newport dave
Chief Petty Officer
- Joined
- Nov 21, 2004
- Messages
- 458
Re: Mercruiser 496 HO - Difficult Diagnosis
Final Diagnosis: Bad plug wire <br /><br />I know, sounds ridiculous. The 496 is a DIS (Distributorless Ignition System) engine, meaning that it has a coil for each cylinder. Four on each side of the engine, mounted on the valve cover.<br /><br />In this case, the #7 cylinder spark plug wire was leaking secondary ignition voltage at the coil tower, arcing to the coil frame. The coil frame and entire valve cover became part of the secondary ignition system. Due to a rubber gasket and appearently pretty decent paint job, the valve cover was not grounded and the ignition voltage found the easiest path to ground (intermittently) was arcing across a gap from a bare spot (chip) in the paint on the valve cover next to a mounting bolt, to a bare spot on the upper part of that mounting bolt. Just so happens that mounting bolt was directly under the ECM . The 40,000 volt discharge (about 2.5 inches away) was creating enough EMI (Electro Magnetic Interference) to momentarily shut down the ECM.<br /><br />I only found the problem because I happen to be at just the right place to actually see the spark under a wire bundle. You could not hear it due to normal engine noise. I pulled the wires away a little and I could hardly believe it, a nice fat blue spark from the valve cover to the mounting bolt. Directly under the ECM . The engine lurch happened at the exactly same times as the spark occurred. I put a jumper wire from the valve cover to a good ground. Problem went away. Took it off problem came back. Still unsure what cylinder it was, I put it on the scope. Found an intermittent misfire on #7. Visual inspection confirmed arcing from the coil tower (under the plug wire boot) to the coil frame. Changed #7 coil, wire and plug. No more problem. <br /><br />Thanks for your responses. Maybe someday this will help someone with a similar problem.<br /><br />Dave<br /><br />
<br /><br />
Final Diagnosis: Bad plug wire <br /><br />I know, sounds ridiculous. The 496 is a DIS (Distributorless Ignition System) engine, meaning that it has a coil for each cylinder. Four on each side of the engine, mounted on the valve cover.<br /><br />In this case, the #7 cylinder spark plug wire was leaking secondary ignition voltage at the coil tower, arcing to the coil frame. The coil frame and entire valve cover became part of the secondary ignition system. Due to a rubber gasket and appearently pretty decent paint job, the valve cover was not grounded and the ignition voltage found the easiest path to ground (intermittently) was arcing across a gap from a bare spot (chip) in the paint on the valve cover next to a mounting bolt, to a bare spot on the upper part of that mounting bolt. Just so happens that mounting bolt was directly under the ECM . The 40,000 volt discharge (about 2.5 inches away) was creating enough EMI (Electro Magnetic Interference) to momentarily shut down the ECM.<br /><br />I only found the problem because I happen to be at just the right place to actually see the spark under a wire bundle. You could not hear it due to normal engine noise. I pulled the wires away a little and I could hardly believe it, a nice fat blue spark from the valve cover to the mounting bolt. Directly under the ECM . The engine lurch happened at the exactly same times as the spark occurred. I put a jumper wire from the valve cover to a good ground. Problem went away. Took it off problem came back. Still unsure what cylinder it was, I put it on the scope. Found an intermittent misfire on #7. Visual inspection confirmed arcing from the coil tower (under the plug wire boot) to the coil frame. Changed #7 coil, wire and plug. No more problem. <br /><br />Thanks for your responses. Maybe someday this will help someone with a similar problem.<br /><br />Dave<br /><br />