Exhaust Explanation

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Rick D

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Can someone describe to me the exhaust system on the 5.7L Merc engine? I am looking for info on the section after the exhaust elbows... basically from the exhaust flappers all the way to the outside of the boat.

I ask because the starboard side of my new port engine runs 30 degrees hotter at the elbow than on the port side of my new engine. New hoses, new water circ pump, new manifolds and risers. The only thing left is a possible blockage downstream from the flappers. And it's not he flappers as I can see them and they aren't even staying closed all the way anyway.

The engine that was replaced was doing the same thing.

--Rick
 
"... The only thing left is a possible blockage downstream from the flappers..."

Or blockage upstream. Are you sure both seacocks are ALL the way open? Are you reading the temp from the dash or from an IR thermometer?
 
quote:
Originally posted by KiDa

"... The only thing left is a possible blockage downstream from the flappers..."

Or blockage upstream. Are you sure both seacocks are ALL the way open? Are you reading the temp from the dash or from an IR thermometer?

Click to expand...


IR Therm. There are no sea cocks... it's an Alpha 1 Gen 2 with and external impellor. The water coming into the engine from the impellor is strong. I have run without a thermostat to see if it was a thermostat issue... same thing. Port side runs 30 to 35 degree higher. Water running to the base of the exhaust elbow is strong. I have checked my gaskets several times and they are both the same on both sides.

--Rick
 
It's a pretty much exhaust/raw water mix from the riser aft with the exception of a separation for the small ports exiting at the bottom in the transom assm. At idle, most water flows thru those ports. The separate 'water ports' extend back into the 'Y' pipe along the bottom, but looking down from the entrance of the 'Y' pipe, you can't see where the start of the separate water ports start.

Tell you the truth, I don't see how blockage of these ports would cause increased temps but they seem to do just that. The same as pieces of broken impeller caught in the drive can do it even though exhaust flow seems good.
 
Counter rotating props? I ask because if everything aft of the water pumps is new and the probem is the same, I have to wonder if one drive is stealing suction (water flow) from the other; especially if the symptoms are identical.
 
quote:
Originally posted by KiDa

Counter rotating props? I ask because if everything aft of the water pumps is new and the probem is the same, I have to wonder if one drive is stealing suction (water flow) from the other; especially if the symptoms are identical.

Click to expand...


yes, BUT... I'm only running the one drive when this is happening. The boat hasn't moved from the boat yard slip in a quite a while.

Tomorrow I will use my inspection scope and look down the exhaust pipe to look for any blockage on the starboard side of the port engine. If I find none, I will swap the circ pump from my other engine and see if the problem stays with the pump or with the engine. If it stays with the engine, I will have to pull the boat out and replace the stern drive impellor.

Shouldn't have to do either change as the both parts only have 6 to 8 hours on them... BUT, they were both on the old engine that failed... so it is possible that they went bad on that engine and are now bringing me the headaches on the new engine.

--Rick
 
I had a similar problem for several years. Finally found that the freshwater hose passing thru the stern, from the outdrive to the engine, was pinched and getting worse from salt buildup around the outside of the hose where is passed thru the stern. Drove me crazy but worked perfect after fixed.
 
Inspection scope showed nothing. Furthermore, I took off the rubber hose connecting the riser exhaust to the Y tube going out. Ran the engine letting the exhaust water run into the bilge. Riser still got hot. So I removed the water circ pump from my other engine and put it on this engine. Ran it, same deal, hot riser. Tomorrow I am pulling the boat out (again) and will replace a impellor that has only 8 hours on it. Will also take a look at the water line coming in per GaryP's experience.

Running out of things to check.

--Rick
 
What about that side of the engine running hotter because of a gasket issue causing a different air fuel mixture making the engine run hotter on one side.

Have you done side to side engine head temp comparisons at the block-exhaust manifold area?

Who knows but maybe your looking at the end and not at the beginning.

Compare engine temps at different places against each side. See if you may have 1 or more cylinders running hotter than the others.
 
quote:
Originally posted by JLC

What about that side of the engine running hotter because of a gasket issue causing a different air fuel mixture making the engine run hotter on one side.

Have you done side to side engine head temp comparisons at the block-exhaust manifold area?

Who knows but maybe your looking at the end and not at the beginning.

Compare engine temps at different places against each side. See if you may have 1 or more cylinders running hotter than the others.

Click to expand...


I don't recall what the thermo read on the heads itself. Will check tomorrow though. This is also a brand new engine and the one that it replaced did the exact same thing. So the parts which were not swapped out are suspect.

--Rick
 
Isn't there another thread with pictures of a engine doing the same thing? What was the final answer on that one? I'm typing from iPhone and Im to tired to read that other thread again. Zzzzz xxxx zzzzz
 
OK, Sounds like you have a raw water cooled engine. A previous broken impeller may have sent pieces into hoses or therm and clogging the water to that riser. Also the therm housing may have corrosion or spring stuck.
 
quote:
Originally posted by jtybt15

OK, Sounds like you have a raw water cooled engine. A previous broken impeller may have sent pieces into hoses or therm and clogging the water to that riser. Also the therm housing may have corrosion or spring stuck.

Click to expand...


Swapped thermostat housings with my other engine and the problem stays with the engine, not the thermostat housing.

I'm really hoping the impellor is the cause. Will know tomorrow.

--Rick
 
Then the hoses that supply water to the manifolds and risers. Pull and flush them.
 
Guys on this forum have said for years that one side of the Alphas runs warmer than the other. My mechanic agrees. He said that while the two sides of the engine appear symmetrical (never used that word in a sentence till now), the water discharge from the circulating pump is not. Combine that with a small water pump in the lower unit and a small hose through the transom assembly, and the outcome is marginal, imbalanced cooling.
 
Are your engines fwc or raw water. You may need to bleed your hose lines might have air in it. Remove one of the hoses from your thermostat and make sure water comes out strong.
 
OK, replaced the impellor today put the boat back in the water. Ran the engines... almost same result. The starboard exhaust elbow of the port engine runs 35 degrees hotter than the port side... starboard is running at around 135 degrees.

I checked the hose from the sterndrive to the thermostat... all clear and no kinks. I pulled the hose coming from the sterndrive just before it runs into the thermostat housing while the engine is running for a few seconds. The water flow appears equal to the flow on my other engine that is not running hot on one side. So I pinched the hose that runs to the port manifold just a bit and the starboard temperature came down very quickly. It seems as though the water just wants to flow to the port side of the engine more than to the starboard side. But the hose to the starboard is brand new. The manifold, riser and elbows are brand new.

So, tomorrow, I will take the lid off the engine compartment and really put the gun on the heads to determine that its not the engine itself running hot on that side... hope not... brand new engine.

--Rick

edit - oh, and I am certain the gaskets on the new risers and elbows are correct as I triple checked them when I put them on.
 
Rick,
I'm reading this post and so far your doing as much as you can to diagnose the higher temp on one side. I have a twin engine boat with Alpha GenII's and one side runs hotter than the other. The temp difference is not as extreme as yours but it's there. My question to you is "is this all happening at idle or does it continue as you increase rpms? The reason I ask, a friend has a boat with Alpha genI drives and his does the same thing at idle but as soon as you add more rpms the risers quickly cool and although one is still hotter than the other they're much closer in temperature.
I'd check it underway and see if it gets better or worse. The 135 degree temperature is not as bad as it seems as long as it doesn't go up underway. Keep us posted.
Rob
 
What if you just put a restriction in the water line to force more water to the hot side? Did the cooler side temp go up at all when you did the pinch test?
 
Could it be that the starboard engine is running cool and the port is fine. I know my engines run about 180. So if you take the 135 on Starboard and add the 35 from the port. The port is running 170. Just a thought.
 
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