was caused by a badly fitting coil cover which let in water. I got no codes
If its missing you want to look at causes for that inc leaks in the induction tract.

 

Is the performance air filter a recent addition? I'm not sure what one of these is

Has it only happened since you changed the plugs? Which plugs did you fit?

If I continue to accelerate then I get a RED warning with 'Reduced Performance'
The engine then sounds sluggish with occasional missing....

The missing is due to the reduced performance thing isn't it? It's not a warning it's a statement that the car is in reduced performance mode capped at 3,000 rpm I think.

 

The misfire is causing the reduced performance ..find the cause.
I lived in Australia for 10 years and the Aussies are obsessed with air filters and Snorkles because its so dusty. Tests showed that the standard paper filters were less restrictive and caught finer dust than the others like K&N.

 

Do you get a CEL with the yellow and red warnings, and if so,,, is it blinking? If it is, stop what you are doing... I don't want to sound all alarmist or like a crazy person BUT disfunctions in the CATs can be a baaaad thing. Just throwing it out there.

 

Yep! Cleaned out the MAF Sensor and throttle housing etc.
Could not get any MAS Sensor CRC Cleaner but used CarLube Electrical Contact Cleaner which does the same thing.
All seems to be ok now... Good acceleration so far.
Will give some more tries again over the weekend.

Many thanks for the advice.