Merc 280 running cold

[H2oSki2Fast]

I have a Mach 22 w/ twin 280's ROS boxes. Boat starts great runs good for the most part.

The temps come up at the same time to around 150*, and stay there. If I cruise around everything is good. If I run it up above 5500 or so rpms, for a long distance (2 plus miles), the starboard motor temps slowly drop. They drop till it is no longer registering on the gauge. At that point the motor starts to sound odd, like it is getting too much fuel. If I go back criusing the temp comes back up.

The motor with the problem has approx 35 hours and the other around 10. I bought the boat used, from a reputable person/ dealer. All Merc parts were used in rebuilds.

Anyone have any idea what could be wrong? The boat is run in Lake Havasu, Az.

Thanks
Dennis

[Bunger]

Maybe a poppet issue? What are the water pressures looking like?

[H2oSki2Fast]

Water pressure on both motors are the same, at 12-13 psi above 6000.

What exactly does thee poppet do? My old motor was a 260 (different boat), the poppet was removed.

[catfish123]

Water pressure on both motors are the same, at 12-13 psi above 6000.

What exactly does thee poppet do? My old motor was a 260 (different boat), the poppet was removed.

Pretty sure Merc 280's do not have poppet valves.

[1BadAction]

No poppet on a 280. Make sure the restrictors are in the thermo housings. Are they 100% stock? Have they always done this? If it's always done this it may be worth taking a look at the exhaust plate to see if the dumps have been messed with.

[Bunger]

Water pressure on both motors are the same, at 12-13 psi above 6000.

What exactly does thee poppet do? My old motor was a 260 (different boat), the poppet was removed.



Its like a pressure relief valve. Once you get to a certain rpm and the pressures at a certain psi it opens up and dumps excess water so the block pressure doesn’t get to high and blow out gaskets.

[H2oSki2Fast]

It has done this since I bought the boat beginning of november. From what I was told, the motors were rebuilt as stock, but they put offshore mids and ROS ECUs on it.

[1BadAction]

they put offshore mids and ROS ECUs on it.

That right there makes it worth checking. Take a look at whats in the thermostat housings first, compare to the "good" motor. Compare the elbows hoses and restrictors from the poppet block off plate and the thermostat hoses. If that's all identical, pull the powerhead and take a pic of the exhaust plate to post here. You're dumping water too fast somewhere.

Ignore the talk about poppets, it's irrelevant because these motors don't even have them.

[Schramer]

Try to put some restrictors in the dumps and see if it helps at all......at least that is a easy experiment to try before you start tearing into chit.

[H2oSki2Fast]

That right there makes it worth checking. Take a look at whats in the thermostat housings first, compare to the "good" motor. Compare the elbows hoses and restrictors from the poppet block off plate and the thermostat hoses. If that's all identical, pull the powerhead and take a pic of the exhaust plate to post here. You're dumping water too fast somewhere.

Ignore the talk about poppets, it's irrelevant because these motors don't even have them.

I forgot to mention, I have already looked at the restrictors in both motors. They are the same. I think it's come down to pulling the powerhead.

[patchesII]

Do you have EGT's?

[H2oSki2Fast]

Do you have EGT's?

I do not.

[tomoad]

Are the mids drystacked?

[patchesII]

Sounds like the cooling system is operating. If they were mine I'd start swapping parts to insure the cool engine isn't just getting flooded with fuel. I'd start by swapping ecu's. Egt's would answer this very quickly