Is the Sterndrive Water Guide Tube just a Guide?

[myoldboat3]

Mercruiser "Late Model I" Stern Drive s/n 3901959

Does the drive housing (upper unit) water tube reach all the way through the gear housing (lower unit) guide sleeve ("guide tube") and seat in the raw water pump seal?

I replaced the entire water pump last year but the impeller failed this season. A few days ago, we replaced the impeller. The rest of the pump looked great, except that the plastic guide tube was widened at the top, sort of like a funnel, probably from an overheat condition. I wasn't able to locate an old tube I thought I still had, so I reused the funnel-like guide tube, believing that the water bypasses it, anyway. Now the flowrate at the thermostat housing is still very low.

We're going to pull the gear housing again. I have a pump kit on the way, but if it's just the guide tube and everything else still looks ok, I may just replace that. Or I may just replace the whole pump again. I get tired of throwing parts at it, but dropping the lower unit is a PITA.

Here are the images from the manual, which make it appear that the guide tube is bypassed once the gear housing is bolted up to the drive housing.

 

[myoldboat3]

After thinking about it, I think the water tube must seat in the pump seal, and the guide must really be just a guide. If the guide tube got deformed from heat, maybe the pump did, too, so I'm going to replace the water pump. Service Bulletin No. 82-4, "Checking Stern Drive Unit Seawater Pickup Pump Output – MerCruiser I Stern Drives", provides a troubleshooting chart. Very few items might apply, but I'm going to recheck the water pickups and while the lower unit is off, I'm going to backflush through the water inlet hose to check for debris and that the drive shaft housing water pocket cover isn't cracked. If you have other thoughts, I'd love to hear them!

Steve

 

[myoldboat3]

Sorry so late getting back to this. I wrote last August (above):

"I replaced the entire water pump last year but the impeller failed this season. A few days ago, we replaced the impeller. The rest of the pump looked great, except that the plastic guide tube was widened at the top, sort of like a funnel, probably from an overheat condition."

So I will now echo the advice from the experts on here: IF YOU'RE GOING TO DROP THE LOWER UNIT, REPLACE THE WHOLE WATER PUMP, INCLUDING THE BASE! Don't be stupid like me. The deformed plastic guide tube indicated an overheat had occurred, and when I pulled out the base of the pump last summer, the gaskets were shot.

Lesson learned. AGAIN.