WOT temp creeps too high

[500dollar744ti]

I have a 1990 MCM 350 Magnum, I don't have the S/N handy but let me get your opinion anyway.

Took the boat out for the first time memorial day and didn't have any issue except the temperature was creeping up past the normal 140* and continued past 180* at wide open throttle. WOT is 4400 rpm for me, pulling back to 3900rpm brought the temp right back down to normal.

Last year I replaced the lower unit at the end of the season but did not have a chance to get out on the water. I bought a used lower and put a new (but had been for a couple years) impeller in the water pump. I installed everything, tested it on the muffs in the driveway then winterized the thing.

Before swapping the lower unit everything was perfect, never had temperature creeping up. The water pump housing didn't look too bad to me but I can't think of anything else to cause the issue. Manifolds and risers are new OEM mercruiser.

I just put another new impeller in it and replaced the housing, noticed a small tear in one of the gaskets, hoping that's the issue but it didn't look like the tear was in a spot that would cause a problem.

Anyway, do you all agree in replacing the pump housing and impeller to fix this? I can't think of anything else.

 

[alldodge]

I have a 1990 MCM 350 Magnum, I don't have the S/N handy but let me get your opinion anyway.

Took the boat out for the first time memorial day and didn't have any issue except the temperature was creeping up past the normal 140* and continued past 180* at wide open throttle. WOT is 4400 rpm for me, pulling back to 3900rpm brought the temp right back down to normal.

Last year I replaced the lower unit at the end of the season but did not have a chance to get out on the water. I bought a used lower and put a new (but had been for a couple years) impeller in the water pump. I installed everything, tested it on the muffs in the driveway then winterized the thing.

Before swapping the lower unit everything was perfect, never had temperature creeping up. The water pump housing didn't look too bad to me but I can't think of anything else to cause the issue. Manifolds and risers are new OEM mercruiser.

I just put another new impeller in it and replaced the housing, noticed a small tear in one of the gaskets, hoping that's the issue but it didn't look like the tear was in a spot that would cause a problem.

Anyway, do you all agree in replacing the pump housing and impeller to fix this? I can't think of anything else.

I think it's the impeller and the reason why, similar thing was happening to me this season. I picked up several impellers 3 to 5 years ago. Allways like replacing them every 2 years. Replaced the one in my cruiser in Dec 2012 when I replaced the mechanical fuel pump. This year first run out the temp got close to 180 at cruse speed, and took a bit longer to cool back down. I have a 160 thermostat due to MPI but it just was not acting right. Pulled the old impeller and upon inspection one of the vanes had started to crack. Also the impeller has started to create a set. So looks like the other 4 impellers on the shelf will be tossed, so much for buying in bulk.. Looks like there is a shelf life and we have no idea of how long it set on the shelf before I got it.

 

[500dollar744ti]

Yep, that fixed it, had it up to 48mph gps with 8 people, didn't skip a beat.

 

[achris]

...we have no idea of how long it set on the shelf before I got it.

Yes we do. The date of production is cast into the rubber of the impeller, right in on the hub.. I'll get a photo and post it...

Chris.....

 

[alldodge]

Yes we do. The date of production is cast into the rubber of the impeller, right in on the hub.. I'll get a photo and post it...

Chris.....

Good to know, thanks Chris

Did find that I have 3 Sierra and 2 Mercruiser type. The Merc's are newer then the other and the rubber feels different then the Sierra's