Mercury 135 Optimax Alarm Issue

"I've done a lot of readin

"I've done a lot of reading and well here's the deal. I recently purchased a boat with this engine, it ran great on the test run... I just got the licensing from the state and took it out yesterday. It started right up, and ran fine at the idle. My uncle was behind me in his boat he said the water flow out the back was looking good. So we moved out through the wake zone and when I went to plane it, the alarm sounded and wouldn't go above the slow moving speed. I backed it down to a slow pace and the alarm stopped. Tried it again and the alarm went off again. I read some people need to go from idle in hard on the throttle, but is this a sign of something wrong or just the way it has to work?

My uncle suggested the thermostats, but the fuel bars are in the way. Can they be loosened and retightened or do they have a gasket that will get messed up? I'm fairly handy with cars,,, trying to learn marine engines...

Any and all suggestions and comments are welcome and thank you in advance.

Ryan"

"Ryan, the cooling system on t

"Ryan, the cooling system on the Opti's is a touch different than every other outboard out there.

Water is pumped up to the base of the powerhead and then split in two - one stream goes over to cool the heads a second goes over to cool the compressor and fuel rails.

It's that second stream that provides the water for the tell tale (pee stream).

So while it does confirm that your waterpump is probably fine, it does not tell you whether the head is getting cooled properly.

Now, when we get to the heads the cooling is kinda divided into 3 stages.

From idle to about 1500 rpms (or there abouts) the thermostats control the cooling according to temperature - the 'stats open/close trying to keep the heads in the 140-160 degree range (roughly) - an overheat occurs somewhere around 190 and the alarm will keep sounding until it falls below about 175.

So if all is fine in the no-wake zone, your thermostats are probably working as they should, so on to the next stage.

The poppet valve. Water has two paths to exit the heads - past the thermostats or past the poppet valve - unlike the thermos which work on temp, the poppet works on water pressure.

In the thermo's range, the psi from the water pump is normally in the 1-4 psi range. The spring on the poppet valve will not start to open until at least 4 psi (4-9 psi range).

So as the rpms increase, and the water pressure increases (and the engine demands more water to run at the correct temp), the poppet starts to open to let the water pass through the powerhead.

The poppet will allow far more water to pass compared to the thermostats - so if the poppet is "stuck" or broken, the water can not exit the powerhead fast enough to carry the volume required to keep the motor cool and you get an alarm (and an rpm reduction).

Although the poppet funtions on pressure, it's operating range is roughly in the 1500-4500 rpms range - above 4500 rpms, the waterpressure will exceed 9 psi, effectively "locking open" the poppet, so water is then passing through the engine just as fast as it can to keep things cool (with the thermostats and poppet having virtually "no effect" on the cooling).

I mentioned the second path earlier (to compressor, rails, tell tale). There is a strainer (filter) on that line and if it gets clogged then you can get an overheat detected at the compressor (which causes an alarm and reduction in rpms), but normally reducing speed will not "unclog" that filter, so in your case, my first suspect would be the poppet valve...."

"wow that was fast and very in

"wow that was fast and very informative. I should have mentioned the motor is a 2001 in my prior post. I have a manual coming in this week, I'll look at both of those tasks or is it something I should bring to a marine mechanic? Thanks again for all the info, I'll post back up what the problem is when I get it resolved."