newport dave
Chief Petty Officer
- Joined
- Nov 21, 2004
- Messages
- 458
This one almost kicked my butt. Since last June (off and on) I have been working on a 39 performance cruiser with twin Mercruiser 496HO / PCM555 engines. The starboard engine had an intermittent problem with momentarily losing power. The engine would lurch (bog for a second and then return to normal) and the alarm would sound as though you very quickly turned the key off and back on. Sometimes it would completely die. It would do it at idle or while cruising, seemed to be totally random. <br /><br />Connecting a scan tool was first. There were no active or stored codes that helped any. I wanted to watch what happened on the scan tool when the power loss occurred, but every time the engine would lurch the scan tool would instantly lose communication. The fact that the scan tool would lose communication suggested that the ECM was losing power.<br /><br />I began a long process of checking the entire electrical system from the battery to the key switch and back to the engine. It came down to the ECM appeared to be shutting down and/or disengaging the main power relay for no reason. I thought it must be the connections at the relay and/or ECM, but they all were good. I thought it HAD to be the ECM. Swapped it with the other engine thinking the problem would switch engines it did not.<br /><br />What now? I have checked everything I could think of, tried every troubleshooting procedure in the PCM555 manual, tested and/or swapped out anything that I thought even might be causing the problem, and no change at all! <br /><br />I finally did find the problem by pure dumb luck. It was something I never even would have guessed. Anybody think they have an idea?<br /><br />Dave
