Some of you may recall that I posted back at the beginning of March
that I was having an engine alarm problem with my 6.2L MPI Bravo 3.

After warming up, the alarm would sound at anything over 3,000 RPMs
and the rev limiter would kick in to slow down the engine. All panel
guages were reading normally, oil level and outdrive level were
normal, and a survey of the engine with a digital heat gun showed no
temperature abnormalities.

My neighbor who has a fair amount of engine expertise tried
disconnecting several different sensors but nothing helped.

The next step was to contact a local mechanic who is Mercruiser
certified. He showed up with his digital diagnostic instruments and
could find no record of tripped alarms. The only abnormality he saw
was a somewhat low reading on raw water pressure. We had already
tried disconnecting that sensor but decided to go ahead and replace
the raw water impeller. It turned out that not only was the impeller
scuffed up but also the internal pump housing so we replaced the
entire pump. Still no joy, same old alarm condition, no read out on
the Mercruiser instrumentation, but raw water pressure was now in the
normal range. Several calls by the mechanic later, several
conversations with Mercruiser, many more sea trials, another weekend,
and we were still no further along. Meanwhile the ECM had been
swapped out with one known to be good, the alternator checked, and
several more sensor swaps were made.

This morning the mechanic came back with one of his colleagues, a Dell
laptop computer with Mercruiser diagnostic software, interface cables,
etc. The laptop showed a low oil pressure reading even though the
panel guage was reading good pressure, and the Mercruiser diagnostic
box showed no oil pressure issues. Even with the low oil pressure
reading, the laptop software was still not indicating an alarm
condition although the alarm horn was howling, and the ECM rev limiter
was engaged. As luck would have it they had a spare oil pressure
sensor with them (the one that feeds the ECM, not the panel guage),
and swapped it out.

Voilla, no more engine alarms, $1200 and 3 or 4 weeks later.

Strange things these computerized engines, it must be running MS Vista
or have a virus. :-)

The mechanics say they have been running into a lot of sensor issues
with the 6.2L engines but it looks like the Mercruiser diagnostic
tools have a few bugs also, or there may be some issues with the way
the ECM is reporting problems (or not).