Jaguar XJ8 - rough idling

smsz@pop.jaring.my wrote:


What does the fault indicate when the computer is plugged in?

"" wrote:
 > smsz@pop.jaring.my wrote:
 >
 > > I own a 1998 XJ8, that has about 170,000 kms on the clock.
 > The car had
 > > been running perfectly until recently. I return from a long
 > outstation
 > > trip doing an average of about 180 kph. The day after that
 > the car
 > > suddenly dveloped rough idling. I sent the car to the Jaguar
 > workshop,
 > > here in Malaysia and after so many days they still cannot
 > find the
 > > fault. They tested the car for everything from, plugs, fuel
 > quality,
 > > vacuum, leakages but came out with zero. They finally said
 > it could be
 > > the ECU. But are uncertain. So the car has to be with them
 > for another
 > > week.
 > > Has anyone experienced this? i would really apppreciate some
 > help as
 > > the guys at Jaguar are as confused as i am. Thanks.
 > >
 > > Syed Mokhsain
 > >
 >
 > What does the fault indicate when the computer is plugged in?

Hi, I dont have alot of experience with ECU’s as I prefere older carb
type engines.  I have recently done a couple of engine swaps on later
model cars.  What I have found out about ECU’s is if part of the
system is not operating then the ECU switches to an average mode in
place of calculating the best running settings.  From this information
learned I would highly doubt it is the ECU that is at fault.

From experiences with other brands of vehicle, I would first go about
checking the condition of the oil.  This I would advise as if the car
ran for a long time the oil would have been at peak temperature longer
causing its viscosity to be very thin.  I have come across problems
where this has resulted in small particles of dirt getting into area’s
like tappets and causing slight knocking noises.

If you can eradicate the oil symptons I might be able to offer further
advice.  Something I’ve just done to my XJS is to add an anti-friction
additive to the oil and a little to the fuel.  This has smoothed out
my 3.6 quite noticably.  It cost me £17 a bottle and with the size of
the engine I used two bottles.  I ran it in for a few miles and as
stated earlier it seemed to quiten down.  

I think this would be beneficial to start off with just incase
anything has expanded a little under the pressure’s.

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Thank you very much guys. My car is still in the garage and they still
cannot give me a definite answer on what is causing the rough idling.
I am anxiously waiting what they have to say now. I will keep you
updated. Cheers.
Mokhsain