What would cause a Mercury Optimax 150 to chug at high RPMs when air temp warms?

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jhfishdad

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I have a 2006 Mercury Optimax 150 outboard, in the cool morning air in Florida it runs 5700rpm Wide Open Throttle (WOT) with no problems. After the morning air temperature rises (11pm) the engine will only turn 5200 rpm maximum and after about 5 minutes will begin to chug and misfire and won't continue at high rpms. This happens whether the engine is cool or warmed up. None of the warning buzzers go off. I can slow down and continue to run at 4800-4900 rpms without experiencing this issue. Thinking it was a fuel issue I replaced fuel filter and insured the fuel line ball is pumped up. All is OK. I even used Mercury fuel additive to remove any water in the gas, but to no avail.

How does a mercury optimax outboard handle the air metering/compression/fuel pressure system? What happens at 5000 rpms in this air/fuel system to be making this difference with the engine? Could this be related to Mercury's air mapping system/sensors?
 
This could be a sensor problem...

The Optimax is a direct injected engine, and has a lot of different sensors that monitor many different things within the engine, and many of them do play a role in fuel delivery.

Optimax engines are designed to inject more fuel on a cold start, which allows the engine to warm up faster. Once the engine comes up to temperature, the ECM will then regulate the fuel back to the normal amount.

What I have seen before, is a fault with those sensors, which in turn makes the engine inject that extra fuel for cold start up at ALL times, even when it is up to temp. This will cause your engine to run poorly.

My suggestion to you is to take your engine to a Mercury dealer, where they can hook it up to the DDT or the Mercury diagnostic lap top, and they can look at the faults that are specifically occurring with your engine. Since the Optimax has quite a sophisticated fuel system, this is the best thing to do.

Good luck.
 
Possibly a faulty air intake temp. sensor, or MAP sensor. Have the motor scanned at a dealer.
 
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