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Re: 1991 XJ6 cylinders 5/6 not firing
Check for spark at the cylinders you mention. Unplug the wires from the spark plugs, put a spark plug in the end of the wires, and attach the body of the plugs to earth, or an exposed metal part of the engine. With the engine running, you should have spark at the plugs. If no spark, get the ignition system tested. If there is spark present, and you think the fuel is ok, then the compression must be at fault. The things that are required for combustion are: spark, fuel, and compression. Get a compression test done on the engine, but first check for any vacuum leaks in the inlet manifold around the cyl 5 and 6 area.
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Fuel, spark, air, compression, timing. Essentials for an engine to run. You've addressed injectors (fuel) and spark (plugs and wires), and I assume it's getting air as it is running. The next thing I would do is a compression check. If you have low compression on a cylinder or cylinders, you may have mechanical problems with valves or rings.
Check the fuel injector to that cylinder for power and ground, its best to use a noid light to check for proper firing. It would likely be plugged, no power or internally open electrically and not opening the injector to fire. See if you get it scanned at a parts retailer for better diagnosis. If its easy to access, remove it and swap with the fuel injector next to it. If the misfire changes to Cyl #5 then you know its no good.
unplug the fuel injectors, check the olhms on all. the readings should all be the same, around 11-11.5 olhms. Any readings that are higher is a sign of a bad injector which is what your problems sounds like., putting to much fuel to a cylinder causing different fuel amounts to each cylinder and causing it to backfire out of the throttle body , excess fuel . Also remove the throttle body and clean with cleaner and a toothbrush after replacing any injectors. Or there's a simpler answer and that the cambshaft is worn-out/round causing the lifters not to open enough to let in all the fuel. A worn belt would not cause misfires only on select cylinders but rather jump timing, slipping a notch on a gear and possibly bending a valve or valves and this cause a loud clacking or a really a sign to quit running the motor because it's broke! Good luck.
You have done a great test but If I assk you to do a swap of the Ist cylinder plug and injector with one from the 3rd , we can come to a conclusion before we can change the full set of plugs and injectors for a good performance.( only if the mileage is time enough to do the change)
If you have confirmed with the spark and the fuel ( hope the fuel pressure is good and the pump calibrated , even though this is not relevant) please also check the wires to the first plug and the fuel line to the first injector.
I hope there is no issues in the pump finally for not releasing enough fuel to the first.( Check for presence of fuel pressure in this line)
I am sure you are confident enough to get to the fault .
Hope you credit me if this helps a bit. Good day
Test the compression on the two cylinders that won't fire. Make sure their plug wiring is not crossed. Check those two spark plugs after running a short time--they should be wet with fuel if not firing. Any injector that won't stop leaking should be replaced. Fuses and relays should not be responsible for just two cylinders not running. The check engine light should be on--read the computer with a scanner. An engine with two cylinders inoperative SHOULD vibrate heavily. Good luck!
is there spark coming out of coil when running? ck compression? and then the fuel injector is next on the list. have a shop that has a tech2 scanner check it, or they they can do a "injector flow test" and see if its opening and flowing when commanded, could be plugged.
Try some injector cleaner in the fuel--might be a problem in # 3 injector. One cylinder misfiring should not cause the engine to stall. Replace the fuel filter, and check the air filter. Does this engine employ a distributor or a coil-pack?
try doing a cylinder balance test take one plug wire off the coil at a time while running the engine to locate the cylinder thats not firing . then check the plugwires with dmv meter should be 7000 ohms per foot also engine running u can disconnect one ijector at a time to see if it makes a difference too locate the problem the engine wont change if the injector is not flowing fuel if feel it pulsate its working electricly may be still plugged.
Mine had a similar problem, there is a wire that goes from the computer to the fuel rail and this wire sends info on how much fuel and when to inject the fuel... if that wire is grounding out then this will cause the lack of cylinders to fire and also explain the inconsistency. So i would start with the fuel rail..hope this is of some help