Jaguar Repair: xjs v12 he miss fire, noid light, gas tank

Question
hi
I have a miss fire I think on the right bank of my v12 xjs 1988 the engion seems to do this all the time but if it idles just above 1000 rpm it runs fine.I have just replaced all the spark plugs and all the ht leeds are on fine. could this be a stiking idle valve? thaks ed

Answer
Hi Edward,
You need to do a compression test first. Be sure to hold the throttle open and let the engine spin over at least 4 or 5 revs.
It is difficult to isolate an idle miss on this V-12 because when the car is running good you can kill a cylinder and not note any change in RPM. So even a cylinder kill test on a scope was difficult to diagnosis a problem on one cylinder. Sometimes we were able to spot which cylinder by putting in a clean set of plugs and let it run at idle (when the miss is noted) for a good amount of time and remove the plugs and look at them. When a cylinder is not firing correctly it will color the plug different than the ones that are running correctly.
More often we found injectors at fault. An inexpensive Noid light will tell you if the power and the ECU grounding are working. Some shops have injection cleaning kits to clean injectors and a few have an off car injector cleaning system and test stand which is the best.
The injection cleaner that you put in the gas tank may help over time but is limited as to what it will do.
Howard