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Jaguar Repair/1985 jaguar xj6 series 3 is over fueling


First and foremost, thank you for you time advise!
I have a xj6 series 3. Fuel pump, and fuel regulator have both been replaced.
The pump was out, the regulator was Leeking. I don't replace parts unless they are proven or test bad.
The car will now run. The problem I'm having is over fueling. 85% of the time when started it over fuels,
Stumbling instead of idling, while putting out thick black smoke. All spark plugs are blacked as well. The other 15% of the time it runs like brand new, but that's seldom to none. The cold start injector,coolant temp sensor have both been tested, they are good working parts and the lines are free of any shortages. Aswell the fuel pressure is good too. I really need some advice here, as I know this is a great car just needs a little t.l.c.
     Mario cheek

Hi Mario,

What was the fuel pressure with and without the vacuum on the regulator? Did you disable the fuel supply to the cold start valve to be sure that it was no supplying any fuel when the thermo/time switch was off?

Did you check the vacuum hose on the new fuel pressure regulator for any raw fuel?

My method of diagnosis is to isolate the source of the extra fuel and since all plugs are rich then you need to disable all possible sources of excess fuel besides the injectors. One or several injectors can fail to close and thus allow fueling all the time, but it is very unlikely that all six will fail at the same time so all other sources must be cut off (not just new parts replaced)

Once all other possible sources have been closed (not just new parts installed)then when it is down to the pulse duration of the six injectors causings the extra fueling, you then look at each thing that the ECU looks to for info to set pulse duration (fuel mixture).

You were right in the coolant temp sensor (for injection). It is a major influence on mixture. The next is the airflow meter which is another major influence on mixture. The air temp sensor and the O-2 sensor are minor influences and usually don't have the ability to over fuel the mixture to a point that it flat blacks the plugs and causes black smoke. (You need to be careful at this point because if it is over fueling to a point of black smoke the two CATs will over heat and burn up very quickly. A simple pressure test at either one of the two exhaust manifolds at there port. High RPM should not exceed 2 PSI. 1.5 PSI is about all a jag can handle. A partially stopped up CAT will show 5 to 7 PSI at a fast idle.

If all of that is correct you will need to take your wiring diagram and test each wire at the ECU to see that they are what they are suppose to be. Example, the ohm meter reading of the collant temp sensor must be seen at the pin on the ECU. All grounds must be a good ground at the ECU etc. That is the last resort. As far as a possible failed ECU, that is very rare. I have only run across one bad ECU in all the years I have been working on Jaguars even though it is the most complex part on the car.

Air flow meters so have an high failure rate. Also,don't forget to check the battery charge rate as a failed regulator will drive a ECU crazy. But first isolate ALL possible fuel other then the injectors. By "Isolate" I mean block off any possible fuel entering the intake, then go after the reception of the signal to the ECU.

If you don't have a good diagram of the injection system let me know and I will up-load a copy to my web site for you to copy.


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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