QuestionI've a RHD '84 Brit Specs XJ6 that hasn't had much use over the last few years. I decided to start using it again. The last 2 weeks I only used the left fuel tank running Lucas fuel injector cleaner thru the left tank and some Seafoam Green in the engine oil. After an Oil change last week and some hwy use she ran strong up to 95mph with an idle going from 1000rpm down to 800rpm. Two days ago I switched tanks and ran off the right one for a few miles, parked it over night, left for work the next morning, on the way she lost power and when I stopped began to Idle very roughly and the rpm dropped below 500rpm. Accelerating was/is very slow, no power up hills, seem to max speed out around 60mph on flat road while I limped her back home.
I'm guessing the fuel injectors have gotten clogged? From reading your many replies, shall I purchase a "Noid" light to check the injectors and replace the fuel filter to start? I'm very much a novice when it comes to cars and working on them.
Thank you very much for all your outstanding replies, I find them very interesting as I try to learn.
Jim
AnswerHi Jim,
I have no experience on Euro versions of the Ser. III Jags but I doubt the injection and fuel system is much different then the US version.
It is good to keep a noid light to check for an injector pulse if you drop a cylinder or two and when you have a no start condition and they are very inexpensive. There are several different noid lights so be sure to ask for the Bosch or Lucas light if you purchase one.
I learned not to think much about symptoms when a poor running engine came in the dealership. I learned to look at the 3 systems first that make an engine run. I learned to test the 3 systems in order and not think about what it might be. This system of diagnosis worked well for me for many years and still does.
The systems are Compression, Fire and Fuel and in that order. Just because the compression was good last week, a day ago or 10 minutes ago means nothing when you have a poor running engine.
You MUST confirm that the combustion chambers are good before looking at Fire or Fuel.
A compression gauge is not expensive and a necessary tool in you tool box. The compression test must be done with the throttle open and at least 5 or 6 full revolutions of the engine.
You need to see from 125 PSI to 170 PSI on all 6 cylinders with very little difference between cylinders. If you do not have that, you are wasting you time proceeding until that is fixed.
If you have that reading you can then look at ignition (Fire).
In some of the Jag dealerships I worked in we had a scope which makes the test of the ignition system a 10 second look. Most people don't have a scope so there are other methods to test the ignition system. When you have a "No Start" condition you just check for a spark but in your case you have a poor running engine so you need to check further. A cylinder kill test is a great test to see if one or more cylinders are not firing however. on any electronic ignition system you can't kill a cylinder by just pulling a plug wire off of a plug. This makes the danger of a spark jumping down the coil tower to the ignition system and destroying the ignition system.
You can take a 12v test light that is mainly plastic except the pointed probe and connect a jumper from a ground and connect the other end to the metal point of the probe and with the engine idling probe the rubber cap on the spark plug so as to short that cylinder out and note the RPM drop. You should see a drop on every cylinder. If you get no drop on one or more cylinders you know now that either those cylinders are not firing due to either fire or fuel. Then on that cylinder remove the plug and attach the plug wire to it and lay the plug on a metal part of the engine and start the engine and watch the spark. It should be thick as a pencil lead and blue in color (not in bright sun light).
If you do not have a good spark you will need to get an ohm meter and test the wire. A rough rule is about 5K ohms per foot of wire. Also look at the condition of the plug and if it looks dirty try switching it with another cylinder to see if the problem switches cylinder too.
A distributor cap can also cause one or more cylinders to not fire due to "Carbon Tracking" usually inside the cap but it can happen on the out side too so you may have to clean the cap very well and examine it in a strong light for what look like cracks. Look at the rotor too but any carbon in it will usually affect all cylinders.
If all of that looked good you can proceed to Fuel.
With an injection system fuel can be good on several cylinders while poor on others. When you don't have an lot of test equipment there are other ways to test. If your UK version has a 0-2 sensor in the exhaust manifold, remove the wire and set the engine speed at what ever make is run the worst and take a volt meter and test the output of the sensor on a low DC volt scale.
.5v is ideal and any reading at idle higher then .7v is a over rich mixture and .3v or lower is a lean mixture.
Another no equipment test is to remove the air filter and put your finger in the airflow meter while it is idling and slowly open the flap very little at a time to see if the engine smooths out any. This richens the fuel mixture. If it does you need to get a fuel pressure test. It should be about 32 PSI running and about 42 with the vacuum removed from the fuel pressure regulator. This is a MUST!.
If you found a dead cylinder or two and the spark test showed good and the noid light showed a good pulse, it is possible you have a clogged injector or a failed injector and that test is difficult but not impossible.
I had equipment at the dealerships but not at some so to test an injector you can remove the injector and in it's place put a longer injection hose so you can place the end of the injector into a jar with it's wire connected and put a plug in the injector hole and have someone start the engine and watch the spray pattern. If it is in a cone shaped spray it is ok. A bad one will usually just produce a thin stream or a partial spray of to one side. A lot of work but some times it need to be done as it is too expensive to just replace all the injectors as a guess.
Let me know what you find.
Howard
