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Jaguar Repair/1981 jaguar xj6 overfueling on cold start


my xj6 keeps over fueling at cold start,18 months ago the battery exploded,i replaced same car then went for a mot,when it came back they said the inertia fuel cut of switch was causing a relay to get hot so they bypassed it.i replaced the inertia switch then noticed the car would only idle at about 1200 rpm. i had a mobile mechanic i know out he cleaned all the air flow meter parts and said the air flow meter was faulty,{ if you can get the engine warm it runs okay} i replaced the air flow meter and coolant temp sensor but car still over fuels,i read about a mod featuring a capacitor soldered on the air flow intake electrical connections which i also carried out. the car is at my local garage having gone in for an mot in October 2014 failed on emissions and is still there 9 months later, can you help?

Hi Melvyn,

You seem to have had a series of problems. I am not convinced that they are all related. Your car is 35 years old. So it is common to have several failures at one time.

Lets cover one at a time. First a battery that explodes does it for a reason. It can be a separated main post inside that produced a spark and ignighted the Hydrogen noramally produced when charging. Or it could be an over charging of the alternator. That don't matter because arching of a main power source or over charging of an alternator can also damage many electrical components. You should test the charging system. Eaily done by connecting a volt meter to the battery posts (not the cable ends) and read battery voltage (12 v +) then start the engine and note it is at least 1200 RPM and then read battery voltage (13.8v to 14.5v) No more no less.

Next is the Inerita switch mounted in the door jam on the right side. This switch cuts off the fuel pump if you bump into somethng. If you do just bump a post in a parking lot it can disconnect the fuel pump but it has a reset button to reset it. It is possible that it's contacts are coroded and not making a good contact thus causing the fuel pump relay to not make good contact but this is unlikely. It is also very unlikely to have any effect on Idle speed.

Next is your statement that the car is "over fueling". A higher Idle speed is not an indication of over fueling. It is the opposit. If you mean a "Rich" fuel mixture as "over fueling", a rich mixture would make it idle extreamly slow not fast.

We need to start from scratch. If you had brought your car to me in one of the Jag dealerships that I worked in, I would have listened to all your symptoms and smiled and nodded my head as you spoke and then forgot most of it and proceeded with the tests necessary to correct the car.

Your injection system is a Bosch "L"-Jetronic system. All injection systems are just carburetors that are spread out over the engine. True they are more efficient and don't waste fuel as much as a carburetor does. The components are separated and some have major influences on fuel mixture and others have only minor influences.

The major influnences are "Fuel Pressure" controlled by the fuel pressure reulator mounted close to front of the intake manifold. It MUST maintain 32 PSI fuel pressure on the fuel rail all the time that the engine is running. It does this by applying manifold vacuum to the regulator. If the regulator looses it's vacuumn supply the regulator will climb to about 42 PSI. This happens each time you open the throttle a lot. This it providing the fuel pump in the trunk (excuse me, I mean "Boot")is supplying sufficient fuel through the filter there with the pump.

So first thing is to measure fuel pressure at the fuel rail with the engine running (32 PSI) and then remove the vacuum hose to the regualtor to see that it goes to (42 PSI). (A "T" fitting added to the fuel hose at the fuel rail will allow you to connect a fuel pressure gauge"

If it does that, then and only then can you go on to the next test.

The next major influnence to fuel mixture is the coolant temp sensor (which you replaced) but didn't test afterwards. Test with the wire off. 2.5 ohm at +20 deg C., 1.18 ohm at +40 deg C., .60 ohm at +60 deg C.

The next major influence on fuel mixture is the airflow meter (which you also replaced) This is more difficult to test as it is a variable resistor with a spring loaded air flap door. But it is not necessary to run these tests on it at this time. The airflow meter does have an adjustment to the fuel mixture at idle and low RPM ONLY.

All other components of the injection system are minor influences like the air temp sensor in the airflow meter, and the O-2 sensor in the exhaust manifold.

The "Cold Start" system it indipendent of the injection system and the ECU (Brain in the boot) has no control over the cold start system.

The cold start system can and often does cause a excessive amount of fuel. The way it operates is that it has it's own injector called a "Cold Start Valve". It is powered by the "Start" position of the ignition key. But it will not operate unless the "Thermo-time" switch is grounded. It is grounded by the temperature of the coolant in the coolant manifold on top of the intake manifold. The "Termo-time" switch also contains it's own electric heater so if you try to start and the engine does not start and you continue to try, the heater in the "Thermo-time" switch turns off the ground to the "Cold Start" valve so as not to flood the engine.

As you can see, this system has the potential of causing excess fuel into the engine if it fails.

Next is the Idle speed control just after starting. This is called the "Aux Air Valve" and only controls Cold Idle speed and it turned off when the engine warms up. The test for this item if you have a 1200 RPM idle after the engine is warm just take a pair of needle nose pliers and pinch closed the rubber hose on the "Aux Air Vale" and then if the idle drops you know the "Aux Air Valve" is causing the high idle. This valve has no control over fuel mixture at any time.

If after warm up you suspect the wrong fuel mixture you have a tool on board. Take a volt meter and set the DC volts to a low volt scale. Remove the wire from the O-2 sensor and measure the voltage at idle of the O-2 sensor.  .5 volts is ideal and a lower voltage like .2 or .3 volts is a lean mixture. A high reading of .7 to 1 volt is a rich mixture.
This can not be done with a cold engine, the engine must be up to operating temperature.

All of the above tests are necessary to diagnose injection problems. They don't show if the ECU is receiving the correct information from the sensors. To supply the correct amount of fuel to the engine the ECU must see all this information to know how many times to ground the injectors and for how long to keep them grounded which controls fuel mixture.

These test only test the sensors. (where 99.9% of all injection problems show up) I have worked on the "L" Jetronic system for many years and use to conduct mechanic training seminars on the "L" Jetronic system in almost every state in the US and three of the Hawaian Islands. and I only came across ONE failed ECU in all those years. This is why I stress to test all the sensors.

If you don't have the test equipment, it is not expensive. Even the fuel pressure gauge can be any industrial 0 to 100 PSI gauge which is not expensive. Volt/ohm meters are also not expensive.

Let me know,


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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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