Mercruiser Alarm staying on at low speeds and Idle

[E_S_A_D]

So our boat has run great all year. All of a sudden yesterday, the alarm stayed on at idle and low speeds. Once increased speed to above 10 mph or so the alarm went off. Any ideas,

all guages showed normal...

 

[Bt Doctur]

Not know what you have there ,the constant alarm sounds for high engine temp, low/no oil presssure, or low/no drive oil

 

[E_S_A_D]

The low drive oil, where would I check that? And what would I add? I’m not very mechanical.

Also, which would meet those symptoms most that the engine stops doing that after the RPMs and speed are up?

 

[Bt Doctur]

Most cases it`s the oil pressure switch,NOT OIL PRESSURE SENDER. Low oil pressure from incorrect oil or a very high engine temp thinning out the oil. A leaking fuel pump diaphram will leak fuel into the oil pan diluting it.
Drive oil bottle res is usually at the motor ,port side ,if equipped

 

[Bt Doctur]

Again , not knowing what your working on makes it tough to give the correct answers

 

[achris]

So our boat has run great all year. All of a sudden yesterday, the alarm stayed on at idle and low speeds. Once increased speed to above 10 mph or so the alarm went off. Any ideas,

all guages showed normal...

Life Of A Partsman
I work behind the counter
in an automotive store,
Sometimes I'm a "genious"
Sometimes I'm called much more

I claim I'm no Mechanic
But, when the job gets sick,
The Mechanic comes and asks me
What makes the darn thing tick.

I'm supposed to know the numbers
Of bolts, nuts and gears,
For every car that was ever made
For more than fifty years.

I'm an Engineer and Machinist
And what not, oh My Lord,
I'm supposed to be an Edison
Combined with Henry Ford.

But life would be a pleasure
And I'd grin from ear to ear'
If the customer would only tell me
The Make, Model and Year.

Chris........

 

[E_S_A_D]

Mechanic so far thinks bad impeller even though it didn’t effect temperature gauge. Make sense?

also, what should I expect to pay for a 96 4.3 Mercruiser impeller replacement?

 

[achris]

Mechanic so far thinks bad impeller even though it didn’t effect temperature gauge. Make sense?

Nope. Bad impeller only causes one thing, overheat. Very difficult to get an over heat if the gauges all read 'normal' (and before anyone jumps on that, yes the alarm switch is different to the gauge sender, but it's very unlikely you get the alarm only at low speed if it was a faulty sender or switch)....

also, what should I expect to pay for a 96 4.3 Mercruiser impeller replacement?

Just to change the impeller, about an hour. But I suspect that is not your problem. What is the service history on this engine? When was the oil last changed, checked, oil filter, quality of oil etc? Have you checked the oil level? Chris......

 

[AShipShow]

But I suspect that is not your problem. What is the service history on this engine? When was the oil last changed, checked, oil filter, quality of oil etc? Have you checked the oil level? Chris......

Agreed...

My assumption would be oil pressure related, as the motor speeds up, your oil pressure will increase assuming your pump is not totally shot (causing the alarm to turn off). The alarm will sound whenver the oil pressure is below 6psi which is too low (obviously). First step would be to verify the oil pressure at idle.. You could just have a faulty switch hopefully, but it could be worse (oil pump, etc)...

 

[E_S_A_D]

I thought I'd provide an update. Mechanic fixed it. It was two things, Mercruiser trim level oil needed oil. Also, even though it wasn't overheating (yet), the water pump impeller needed to be replaced. All good to go now... I would've never guessed overheating alarm when it stayed in the cool range, but I guess at idle it didn't have enough water going in or something with the bad impeller.

 

[AShipShow]

Why is your outdrive oil level low? It doesn't just "go down"... Hopefully its a leak, but you might want to inspect to verify that your drive is not taking on water.

 

[E_S_A_D]

It wasn't the outdrive oil. It was the trim oil

 

[AShipShow]

DOH! misread that...
ON a side note, the trim pump reservoir doesn't tie into the alarm system... so that doesn't have anything to do with your alarm.

 

[E_S_A_D]

Yah, I'm still not quite sure how the alarm went off for the impeller when it never showed being 'hot' on the gauges.

 

[AShipShow]

have you checked the drive reservoir to see if its full?

 

[achris]

Yah, I'm still not quite sure how the alarm went off for the impeller when it never showed being 'hot' on the gauges.

Because the gauge sender and the alarm switch are 2 separate circuits. Not related to each other. Even if the gauge were to completely fail, the alarm system would be unaffected.

Chris.........