Testing impeller flow?

[nola mike]

Getting ready to try to chase down a possible overheat (after I verify it and clean out my heat exchanger). It's a new/old gear housing that didn't have any issues (that I recall) when last used time around 2 years ago. I only used it for about 10 hours at the time, and had just replaced the impeller. Before I pull it yet again, I wanted to test the flow of the impeller.
1. Plan on pulling the line coming off the HX and timing and measuring output, I guess at idle. I've seen different specs for the numbers--anyone have anything definitive?
2. I assume this has to be done on the water v. muffs?
3. What's cooling the exhaust while I'm doing this? Won't I fry my exhaust bits?

 

[harringtondav]

Got the Merc. engine manual right here. Pull the raw/sea water hose where convenient to get a least a gallon bucket to catch all of the inflow. They state in bold text, do this in the water. A garden hose could distort actual flow. Depending on your O/Ds gear ratio, the pump should deliver from 3.0 to 4.5 GPM @ 1000 rpm. 1.98:1 is 3 gal, 1.32: is 4.5 gal See the image. Sorry about the bad scan, but you can figure where your drive is. But they stress to take a 15 sec. sample, shut off the engine, and measure the flow in qts. They're worried about overheating the exhaust, I think.

Not to insult you, but 3.3 qts/15 sec = 3.3 gal/min. I've had way too much book learning to have had to go to the calculator for that one.:facepalm:

 

[nola mike]

Thanks. Which manual is that in?

 

[kenny nunez]

Mike, Another way to check if the delivery to the heat exchanger is good is to take the hose off from the drive pump. At idle you should not be able to stop the flow with your hand, you will get wet if the system is good. This also checks the water pocket cover, impeller and the hose in the gimbal housing for a pressure loss.
This along with the volume flow test should let you know of any problems.

 

[harringtondav]

nola mike , it's my #18 GM 262 CID (4.3) manual. I'm guessing any Merc. engine manual coupled to an Alpha has this chart and test in section 6.

 

[nola mike]

harringtondav , thanks, was looking in the Stern drive manual.
Kenny Nunez, which hose are you talking about?

 

[kenny nunez]

The raw water hose from the drive pump to the heat exchanger.