Check compression on cylinder #4. If ok try swapping the coil from cylinder 4 to one of the front cylinders and redo the Key On Engine Running (KOER) test. If the code changes to the cylinder where the coil was moved to, replace the coil. 1. Do a running compression test on cylinder #4, and compare the reading to the running compression readings on one or two of the other cylinders. To do this, remove the Schrader valve from the compression gauge (from the part that screws into the spark plug hole), install the gauge into the cylinder to be tested, disconnect the COP, start and run the engine at idle. monitor the gauge needle. The needle will typically be bouncing so rapidly that it looks like a blur, but note the high side of the bounce, see if it is consistent and compare it to the good cylinders. The results will vary depending on the gauge itself and the length of the hose, but on average, the running compression reading is usually about half of the cranking compression reading. If the cranking compression is good, but the running compression is low in comparison to the other cylinders, this usually indicates a valve train problem, such as a worn cam lobe, a broken valve spring or a valve sticking. Install a vacuum gauge and observe the needle. A regular, evenly spaced downscale flicking of the needle indicates one or more burned or warped valves. Insufficient hydraulic lash adjuster clearance will also cause this action. The vacuum gauge should read between 15 and 22 inches depending upon the engine condition and the altitude at which the test is performed. Subtract one inch from the specified reading for every 1,000 feet of elevation above sea level
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