Mercruiser 5.7 Port Side Manifold Strange Behavior

[Begester]

I had my boat running on the muffs yesterday after a day in the salt water and noticed a slight difference in the amount of water coming out of the port and starboard exhaust when I first turned the boat on. It wasnt enough difference to cause concern but I kept an eye on it. Over the next ~8-10 minutes, everything looked fine. Then, just as the boat was getting to ~145 degrees according to the gauge, water stopped coming out of the port side exhaust. I immediately jumped up and turned the boat off and havent turned it on since. The risers on the port side were almost too hot to touch, and the starboard sides were warm but felt normal. Water was coming out of the starboard side at a consistent and healthy rate this entire time.

Any insight here? I'd think a blockage would be highly unlikely when the boat was on the muffs, right? We had the boat out for an hour or 2 on the water and everything felt fine. The exhaust manifolds were new this spring, have been used ~10 times, and have been religiously flushed every time by running the boat on the muffs for 20+ minutes.

I'm a little confused as to how this could happen, anyone have any insight?

 

[wahlejim]

As far as the issue occurring on the muffs, the water getting pumped has to go somewhere. If something knocked loose and got stuck, it wouldn't matter if it was on muffs or water.

With that being said, you have new manifolds, how about risers? Something could be stuck in there if those weren't changed out. Otherwise, follow the flow of water and disconnect hoses to see where the water stops.

 

[Begester]

Yep, risers are new. If there's adequate flow on one side, that rules out something like the thermostat or impeller, right? How do I know that the water is getting into the port side of the engine in the first place? thanks very much for the insight!

 

[wahlejim]

Pull the drain plug on the port side manifold, if there is water in there, water has made its way through. It's not uncommon for pieces of old riser to be stuck in the exhaust system. If there is water in the manifold but not leaving the exhaust, there is something stuck in there

 

[alldodge]

Not positive but I'm leaning toward there is no issue. Water will always follow the path of least resistance, so the motor was running and water was flowing. Most the water was going out more on starboard then port. Then the thermostat opened and water about stopped on the port. If it ran a bit longer I would guess the water would return, maybe not as much but don't know.

Garden hose flushing does not provide all the water a boat can take in

 

[Bondo]

that rules out something like the thermostat or impeller, right?

Ayuh,.... What drive ya got,..??

'bout what vintage,..??

How old is the impeller,..?? over a year, change it,....

Both sides, Never ever run the exact same temps,....

But I wouldn't expect water to Stop flowin' on one side either,...

 

[Begester]

I've got an Alpha I outdrive, boat is a 5.7 liter (original engine) in a 1989 SeaRay 220DA. I had it out on the water this weekend, everything ran great. I periodically stopped to touch the risers and manifolds and everything felt normal and consistent. Risers were warm and felt about the same, nothing too hot though. Temp gauge read about 155-160 degrees.

BUT, when I got the boat home and was flushing out the outdrive, the same thing happened. Water was coming out both sides just fine, then went to just a trickle on the port side. The manifold and riser got hot but after a few minutes water started flowing out the exhaust just fine, cooled down the manifold/riser, and all was well.

I'd assume an issue with the impeller would prevent water from flowing at any time, right? If this is correct, what could it be? I'm completely scratching my head on this one! Am I losing my mind over a non-issue or is my engine trying to tell me something?

 

[alldodge]

I'm still leaning toward no issue because, My guess is you have the early design thermostat housing and your garden hose volume may be part of the issue.

The early design thermostat housing had issues flowing water. Start the motor and the thermostat is closed so it bypasses the block and goes into the exhaust ports. Once stat opens there is an air pocket and the water needs to fill it up first before flowing again

Early
http://www.mercruiserparts.com/bam/subassembly/31780/1014/210
Design two
http://www.mercruiserparts.com/bam/subassembly/31780/1014/220

New design
http://www.mercruiserparts.com/bam/subassembly/31780/1014/230