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Exhaust manifold overheating

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timo12345

New member
"I have a 1998 Mercruiser Alph

"I have a 1998 Mercruiser Alpha "MCM" 5.0 liter engine. Problem is intermittent heating up of starboard exhaust manifold and riser. Sometimes it cools down other time it gets hot. Engine temperature stays constant at 175 F. Port side exhaust manifold and riser stay cool. I installed a new impeller kit, and thermostat kit. I did not find obstructions in thermostat housing. Hose and connection to exhaust manifold is clear.

Any ideas as to cause and how to rectify. Your help is greatly appreciated!!"
 
"Tim:

This is the year of s

"Tim:

This is the year of starboard manifold/riser getting hot.

Some have found sand, dirt, rust particles partially blocking the riser or manifold. Heavy rust inside the T'stat hsg. and slight hose/nipple blockage. Take a look at the exhaust shutter too.

Reduced water flow seems to be the main issue from above findings. You can read the numerous postings for more info."
 
"Guy,

Good news! After talk

"Guy,

Good news! After talking to a customer service representative at Mercury Marine, he informed me that the starboard side manifold and riser will be 40 -50 degrees F warmer than the port side.

With an infrared temperature gun, I took temperature readings at both exhaust manifolds and sure enough the difference was within the 40 degree range. No temperature reading on any engine component was above 160 degreees F, which is the normal operating temperature.

What a relief! Thanks for your help.

Tim"
 
"Guy,

The explanation was t

"Guy,

The explanation was that most of the engine cooling water (at 160 degrees) exits primarily through the starboard manifold, whereas the port manifold has mostly the cooler lake water going through it.

Tim"
 
"Tim;
Sorry if I sound skepti

"Tim;
Sorry if I sound skeptical, but did your engine always behave this way?

Merc has been building V-8 powered boat packages for several decades now, and they all dispose of the cooling water in a similar fashion. Although cooling water distribution is not perfect, it is never that lopsided. I don't buy that "explanation" for a moment.

Rod"
 
"Rod et al:

I am somewhat

"Rod et al:

I am somewhat bewildered by the Merc. Tech. statement since it did not include/exclude all engines and cooling systems, i.e., T'stat hsgs. w/or w/o check ball valves and full/partial FWC engines.

If you remember last year I had a hot running PORT riser. I replaced the 20 yr. old corroded T'stat hsg. and it made a big difference based on temps. taken w/an laser infrared temp gun. All readings from both sides of the engine were nearly the same but the port man./riser temps. did fluctuate between 15-30 deg. F.

I did a minor tuneup on the engine last Sunday and rechecked the temps. again w/the laser gun. The same fluctuations occurred. After some thought I started comparing the temps. of the T'stat hsg. and assoc. parts w/the mans. and risers. What I believe that I found was a relationship between the T;stat opening/closing affected the variation in the PORT man./riser temp. I mainly focused on the PORT side but the same MAY have been occuring on the STARBOARD side.

Some side notes: the port man. is 1 year old Merc. and the starboard is 4 year old Merc. Both risers are original 21 yrs. old, have been inspected and pressure cleaned--port riser was also cleaned w/mild acid. Engine temp. remains constant @ 143 F. at RPMs above 1,000 and increases to about 150 F. but returns to 143 F. when the T'stat opens after long idle speed trolling periods.

Should I change the port riser? I don't plan to do it since all of the engine temp. readings are between 60 F. at the water inlet and 139 F. at the port man./riser."
 
"Guy;
In my humble opinion yo

"Guy;
In my humble opinion you have no worries with those temps at all.

To me a riser that it running so hot you can't put your hand on it (as with some cases on the forum); is simply not getting enough water flow for whatever reason. The manifolds should usually be 10-20 degrees higher than the stat set temp, as they are getting mostly water that the stat has released.
The risers on the other hand get a mixture of hot manifold water plus cool intake water that is excess to what the block needs for cooling. They most always seem to run a good 30-50 degrees cooler than the manifolds; ie 120-150 degrees.

And I just don't buy that differences from side to side of 40-50 degrees or more is in any way "normal". This says to me that the water flow to the risers is very lopsided for some reason. Could be partial plugging at the flow splitting T, plugged hose(s), partially plugged riser(s), missing or jammed flapper(s), etc. Water will take the path of least resistance, so if there are two parallel routes and one is partially restricted, it will go the other way.

Just my thoughts based on what I have seen, right or wrong.
Rod"
 
"Rod:

Thanks. To me, the o

"Rod:

Thanks. To me, the opinions of you, Bondo, Bt. Doctor, Mista Hays, Troy, Peter Chamberlain, Wrench, Pugent Sounder, Hy Stat, Rick Sweeten, Kurt S., Bert Goodrich, Fast Jeff, Scott Stevens and anybody else I left out are more BOAT BIBLE verses to be taken as cast in stone.

I think that all the members of this board are a group of good people helping out each other in any way they can contribute."
 
Guy;
I could not agree more.

Guy;
I could not agree more. Your input as well a that of countless others is invaluable to the many Merc buffs that frequent this forum.

New visitors always seem to be very thankful of the help they get. I just hope that our information is correct most of the time!

Rod
 
"Guy and Rod have been extreme

"Guy and Rod have been extremely helpful. Although I see Rod's point about lopsided temps (remember I have a hot as heck STBD riser and a cool port), I am forcing myself to believe what the Merc rep is saying about the differences in temp on both sides. Come on, I NEED to sleep at night and stop worrying about this!!"
 
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