As part of my refurbishment of this high mileage 4.0 inline 6 (208,500 miles), I changed out the spark plugs and the coil rail (see pic of old plugs below)
I removed the coil rail first and took each plug out one by one and used a Harbor Freight compression tester to test each cylinder, but since the coil rail was off all I could do was crank the engine for a few seconds. I'm unsure if that produces a reliable result or not. But when I was all done, the engine roared to life and sounded more powerful than before the new plugs and coil rail.
By the way, the plugs I used are the Autolite XP Iridium Spark Plug, gapped to the manufacturers .035
Four of the cylinders reported about 95 lbs of compression, one at 105 and one at 110.
I removed the coil rail first and took each plug out one by one and used a Harbor Freight compression tester to test each cylinder, but since the coil rail was off all I could do was crank the engine for a few seconds. I'm unsure if that produces a reliable result or not. But when I was all done, the engine roared to life and sounded more powerful than before the new plugs and coil rail.
By the way, the plugs I used are the Autolite XP Iridium Spark Plug, gapped to the manufacturers .035
Four of the cylinders reported about 95 lbs of compression, one at 105 and one at 110.