over temp horn

[Lew Taylor]

I have a 90 90hp yam that has the horn go off when I run at over 4000 rpm.I have had the temp sensor replaced, new water pump done and a complete themostat replaced inc popet.
Any ideas

[Hamfisted]

Do you have a water pressure gauge installed? Whats it showing when you get this alarm?


Mike

[Lew Taylor]

No I don't is there a fitting in the block that I can fit a gauge to and what range of psi/kpa would it need to be

[Lew Taylor]

[Lew Taylor]

[Ken]

Lew,
There should be a brass bolt (plug) in the top of the cylinder assy just in front of where the thermostat sits in the head. You can install a water pressure coupling or temp sensor in this threaded hole.
The pressure relief valve (poppit valve) is designed to open at around 20 psi, so I would guess at idle, the pressure should be around
8-10 psi, and increase up to 20 as rpm approaches max. This assumes, of course, that the water pump is doing it's job, and the PRV is not stuck partially open, allowing cooling water to pre-maturely dump out the relief holes.
If the pump, thermostat, and PRV are doing their job correctly, and you are still getting an overheat alarm (and assuming the thermo switch is doing it's job by closing at 180 degrees f.), then the only thing left would be clogged up water jacket. The head would have to be removed and the water jacket cleaned out. Really, not a major task, but the head gasket would have to be replaced and the head bolts torqued per the shop manual.
Also, you say the water pump was just replaced.
I have read where pieces of the impeller have broken off and got stuck in the water pump tube leading from the water pump housing to the water jackets - something else to consider. A good reason to inspect the old impeller when doing a pump repair.
Good luck [img]smile.gif[/img] ,
Ken K

[Lew Taylor]

Ken many many thanks I have inspected the old impeller and it is in worn but intact.I will fit up a PSI gauge and run it up in the weekend .I also think you may be right about the blockage as the motor sat around for 2 years as the owner died and his wife did not use the boat. when I first used it the horn went off and the service agent I purchased the boat from fixed it (thats when the sensor was replaced .