Mercruiser EC Exhaust Manifold Overheat - Help Please

V

vasilios

Recruit
Thread starter
Joined
Sep 29, 2026
Messages
1
Hey, looking for some insight from the Gurus

I have a 2018 Cruiser yacht with twin Mercruiser Seacore 377 EC

Closed-loop cooling; the exhaust manifolds have coolant going through them

The elbows have fresh water running through them

I have a problem where, after four minutes at full throttle on plane, it goes into guardian mode

The error is port exhaust manifold coolant temp too high; it gets to about 214 degrees

I took off the port elbow, scraped out some rust from the ports, tested the flow, and it flows as it should

Reinstalled with a new gasket, the issue got a little better, but it still overheats after about 4 minutes at full throttle

The starboard side is staying at about 170 degrees

I also have a brand new seawater pump

I also swapped sensors side to side. I inspected the heat exchanger, and it's not clogged

I'm also kinda confused because, since it's all closed loop, the gasket between the exhaust manifold and the elbow has the block-off plates. So that sensor doesnt even get touched by coolant, and it's in the elbow

Any insight?

Thank you!
 
Lou C

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
14,456
Don’t know your system but is there some kind of a control valve regulating flow to the elbow?
 
alldodge

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
44,565
First need serial number?
When was impeller last replaced?
Might be the poppet valve
 
tpenfield

tpenfield

Moderator
Staff member
Joined
Jul 18, 2011
Messages
19,513
BTDT . . . I had (past tense) a Cruisers Yacht 338 with the Mercruiser 6.2L Seacore engines. . .{converted it to outboards} . . . I started getting exhaust overheats and it would go into guardian mode (3000 RPM limited). I tried to clean out the elbows using oxalic acid. It improved it somewhat but not entirely. I believe the remedy is to replace the elbows . . . IIRC they will run about $1500 for a pair, but the block-off gaskets are $200 each (total rip-off).

If you take the elbows off, maybe you can try to clean them out some more, but they may still overheat.

There is (should be) a poppet valve at the back of the engine which regulates the raw water flow to the elbows. I had considered taking out the valve from inside the valve body, but never got that far.

In the near term . . . perhaps you can run at less than full throttle and see where the exhaust temp stays within its limits.
 
You must log in or register to reply here.
Top