no water flow to raw water pump in an Mag 496 with a Bravo III outdrive

[highlander86]

I tracked down the an alarm to the temperature sensor on the exhaust manifold. Both sensors starboard and port slowly seamed to silence the alarm but the starboard sensor silenced the alarm the quickest.
The problem is no sea water at the raw water pump inlet, I removed the discharge pressure sensor and installed a pressure gage, start engine with full hose pressure to the raw water intake ports on the out drive, no pressure recorded on the gage.I removed the hose up stream of the air actuator, with full water pressure, still no water flow. I inserted a hose in that line to see if there is an obstruction and with full hose water pressure held the inserted hose while the raw water inlet was observed by others, water did flow out the intakes. My conclusion is that the impeller in the drive is shot although I don't understand why there would be no flow with full water pressure. Any ideas?

 

[jpk33]

Could the impeller fins have broken off and are plugging the water passage?

 

[tbirdvet]

No impeller in the B3 drive. Only impeller is at the raw water pump on the lower side of the engine.

 

[Henry Boyd]

The 496 / BIII raw water system plumbing is terrible. Apparently the pump just barley generates enough head to pull water up through the drive in sufficient volume and even the slightest imperfection causes it to fail. I've been told its well known to guys who have boats with the same engine drive combo that are dry stacked or trailered.

I went through this three years ago with intermittent low water and overheating alarms. First we tightened hoses and re-built pump, no joy. Then replaced hoses and pump, again no joy. Finally, I had a through hull water pickup installed with flush out valve plumbed between the drive and the pump - problem solved. Turns out there is a Mercruiser service procedure that covers this.

Henry