Water Pump losing prime???

[NC_Coastie]

So I have an 86 OMC Cobra with the 4.3. I mainly use in salt water so it has the heat exchanger style cooling system. When I purchased the boat, it had not been used for 2 years. I realized it had the old style exhaust manifolds on it so I changed both of those. I also changed the water pump impeller and housing with the kit. The old one actually still looked fine so I'm keeping it as a spare. I hooked it up with the rabbit ears and hose, had good flow out of the two holes on the right side of the lower unit. Started the engine but had no flow getting to the heat exchanger. Thinking I just burned up an impeller, I pulled the housing off but everything was nice, wet, and cold. I poured water down the line that goes from the heat exchanger to the stern drive and it's clear as a bell. No blockage. Put everything back together and still no flow to the heat exchanger. Water pump stays nice and cool. If I give it some throttle. (Like 2500-3000 rpm) then water starts to flow and will flow even at idle. Does the same thing when the boat is in the water as well. I have to give it some juice every time to get the water flowing. This can't be normal?? Any help would be greatly appreciated.

 

[Scott Danforth]

Pull the end caps of the heat HX and check for fouled tubes.

 

[NC_Coastie]

I'm not even getting water to the heat exchanger. I pulled the input line to it.

 

[Scott Danforth]

Are you running on muffs or is the boat in the water?

 

[NC_Coastie]

Either way. Seems to be a little better on muffs but I still have to accelerate the engine pretty good to get it to start flowing.

 

[Scott Danforth]

if you pull the line to the HX (from the raw water pump) with the motor running at idle on muffs, you shold be able to fill a 5 gallon bucket in 16 seconds or less at 1000 RPM. if you do not have water flowing, I would suggest pulling the drive and looking for the problem.

the first cooler in-line after the raw water pump is the power steering cooler. they make great strainers for the rest of the system. I would also pull the PS cooler and flush it out.

 

[NC_Coastie]

Appreciate the help. My boat doesn't have a power steering cooler. It's a hose straight from the transom which connects to the thru hull fitting in the gimbal housing and a small hose from there to the nipple that goes into the sterndrive. I had the sterndrive off so I could replace the gimbal bearing so I also changed out that small hose. No blockages from the heat exchanger down to the water pump. I can fill a five gallon bucket up pretty quick, but not without having to accelerate the motor first.

 

[Scott Danforth]

The only thing that makes sense is if your trying to run the outdrive in a tub. If that is the case, then yes, the pump will not self prime.

 

[NC_Coastie]

Roger that. No tub. In the water I start the motor, have to idle out pretty quick so I can get out of the no wake zone so I can throttle it up really quick before my engine or manifolds overheat.