Mercruiser 350 Riser Overheating

[dlkb1240]

My friend and I just recently purchased a Nautaline with a Mercruiser 350 outdrive. Starts up and runs well for a few minutes then the riser and ruber hose that connect the riser to the exhaust pipe through the transom starts to smoke. The impeller and pump housing was recently replaced and it's as cool as a cucumber. I've read a few of the threads of people with similar problems but not exactly like this. Any suggestions? Thanks.

 

[Bt Doctur]

Re: Mercruiser 350 Riser Overheating

Most likely a clogged manifold or riser. 5 year lifespan on average for risers.

 

[dlkb1240]

Re: Mercruiser 350 Riser Overheating

OK so i pulled the risers and manifolds. All clear. Put it back together and ran it. Water is flowing through the system (I can see the discharge). But the risers still get hotter than heck after about 10 seconds and stay hot after shut down for about an hour. How much water should be comming out of the exhaust port? Any other suggestions for what might be causing this?

 

[bruceb58]

Re: Mercruiser 350 Riser Overheating

Is only one side getting hot and having the problem? If so, check the thermoststat housing to see if there is a blockage in there.

Also, you can take the hose going to the manifold off for a moment, start the engine for around 40 seconds or so to check what the flow of water is that is going to that manifold.

 

[dlkb1240]

Re: Mercruiser 350 Riser Overheating

I have checked the thermostat housing. A little rusty but water runs through it just fine. I also ran the engine with the exhaust pipe from the riser unhooked to see the water flow and it's comming out but I have never worked on this particular engine before so I'm not sure how much of it should be comming out. The temp sensors looked pretty rusted. Do thoes have to be intact for the engine to properly cool or is that just for the thermostat in the cabin? And I think it's both sides that are getting hot but the starbord gets the hottest.

 

[bruceb58]

Re: Mercruiser 350 Riser Overheating

Who replaced the impeller? If you didn't do it and it was the previous owner, not sure I would really trust his statement.

The temp sensors are only for the gauge. What does the engine temp run at?

Are the exhaust flappers in place? Very possible one of them got burnt up and lodged its way down in the exhaust tube.

 

[dlkb1240]

Re: Mercruiser 350 Riser Overheating

The impeller was replaced by the previous owner. I will check it today. By the flaps do you mean the pieces of rubber covering the exhaust on the stern? If so they're intact. It gets hot at idle and I'm not sure what the temp is but the ruber exhaust pipe starts smoking and the risers get ridiculously hot.

I replaced the tubing from the riser to the transome exhaust pipe. The old ones were all messed up. Also I don't if this matters but everytime i start it up I have to spray starter-fluid in the air-intake.

 

[bruceb58]

Re: Mercruiser 350 Riser Overheating

Sounds like you have through hull exhaust. Is that correct? If so, I doubt you have the flappers I am thinking about. They would normally be in the exhaust tube right after your rubber coupler that is burning up.

What is the temperature of your other manifold? If it is cool, you have a water flow problem through that one manifold. One will always be hotter than the other just becuase of the waterflow through the thermostat housing.

You should invest in an IR thermometer to check your temps.

Throw away the starting fluid. You can damage your engine with that. Just pour some gas down the throat. That has nothing to do with your overheating problem.

 

[rebars1]

Re: Mercruiser 350 Riser Overheating

Check the hose(s) path with the manual to make sure they are routed correctly. You said they were messed up..how so?

Are the restrictor plates (if any) between the exhaust manifolds and risers improperly installed?

You should be getting just about the same amount of water out as is going in.

During one demo engine run for a boat I was looking at (on muffs) I noticed that not much water was coming out. The risers got so hot that we watched them actually crack as they cooled off.

 

[lmannyr]

Re: Mercruiser 350 Riser Overheating

Test the THERMOSTAT for proper opening. It should open at 160 or 140*. Suspend it in a pot of water and heat it up while monitoring if at what temp it opens - if at all. You may have a stuck thermostat.

Is the water coming out of the exhaust hot or cold?

Do check the impeller while your at it as suggested.

 

[dlkb1240]

Re: Mercruiser 350 Riser Overheating

Yes it is a hull exhaust system. I was the last person to install the risers (put new gaskets on as well) and I dont hear or see any water going into the wrong places. I will check what the temp. of the water comming out is today as well as the impeller. The guy we got the boat from was describing what he did and he got a little confused when he came to the hoses that go into the pump assebly so maybe he did mix thoes up. I'll post my findings tomorrow. Thank you all for your help and advice.

 

[dlkb1240]

Re: Mercruiser 350 Riser Overheating

Ok. So I took off the raw water pump and the impeller is in fact new. I went ahead and started systematically back-flushing from various points and everything seem clear. But when I back flushed the engine pump assembly the water came out brown/rusty. It looks like my next step is to take that puppy out and see if there are any clogs. Anyone have any other suggestions or does my game plan sound pretty solid?

 

[lmannyr]

Re: Mercruiser 350 Riser Overheating

Pull the thermostat and test it. Suspend it in a pot and heat it up. With a thermometer, check at what temp it opens, if at all. While the thermostat housing is out, check for blockages. Post pics.

 

[Dshow]

Re: Mercruiser 350 Riser Overheating

I believe these are the flappers that were referred to. Check to make sure they haven't fallen down and are blocking your exhaust.

Also, you mentioned riser gaskets, did you use the correct ones to allow more coolant flow?

 

[dlkb1240]

Re: Mercruiser 350 Riser Overheating

Here is the thermostat housing and cap

If there was a thermostat in this housing it is long gone. It doesn't seem like that would cause the riser to over heat

But this does.

I went ahead and took the Engine Circulation pump off and back flushed it

It looks like the port side is fine but the starboard side had hardly any water coming out of it. So looks like I need to get that unplugged somehow. Any suggestions?

 

[rthomas]

Re: Mercruiser 350 Riser Overheating

it could be old impeller blades stuck in the risers and slowing water flow, ive never seen this but ive read that they end up there on yacht survey.com

 

[intrepidvoyager]

Re: Mercruiser 350 Riser Overheating

OK ...if your risers are clear and thermo housing OK you either have a pump issue or bypass issue so as stated in a previous post make sure you have the right hose set up and routing for your engine.

make sure you understand the bypass system and that it is set up right.

saw you backwashed ....are you sure no blockage at power steering cooler ?

As you say the risers get red hot in ten seconds after start up , that indicates they are getting virtually no water.

the only time I have seen this before based on all you have said is when the intake and outlet on the raw water pump were reversed ..... it is actually possible when hooked to muffs to get a small amount of water past the pump if it is hooked up wrong so check on that.

You also may have a considerable air leak ....crack in pump housing, etc

check belt to pump is tight.

 

[dlkb1240]

Re: Mercruiser 350 Riser Overheating

No leak, the hoese are right, and water comes out of the risers... as previously posted.

 

[intrepidvoyager]

Re: Mercruiser 350 Riser Overheating

Ya but how much water ....... saw your pic of water coming out of block ...if thats full pressure you don't have enough water hose pressure on muffs.

MAN THIS IS WEIRD ....

read thru this then try what follows ...

For example the circulating pump on a standard small V8 GM based engine will flow approximately 50 gpm through the engine. The typical raw water supply pump is capable of flowing about 18 - 20 gpm depending on the gear ratio and restriction the system experiences. A standard 300 Hp small V8 engine dissipates approximately 300,000 Btu/Hr of heat into the coolant. To maintain a thermostat temperature of 160 degree F at wide open throttle this engine would need a pump to flow about 8 gallons per minute (GPM) of 70 degree F water into the system to maintain this engine temperature. Since the Alpha raw water pump can flow 18-20 GPM there is significant bypass (excess) water supplied, which therefore has to be dis-charged somewhere.

In a cold manifold system the warm water that flows through the thermostat, as well as this excess bypass water are joined together in the thermostat before exiting and being introduced into the bottom of the exhaust manifold. The fact that your are mixing together a lesser amount of 160 degree F engine water (8 GPM) with 10+ GPM of cold lake water allows the aggregate coolant temperature in the manifold to fall below the 110 degree F threshold needed to prevent the accumulation of condensation in the exhaust system.

put the boat in the water and see what happens ..... I have thrown this link in just for reference RE RISER TEMPS ...

http://www.acms-usa.com/bulletin_files/hotItem.htm

MAYBE ITS YOUR MANIFOLDS THAT ARE THE BLOCKAGE PROBLEM ???

maybe your hose pressure is way too weak and the boat needs to be in the water .......

 

[dlkb1240]

Re: Mercruiser 350 Riser Overheating

The boat is in the water. The manifolds are clear. I checked them at the same time I checked the risers. That picture is just me putting water through the thermostat housing with a garden hose. I finally got that passages clear and have the engine back together. I have the exhaust pipes off so the exhaust and water go straight into the engine room. I ran it for about 10 seconds last night and the water was shooting out of the risers...good sign. I had to shut it off because the water/exhaust was shooting all over the starter and I didn't want it to cause an electrical problem.

My plan is to run it again today and measure the water coming out as well as the temperature of the risers and manifolds. I'll also try and get some video posted for you guys.