Mercruiser Exhaust Overheat

[HornetDriver]

I have an '88 5.7L Mercruiser with an Alpha 1 drive, thru hull exhaust, stock manifolds and risers, and RWC. After a one month lay up, stbd exhaust system has begun overheating. At idle, stbd and port riser temps and waterflow are equal and normal. Above 1000 RPM, waterflow out of stbd exhaust completely stops and appears to increase on port side. Engine temp remains steady at 160 degrees. Manifolds, risers, and impeller have 2 years on them with 50/50 use in salt and fresh water. All theories are welcome.

 

[waterone1@aol.com]

Re: Mercruiser Exhaust Overheat

Are you getting this in the water....or on the muffs, in the driveway ?

 

[arfaname]

Re: Mercruiser Exhaust Overheat

is this a new occurance? What has changed just prior to this happenning? Is your cooling running through a T piece or a Y piece to your manifolds?

 

[HornetDriver]

Re: Mercruiser Exhaust Overheat

My problems are occurring on the water. Everything works normally when the drive is connected to a hose while sitting in the driveway. The only thing that has changed since I last used the boat is that I moved from Florida to Virginia and pulled the boat about 1000 miles. I have a combination thermosat housing. A Y-piece feeds to the bottom of the manifolds from above the thermostat (this supply routes water to the manifolds once the t-stat opens) and a T-piece (with ball check valves) feeds raw water directly to the risers. I simply can't figure out why I'm getting good flow at low rpm (both sides) and not at high rpm (port side only).

 

[Phantom17]

Re: Mercruiser Exhaust Overheat

Possible the ball check valve isn't opening enough to let a sufficient amount of water through at higher rpm? That is if I am understanding you correctly.

 

[Don S]

Re: Mercruiser Exhaust Overheat

Change the impellers, 2 years is normally max life on them anyway.<br />Besides, on a raw water cooled engine the impeller would be the first thing to check in any type of overheat problem.