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I have been getting an intermittent “Check Engine” light on 1999 135 Mercury Optimax. According to Mercury this light by itself with no warning horn indicates either the horn itself is not functioning, or a problem with either Air Temp Sensor, Coolant Temp Sensor, the MAP sensor, or a malfunctioning coil or fuel injector.
Prior to the start of the intermittent light I had two other brief anomalies: First I had an afternoon a few weeks back where suddenly the motor was ‘breaking up’ or bogging down at very wide open throttle. I run about 5250 RPM at WOT and the problem did not occur until I tried to get to the last few 100 RPM’s. It almost seemed like a fuel starvation issue. On a hunch I replaced the old water/fuel separator on the motor and the issue hasn’t occurred again. I had no warning lights during this episode.
Second issue was an engine temperature light that came on one afternoon again at WOT. I throttled back, shut down, restarted the light cleared and hasn’t happened again. What seemed odd about that event was my temperature gauge did not indicate overheating and the motor did not automatically throttle down to 3000 RPM’s as it supposed to do in an overheat situation.
The warning horn is sounding on start up/self check, so that’s not causing the light. My suspicion is the problem lies in the either one of the temp sensors or the MAP sensor and maybe even comes down to a connection/ground issue which have yet to have the time to troubleshoot.
But my questions are can I have a bad injector or coil that’s activating the light but not apparently affecting engine performance in terms of RPM’s or speed? Or can an injector or coil be “intermittently” bad?
Update: Mechanic put the motor on code reader and check engine codes are "injector related". Before I go and pull the boat, have the injectors removed and sent out for cleaning/testing anyone have any thoughts about trying a strong dose of injector cleaner in the fuel system first?
Before letting first mechanic tear things apart I got a second opinion.
Had Mid Coast Performance Marine in Patchogue, New York look over the motor. Found a dead Direct Injector on cylinder three, replaced that, replaced the standard duty spark plugs with recommended high performance ones, replaced a wonky battery switch and all is right in the world.
Eric, owner of Mid Coast knows Mercs! Anyone in the area with Mercury outboard or I/O's should check this guy out. Clean shop, well organized, right tools, and when he's not working on Mercury's he's racing them on his offshore powerboat. Funny thing is he's not all that well known around here among boaters.
