The car has a severe backfire through the intakes when cold,
hesitates and stumbles badly on any attempt at a fast acceleration.
On a slow hold your foot in one position the car will slowly
acclerate to over 160 KM per hour. When warm the backfire and
hesitation are less severe but still bad.
Prior to the onset of backfiring the car 0-100km times were 14
seconds, very slow. The development of the backfire severity
developed over about 4 days from the odd backfire to a sure bet.
In search of an answer the following has been changed with new
parts from Jaguar:
a) fuel regulator
b) throttle potentiometer
c) distributer cap and rotor
d) timing checked
e) manifold bolts torque checked
f) new fuel pump (not OEM) 3 months ago
No improvement whatsoever.
Any suggestions would be very much appreciated.
The car has been modified with the AJ6 torque plus manifold/
throttle bodies/ air filter/ ECU and intermediate silencer bypass
pipes but the issue started several months after the change so they
are not suspect.–
gjopson
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At 07:02 AM 31/03/2010, you wrote:>The car has a severe backfire through the intakes when cold,
hesitates and stumbles badly on any attempt at a fast acceleration.
On a slow hold your foot in one position the car will slowly
acclerate to over 160 KM per hour. When warm the backfire and
hesitation are less severe but still bad.
gjopson
A faulty temperature sensor can fool the ECU to keep the mixture too lean.
You did not change that item, it is easy to check resistance against temperature.
Make sure the vacuum signal from the inlet manifolds is getting to the
ECU, and the line is not partially blocked.
If the ECU stays in closed loop mode instead of open loop for
acceleration it will take a while to get up to speed.
Partially blocked fuel pump or lines to/from can also impede
acceleration, but the effect is not likely to be so consistent.
Richard Dowling, Melbourne, Australia.
1979 XJ-S coupe + HE V12 + 5 speed, 1988 XJ-S V12 convertible, 2003 XJ350
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In reply to a message from Richard Dowling sent Tue 30 Mar 2010:
I have now replace the temperature sensor, checked the vacuum lines
by vacuum test and replaced the fuel filter.
No change in backfire, power or hesitation issues.
It is at the local Jag dealer but after three days no luck
resolving it. To be fair there is not one mechanic there who has
seen a V12 so it is not surprising.
Any other ideas, suggestions or checks are welcome.–
The original message included these comments:
The car has a severe backfire through the intakes when cold,
hesitates and stumbles badly on any attempt at a fast acceleration.
A faulty temperature sensor can fool the ECU to keep the mixture too lean.
Make sure the vacuum signal from the inlet manifolds is getting to the
ECU, and the line is not partially blocked.
If the ECU stays in closed loop mode instead of open loop for
Partially blocked fuel pump or lines to/from can also impede
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In reply to a message from gjopson sent Tue 20 Apr 2010:
Have you checked the timing? Possibly the timing chain jumped.–
Maynard 94 XJS V12 Coupe 91XJS(RIP) 86XJ6 78MGB 67MGB
Niles, IL, United States
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In reply to a message from Maynard sent Tue 20 Apr 2010:
I had a similar problem some while ago with miss firing and running
fine up to 50 mph ( ish ), This did work for me I unplugged the ecu
( four plugs ) and took the unit out of the car and cleaned all the
contacts with some contact cleaner and used cotton buds to do it ,
put it back and hey man she has been fine ever since.The ecu is
down in the footwell near the gas pedal to the side (RIGHT HAND
DRIVE).You will be surprised how much grunge they accumalate over
time. time taken was around 90 mins.I presume that the ecu did a
soft re-set , ? .good luck.I did not disconect the battery and the
ignition keys were not in the ignition .worth a try .–
The original message included these comments:
In reply to a message from gjopson sent Tue 20 Apr 2010:
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