mercruiser 260 emitting steam

[Marshall1]

I have been reading this website for some time and found a great deal of useful information. Maybe someone can help me with a problem I am fighting. Let me start with a little background. I have a Mercruiser 260 (350 ci) that is 1982-1985 vintage. I have owned it for 5 years. I have had a nagging issue with excessive steam coming from the exhaust since I owned it. I have replaced the lower unit impeller, inspected the risers, pulled the circulating pump, thermostat replaced....all looks good. This moring I connected a garden hose direct to the waterline to the motor and put water to the lower unit via the muffs. I am still gettinig excessive steam even though I know I am pumping plenty of water to the engine.
Using a infrared heat gun, I noticed the the left side of the engine is runnig 50 deg hotter at 1000 rpm. When checked the following temp were detected:

Top of Riser: Left 174 deg Right 127
Top of Exhaust Manifoild: Left 180 deg Right 137 deg
Side of Exhaust Manifold: Left 185 deg Right 135 deg

I suspect there is blockage of some type on the left side that might be causing the steam to develope.

Your wisdom is appreciated.

Marshall1
Dayton, OH

[stonebreaker]

Is the steam coming form one or both sides?

[Marshall1]

Good question...not exactly sure. I pulled the manifold/riser off to check for buildup and it looks OK.
I am going to divert additional water to each manifold to see if I can tell which side or if both sides are producing the steam.
Tom

[Marshall1]

Forgot to mention, I am also getting steam coming from the center of the prop. DOes anyone know why steam would come from the center of the prop?

[stonebreaker]

Do you get steam if NO water is running through the manifolds?

[Marshall1]

I am hesitant to try this. Will it not overheat the manifold?

[stonebreaker]

Well, the reason I ask is because there are other causes of steam in the exhaust - a blown head gasket, for example. That would account for the steam from the through-prop exhaust.

[Marshall1]

I checked the compression when I removed the exhaust manifold and all 4 cyl reached 130-135 PSI. I did not check the other four. Specs for a new engine are 150 PSI.

[stonebreaker]

Well, check the other four.

[Marshall1]

Stonebreaker,
Checked all 8. Compression ranged from 130 to 150 psi, with most in the 140-150 range.
I was considering doing a cylinder leak-down test....compressing the cylinders to 60 psi at TDC to see if their is a leak. I just attempted to do it with the plugs out but the compressed air is compressing the cylinder opening the valves. Considering doing it with plugs in.
However, will this tell me anything with the compression check that I did shows no noticable problems?
Another data point...plugs are not running hot.

[Frosty]

Yes a cylinder leak test can be done on a car because you can lock the engine in gear.

I really doubt a head gasket leak as steam would mean water in the cylinders and therefore a misfire.

To be honest I am not sure if you have a problem. A difference of 50 degrees!!!.

I would like to see the temp test at 2000 RpM or more.

The centre of the prop is an exhaust outlet, steam will come from there.

Is your drive way level,-- ie the place where you did the test. If the engine is not level water will prefer one side or the other.

[stonebreaker]

Originally Posted by Marshall1

Stonebreaker,
Checked all 8. Compression ranged from 130 to 150 psi, with most in the 140-150 range.
I was considering doing a cylinder leak-down test....compressing the cylinders to 60 psi at TDC to see if their is a leak. I just attempted to do it with the plugs out but the compressed air is compressing the cylinder opening the valves. Considering doing it with plugs in.
However, will this tell me anything with the compression check that I did shows no noticable problems?
Another data point...plugs are not running hot.

OK, then sounds like it's an exhaust problem (heave big sigh of relief). Maybe a cracked manifold or something letting water into the header too close to the cylinders. Do you have any way to pressurize the header while it's off the engine? Maybe you have access to a boroscope?

On the temp difference, I agree with Frosty - 50 degrees isn't really all that much, particularly at idle. Coolant doesn't go to both sides of the block exactly even. Partly this is the block design, but mostly it's the pump's design, and isn't anything to worry about, as this was taken into account during the engine design. I'd bet the temp difference will even out somewhat at higher rpm due to increased coolant flow.

[Marshall1]

Boat is fairly level on the driveway. I have the hose pushing water throught the block bypassing the lower unit pump....so I know I have pleanty of water going into the circulating water pump. (But also have a hose on the lower unit as well.)

I do not have a boroscope....but would be nice.

I attempted to pressurize the exhaust manifold last year with water and did not see any leaks.

I am planning to pump additional water into the manifolds to see if put a stop to the steam.

I might be trapping steam in the optional exhaust diverter. The choice exhaust diverts from the normal exhaust route to the side of the boat.

Photo's attached.

[stonebreaker]

Actually, a water hose doesn't pump all that much water - about 5 gallons per minute. The water pump on the engine pumps about 9-10 gallons per minute per 1000 rpm. So at 4000 rpm you're pumping roughly 40 gpm, maybe a little less as the pump begins to lose efficiency at higher rpm.

Have you got any pics showing the steam coming out of the exhaust?

[Frosty]

But youve got swop overs on it??

It looks like you have a system where you can have open pipes or go throught the leg. Did you know?

Those rubber hose could easily be removed and the engine run momentarily to check wich side is steaming. Although it quite likey it will be the hotter side.

I really dont like those exhausts. Exhaust gases dont like going through 180 degrees like that, and it does it twice.

Did you fit those yourself. They look to be very nicley made but!!!