Mercury 90 hp 1992

[sly36]

I have mercury 90 hp 1992 with high speed overheat alarm, new impeller, heat sensor, thermostat I clean the water pressure valve.
Strong water flow at peehole.
I know they have ? lot reason to have overheat alarm but where to begin ?

Thank !

 

[Texasmark]

Below 2500 rpms (service manual number) the tstat is in control....starts opening at 143F.......that's about as hot as your hot tap water in your home......you can hold your hand on the top rear of the engine block but it will be uncomfortably warm. Above 2500 rpm, water pressure knocks the poppet off it's seat and bypasses the tstat with higher, non temp controlled flow. OT alarm is 195F. That will require you to remove your hand form the top rear of the engine block in about 5 seconds or less. If you have an alarm below it's the stat., if above the popoff. Obviously it takes a couple of minutes operating in the range listed for it to come on or if on to go out.

Pee comes off the exhaust manifold (cooling) water jacket and has nothing to do with the engine block internal cooling.

To begin consider the above. Then go to the rear of the engine block, just to the left of #2 sparkplug and upon locating a tan or tan/light blue stripe wire, follow it towards the wiring harness till you get to a bayonet connection and disconnect it. Horn should cease. If not wiring problems between that wire and the horn which is in the control box or near it tied to the wiring harness there.

 

[sly36]

I unplug the heat sensor and no alarm come at high speed the top of engine is not to hot I can leave my hand long time but if I run the boat 5 minutes at WOT after when I return at idle speed the engine run poor look like mistfire or because engine overheat I dont know.
I try to mix oil in gas and samething appen.

If the engine is mounting to low on the transom I have 1 1/2 under the V of the hull can have something water restriction or something ?ter.

Normaly the water not supose to be warm at the peehole when I run ? high speed.

If the ppopet valve not work what appen

Thank you !

 

[Texasmark]

If you unplugged the heat sensor and didn't get an alarm get an ohmmeter and ohm out the sensor. From the tan wire to the engine block (shiny aluminum) should be an open circuit till you get up to around 195 and then it will show a short.

If you run at WOT for 5 minutes and can still leave your hand on the top rear of the block you are NOT overheating! When you do what you said in the run 5 min at WOT....... If you go back to WOT does the engine run ok or it runs bad at slow and high speed after the 5 min run?

When you are talking having 1 ? inch under the V of the hull, what are you talking about?????? the horizontal plate just above the prop that extends across the top of it? If so, that's the anti-vent plate and you are ok on being low enough to pick up adequate water.

If the poppet stays closed at WOT and you run long enough and it's hot enough, your thermostat will open but will not be able to allow adequate water to flow through the powerhead so you will get an OT alarm. If you slow down, 2500 rpm, it should go off. Go back up and it comes back on....poppet valve....first consideration.

 

[sly36]

Thank you Texasmark !

I will test the heat sensor like you expla?ne..

When I run the engine 5 minutes, after in idle speed, the engine run bad, but if I return at high speed no problem.

You right the plate is under the V hull.

For the poppet valve you right thats what happen when I in WOT overheat alarm on when I return under 2500rpm after 1 minutes alarm come off...

So ist mean I have to replace the puppet valve?? Or I have to do another think ?

Thank you for your help and sorry for m'y bad english !

 

[Texasmark]

Replace the poppet. Iboats may sell it, look at the top of the page under Boating Parts and Accessories. They sell a lot of Sierra after market parts and OEM. I have used both from them and both work for me.

Get several cans of Sea Foam engine cleaner. Wall mart and most auto parts stores sell it....$6.50 per pint range at WM. It says to add 1 oz per pint of your regular fuel mix if premixing, or if engine has an internal oil pump and you are using that then 1 oz per gallon of gas. However for the first couple of cans, triple that amount. Give that time to clean up your fuel and combustion systems and then come back with how it runs after that and we'll go from there.

Having answered my question as you did, about after idle WOT ran fine, I don't anticipate any major problems with your engine. I don't know what oil you run in your Merc. currently, but I like the Pennzoil Super Premium TC-W3 available in the WM sporting goods section. It's semi-synthetic and half the price of Merc oil. It burns real clean, almost no smoke.