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Alarm keeps sounding over certain RPM's

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Kim C

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I have 2004 Mercury Outboard 90 HP. Last year the alarm started going off if you exceeded 4200 RPMs so we replaced the water pump impeller and gaskets. Ran approx 10 minutes and solid alarm would go off again. Pulled water pump back off and impeller was burned at the hub even though it was run in water. NEXT: replaced water pump base an gaskets, impeller, base plate and gaskets and water coupler. Same results. NEXT: we replaced the thermostat and power head base gasket. No change. NEXT: we replaced the exhaust plate gasket if ran under a load of 4200 RPMs it would run all day long, but exceed that for any length of time the alarm would go off. NEXT: replaced the sensor in the engine. No change. NEXT: I took it to a aluminum welding place that specialized in Pontoon Boats and he drained the pontoons, but said that there wasn't that much water in them and that he could not see any damage to the pontoons or anything that would cause any water diversion or issues when he had the boat off the trailer. NEXT: I had the water jacket gasket poppet valve kit with gasket replaced and theromostat replaced. And did tune up while there at the end of year as well including sync and link on engine. ALL of the above repairs have been done by a Mercury double certified licensed mechanic so not just a back yard person doing the work.

Put in the water the first time last weekend. Now I can not exceed 4000 RPMs so I lost max RPM;s that I can drive around at before it goes off. But water pressure is more steady and not bouncing around like it did last year.

Does anyone have any other ideas of something to try??? Getting very frustrated. We do not abuse the boat or drive in shallow water, never in sandy areas to pick up anything in the engine.

Thanks for any help you may have
 
You say water pressure is "more steady" - do you have a gauge and what readings are you getting?

I would expect somewhere in the 1-3 psi range at idle. Above 1500 rpms upwards of the higher 3K range I would be looking for 4-9 psi and you would need about 12 psi to keep it (cooled) running at full throttle.

You don't mention if you run in salt or fresh water. Both, in their own right, can cause issues with the water tube. If your pressure readings are low (even if they are steady) you could have a restriction (calcium, lime other minerals) blocking water flow up the tube or a hole burned in the tube (caused by salt) letting some of the water out (into the leg) instead of getting it up to the power head.

You could also have mineral/salt build-up in the water channels themselves in the power head. Even if the whole water pump system is working fine, if there is crud in the channels it will never cool properly.

Crudded up channels or toasted water tubes are not as uncommon as you might think (in certain areas). I boat in water with heavy lime concentrations so once a year the water channels get soaked out with white vinegar (a very cheap, very safe acid that eats minerals - slower than commercial products, but only costs a buck a gallon).
 
Graham,

Thanks for your response. We are in fresh water and I never though of calcium or lime build up. We do have harder water in this area of Missouri so that could be a possibility. I tried getting a hold of the mechanic that had it apart last year to see if he saw any build up. Exactly how do you soak your channels each year?

As far as the water pressure goes. Our water pressure gauge on the boat is showing it running around 8 at 3600 rpms and never went over 10 at all. Saturday when I took it out the alarm kicked off at 4000 rpms and then at 3800 rpms. So I have pulled it off and taken it back to the dealer to have them try to fix it again.

But if you would tell me how you soak your channels I would like to do that to prevent any calcium lime build up. That would be great. We are newer boat owners and really don't know much about the engine, except from what I have been learning over the past year dealing with the problems.

Thanks again.
 
It's a little tougher on larger motors but here is how I do it with my 140 which weighs close to 400 pounds.

With the boat on the trailer I put the trailer nose on the ground with the help of a jack (fold up the tongue jack and lower until the coupler is almost touching the ground.

I pull the lower unit and then plug the water tube and stuff a couple rags up the exhaust housing (trying to half decently seal them from having liquid leak out - doesn't have to be water tight, just enough to keep liquid in there for 24 hours or so without all of it leaking out)

Then remove the hood.

Then put the motor in the "trailer" position (so max raise with the power trim). Motor should be sticking almost "straight", but with the trailer nose on/near the ground you get a decent "upward" angle (if you are following me here).

I then pull one of the thermostats and start pouring in the vinegar (ya, just plain old white vinegar - a buck a gallon - 1/20 the price of the lime/calcium away products and does the same job - just takes 24-48 hours to do it). Keep pouring in the vinegar until it won't take anymore and then leave it as is.

After a couple days, I tilt the motor back down and "pull the plugs - (rags)" and let her drain out.

Clean as a whistle and the vinegar doesn't hurt anything but sure does a number on the (lime in my area) - nice clean metal.

Total cost - about 2 bucks for vinegar and a couple days of sitting.

(with a smaller motor, I remove it from the boat and simply turn it upside down and pour the vinegar down the water tube with a funnel - no need to plug things - but a little tough to do one the motor cross that 150/200 pound mark)
 
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