posted 08-11-2015 08:23 PM ET (US)
I am hoping I can reach out to some of you vintage OptiMax experts out there. I have recently been having problems with my [1999 Mercury] OptiMax 200-HP outboard engine on my 21 Conquest. [The engine] has been very difficult to start and keep running at idle. [The 1999 Mercury OptiMax 200-HP outboard engine] will catch for a few seconds then die out. Sometimes over 10 starts [are necessary] to keep [the 1999 Mercury OptiMax 200-HP outboard engine] running, and I have to quickly put [the shift into] gear for [the engine] to stay running. [The 1999 Mercury OptiMax 200-HP outboard engine] also surges a bit between around 700 to 1,100-RPM. [An engine speed greater than] 1,100 RPM is great, no [problems with stalling].I have the shop manual and followed the following recommendations. None of these corrected the [stalling and idle problems].
--replaced tracker valve diaphragm
--replaced Fuel Regulator Diaphragm
--fuel pressure and air pressure within specification
--replaced primer bulb, and
--checked fuel pumps
I finally took [the 1999 Mercury OptiMax 200-HP outboard engine] to a certified Mercury shop. The [certified Mercury shop] noticed a lot of carbon build-up and the [peak cylinder pressure under test] was only in [about 85-PSI]. Then the [certified Mercury shop] [performed a procedure to remove carbon] with [a solvent called] Powertune. They were able to increase the [peak cylinder pressured under test to] around 90-PSI on all cylinders. The [certified Mercury shop] also replaced the starter because they said it was going bad and dropping the voltage down to 5-Volts during startup. And they replaced an ignition coil that was suspect.
When I got back [the 1999 Mercury OptiMax 200-HP outboard engine], it did seem to run better at first. But it quickly went back to the original [behaviors, that is, it was hard to start and stalled at idle speeds]. I started playing around a little bit more by checking the temperature sensors and seeing how the engine would react to disconnecting one at a time. Disconnecting any of the sensors really did not made a difference in how [the engine] ran, except for the starboard cylinder head temperature sensor. When I disconnected [the starboard cylinder head temperature sensor] the engine picked up about 100-RPM and ran beautifully. I turned it off and restarted it and it fired up perfectly! I took an ohm meter to the sensor leads and it appears the sensor is still working correctly based on the resistance graph in my engine manual.
So I am stumped! Based on the wiring diagram of the engine, the sensor is connected to the ECM and the Throttle Position sensor. Could there be an [malfunction] with my ECM or TPS that is interpreting the signal from the temp sensor incorrectly, and it's messing up the fuel or injector mapping?
I am loosing my mind over trying to fix this problem! I appreciate an feedback.