'99 7.4L MPI Manifold/Riser/Elbow Temperature Difference

[Kay Lee]

I don't know if I have a problem or not. Here's the history: boat has been a saltwater boat all of its life until I purchased it last year. First outing this year resulted in a tow back. The port exhaust overheated and burned a hole in the lower rubber exhaust connector. I found that the manifolds and elbows were almost completely clogged with rust. I have replaced both manifolds, risers, elbows, and both connectors on the port side. Testing on the trailer with flush muffs and a garden hose everything seems fine while warming up. Both sides stay nice and cool! After the engine reaches operating temperature and the thermostat opens, the starboard side starts receiving hot water and gets considerably warmer than the port side. I can't say for sure what the temperatures are, but I'll put an IR thermometer on it this weekend. The port side stays so cool that condensation forms on the surfaces and the starboard side gets hot enough that I can't hold my hand on them comfortably for very long. I get concerned about the temperature and shut the engine down. Engine temperature is normal at about 160F. Is this difference normal? Is running from a garden hose part or all of the problem? Could the thermostat housing be rusted through and causing the problem? I plan to drop it in the river this weekend and see if it does better than being on the garden hose.

Thanks,
Larry

 

[northern]

I have two 7.4L and all four top of the cast iron elbows are the same temp. Do not have my book with me but I think it is 60F. The hoses are different. I think the hotest I have ever seen is 130F in one spot. Normal seems to be 90 to 110F. RPM 1200 to 1600. Never take temp at higher RPM but have felt the elbow at 3800RPM and they seem cool. Did not feel the hoses.
Based on what you say I would think you have a partial blockage. Others maybe able to help you where to look. I do not know.

 

[Kay Lee]

I'm confident that there is no blockage. All new parts from 90 degree NPT pipe fittings at bottom of manifolds to the elbows. I've sighted through the thermostat housing from the port manifold hose connection through the other side out the starboard manifold hose connection. Both hoses were removed and flushed first, before I found the clogged exhaust parts. Both manifold cooling water hoses flow cold water until the thermostat opens, then the starboard hose starts receiving hot water. The port side of the thermostat housing is cold and the starboard side is hot. Is that normal as well?

Thanks again,
Larry

 

[northern]

All sides on my engines run close to the same temperatures. Old mechanic trick I learned long ago was to drop a thermostat in hot water and see if it opened you might want to try that and see what happens. Do not think what you describe is normal.

 

[Kay Lee]

Took the boat to the river today. Just like on the trailer, both sides stay cool (about 90F, that was about the air & water temperature!) until the thermostat opens. When the engine reaches normal operating temperature (160F) the water flowing to the starboard manifold changes from cool to hot. The port elbow at the top was between 100-110F depending upon engine speed (anywhere from idle to 2000 RPM) while the starboard side was 30-35F higher and the temperature increased as I pointed the IR thermometer down the riser and onto the manifold. At this point, the only thing I can see that is left as a possible cause is that the thermostat housing is rusted through somewhere internally. Anybody got any other ideas before I replace the housing?

Thanks,
Larry

 

[Kay Lee]

I pulled the thermostat housing off and looked it over thoroughly. It's rusty, but I don't see any rusted through passages. All of the passages eventually connect together anyway, they are just partially separated by baffles. The cold water from the seawater pump is directed very close to the outlet hose connection for the port manifold. The starboard connection will definitely get mostly hot water from the engine when the thermostat is open because of its proximity to the thermostat and the fact that there is no cold water being forced right in front of the outlet opening! Seems that this might be normal, but the starboard side sure does get VERY warm.

Larry

 

[Kay Lee]

Update: I've found out that the '98 to '01 454 Magnum MPI, 502 Magnum MPI, and 8.2L MPI all have the exact same cooling configuration. If you own a boat with one of these engines do you experience the same temperature difference I'm seeing?

Thanks,
Larry

 

[scoflaw]

I have a 2000 305 efi. Same issue. Noticeable at idle, but will cool right down if I increase rpm's

 

[dpmulvey]

...I have replaced both manifolds, risers, elbows, and both connectors on the port side. ...
Thanks,
Larry

Are those same connectors on the starboard side? Could they be clogged?

My manifolds all stay within a couple of degrees of each other.

 

[Kay Lee]

Are those same connectors on the starboard side? Could they be clogged?

My manifolds all stay within a couple of degrees of each other.

David,

There is no clog. All new exhaust parts as stated and the port side got two new 4" diameter (rubber) connectors because they melted from the heat. The temperature difference occurs when the thermostat opens and is due to the hot water from the engine block exiting through the starboard manifold. According to Mercury Marine, it is designed to work this way and I do not have a problem.

Here is the quote from MerCruiser Customer Service:
"The water coming out of the engine block exits only on the starboard side of the engine. Therefore the starboard exhaust manifolds and exhaust elbows will be warmer than the port exhaust manifolds and exhaust elbows. This is normal for your engine. If you have a 160 degree thermostat installed the temperature of the exhaust manifold on the starboard side of the engine can get up to 176 degrees under normal operation after the engine has reached 160 degrees. The temperature of the exhaust manifolds and exhaust elbows can also vary depending on the temperature of the water, the ambient temperature and the humidity."

Remember, this is only applicable to the years and engines I mentioned earlier.

Thanks for your assistance guys and I thought you'd want to know the official answer.

Larry

 

[ardeprint]

That Merc responce should be archived. This is a common concern and that is a good responce from a reliable source.