my 496's have to be primed every year after winterizing. if i just took it to the ramp without priming, they would overheat. my boat has flushing ports that work well for priming.

After putting it all back together this morning I just took a shot and tried it on the flush muffs. The Smartcraft gauge stepped up to 2.5 psi like normal within a few seconds and everything worked great so problem solved.

I can't say exactly what that problem was. All I did really was replace what looked like a perfectly good 1 year old impeller with another new one.

Oh well, thanks everyone.

You said you used anti freeze to winterize so I wonder if the anti freeze affected the impeller by swelling it or shrinking it a tad.

I know we are talking about Merc here but Volvo Penta had SB years ago about certain anti freeze for winterizations can swell the impeller.

Remember these type of anti freezes contain a form of alcohol inwhich can affect rubber products over time.

It's possible I guess but I've always used the antifeeeze in the past. Funny thing was the water pressure was .3 PSI and stayed there even when I reved it a little before I changed the impeller.

When I worked for SEA RAY when I was just a wheeee boy, the old man Mech. there told me never to start an engine on the muffs with your water on full pressure, That when all that water is pushing past your muffs and drives it gets air trapped and the pump wont draw sometimes.. So you start with 1/3 water and you will see it coming-out the exhaust and then turn the water up.. But never full on.
Had to do the same to a buddy of mines 312 this last Sat.. did same thing,, slowed water down and was all good.
ME NO BULL SH-T YOU EITHER...