QuestionI have a1987 xj6 4.2l. The car starts but has little power and could only rev to 3grand. When idling,(roughly if I may add), if I remove plug wire #1,4 or 5 (one at a time), there doesn't seem to be much difference in power. Could this be an indication of a faulty distributor cap?? Or what is your opinion?
AnswerHi John,
There are standard procedures to diagnose engine problems. A dist. cap is one of a thousand possible so you must forget what it might be and go through the procedures and diagnose the problem.
You must first determine if the engine itself is OK. First is a compression test. A compression gauge is not expensive and will work on any piston engine from your lawn mower on up to you Jag.
You must spin the engine at least 4 or 5 revolutions and you MUST have the throttle open. You want to see at least 125 to 160 PSI on every cylinder with not much difference between cylinders. Then you should do a "Wet" test after each regular compression test called a "Dry" test. The "Wet" test is just a second compression test with about a teaspoon of engine oil put in the spark plug hole and this reading will be higher than the first test but it should not be more than 10% to 15% higher. That is a test of the piston rings.
The next step is to set the timing with a timing light. If you search you can find a timing light that is not expensive also. 12 degrees BTDC with the vacuum pipe off of the distributor if I remember correctly but look it up to be sure.
Next confirm that the plug wires are in the correct order in the cap. The firing order is 1,5,3,6,2,4. You can start at the front or the rear as Jaguar does. The danger of removing a plug wire to kill a cylinder is that the spark still tries to jump to ground and when a wire is removed the spark looks for the next easiest place to jump. This can be inside of the distributor down the rotor or the cap or worse yet down the coil tower to the primary which destroys the ignition module mounted on the front of the intake manifold. To test a plug wire to see if it has failed just remove it from the cap and the plug and switch it with another cylinder. For example, you already suspect #1 so remove that wire from the cap and plug and if you don't have an ohm meter to test it, put it in place of the # 2 wire and put # 2 wire in place of # 1 wire. Except this time place a piece of thin wire on the #1 plug along with the plug wire and hold the thin piece of wire with with either insolated gloves or you can slide a piece of rubber hose over the wire so you can handle it and start the engine and when it is idling touch the end of the thin wire to the head to short out #1 plug. If the RPM drops when you short it out than the plug wire was at fault. If you still don't get a RPM drop then you need to remove the plug wire and place a extra spark plug on the plug wire (remove your thin wire) and lay the plug with the wire attached on the cam cover so you can watch the spark when you start the engine. It should be a thick blue spark (not in sun light) If it is no spark or it is a thin yellow or orange spark then you have an ignition problem. Remove the cap and rotor and with a strong light look at the inside and outside of the cap (after cleaning it very well) Look for carbon tracking. It will be a crooked black line running from a plug position inside or outside down to the edge or over to another position. Next remove the rotor and look closely at it too for carbon tracking.
If all is OK up to this point you could have an injection problem. You can check to see if there is a injection pulse by purchasing a "NOID" light form an auto parts store. You need one that fits a Bosch/Lucas injector lead. They are not expensive. You just remove the lead from the suspected cylinder (1,4 & 5) and plug it into the injector lead and start the engine. It will flash if the signal is there. If they are all OK you could have a bad injector on #1,4 & 5 cylinder. These are more difficult to test but it can be done by just switching the suspected injector with a known good position. (use the thin wire attached to the plug with the plug wire attached to short the cylinder for a cylinder kill test, don't just pull a plug wire)
Don't forget to switch a spark plug with a known good one too to test for a shorted spark plug too.
let me know,
Howard
