Vapor Lock? launching 8.1's and not pushing water

[onthejob]

2003 (late) Sundancer with 8.1 Mercruiser V-drives 500 freshwater hrs. Fresh impellers and belts. original pumps.

Second year in a row I've launched and the raw water pump appeared not to be pushing water on one of the motors. Once I force water into the system it's fine. Yard crew throw words around like vapor lock but it just seems like air gets into the pumps and it won't prime its self. Anyone experience this? Is it possible the pumps backfeed air somehow or it drains out so there is air in the system after winterization? I leave the seacocks closed all winter after winterizing. After I get her running from the launch no problems the rest of the season.

I just hate being that guy at the launch that can't get his motors running right when I'm put in the water.

Sincerely,

LimpLaunch

 

[PMC]

What is the condition of the hoses on the suction side of the pump? The impeller should self prime, and sounds like it's suckling air. Loose clamp, or cracked hose is somethig to look for first.

 

[dc380]

I had the same problem at the start of every season. The Starboard engine would prime itself, the Port engine wouldn't.
I would crack the valve on the water pump while someone started the engine, bleed out the air, and then close it.
The engine would stay primed until the following season.

Dave

 

[Jimmy Buoy]

Never had an issue with mine, but there is always a tense 30-45 seconds between firing up the engine and waiting for the water to begin to show up at the thru-hull exhausts (not underwater exhaust like you have). Is it possible that the one engine just takes a little longer to reveal the cooling water at the discharge site?

 

[onthejob]

Well it's been a month and no issues at all pushing water. Really good pressure. Twice I went through all the hoses and clamps checking condition and tightness and didn't find anything of concern.

 

[onthejob]

Figured it out. The 1 1/4" hose between the sea strainer and the water pump was collapsing at a bend from the suction created by the impeller. It would pop out after the engine was shut down so you could only see it collapsed when the motor was running. For some reason it only happened in the beginning of the season and then fine all summer. Replacing all the engine water intake hoses and double checking the impellers this winter.

 

[Lazy Daze]

It sounds like maybe somewhere along the line someone replaced that hose with a regular, rubber hose. That hose should be a reinforced (typically a spiral-wound, metal wire) hose for the exact reason you mentioned. If it is a reinforced hose, then it might have too much of a bend and it created a weak spot (or a defect in the hose) - but that's actually quite rare. Not to say it can't happen, just that it's a rare. For what it's worth, the hose AFTER the raw water pump can be a normal, rubber hose.

 

[onthejob]

It appears to be an OEM reinforced hose but somehow it was kinked (at a bend) at some point and then straightened back out so the reinforcing wire was weakened.