Mercruiser EC Exhaust Manifold Overheat - Help Please

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vasilios

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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

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Don’t know your system but is there some kind of a control valve regulating flow to the elbow?
 
alldodge

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First need serial number?
When was impeller last replaced?
Might be the poppet valve
 
tpenfield

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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.
 
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vasilios

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tpenfield said:
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.
Click to expand...
alldodge said:
First need serial number?
When was impeller last replaced?
Might be the poppet valve
Click to expand...
SERIAL NUMBER 2A450559

I literally just changed the whole seawater pump, so the impeller is brand new

I swapped the poppet valves side to side; still the same issue
 
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vasilios

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tpenfield said:
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.
Click to expand...

Wow, thats crazy! My boat is a 2018 Cruisers Yachts 338 CX South Beach edition. Same boat

My engine serial number is SERIAL NUMBER 2A450559

So I have the G3 software, and after doing some sea trials this week, I noticed that the port motor is doing the same thing, just not heating up as quickly

Starboard motor throws the boat into guardian mode before the port does, but the port manifold on the port engine is still getting up to 200 degrees

Port manifold on starboard motor is the one that throws it into guardian mode

Basically, what I'm saying is that the problem is happening on both motors

Both motors have new seawater pumps, and I swapped the poppet valve side to side, and it didn't make a difference

These motors don't have the popet valve on the back; they have it right above the alternator

I took that port elbow off, cleaned it out, as it was pretty clogged up with chunks of rust

It made the problem better; it takes a long time for it to heat up

But the issue is still there

What's crazy is that, since the manifolds are closed-loop cooling, that sensor reads the overheat; it isn't in any sort of liquid

I cant understand if it should be reading the raw water from the elbow, or the coolant thats in the exhaust manifold

What do you think about it being Bravoitis?
 
alldodge

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alldodge

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Sound like more than enough pressure, only thing left is flow rate I would think. Bravoits and or a leak will cause bubbles
 
tpenfield

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I am not thinking Bravoitis since I think it was fixed on the later year Bravo drives.

i’m in the camp of clogged elbows. Take them off and clean the daylights out of them.

Also run a descaling solution through the raw water side to clear out the heat exchanger. If the water going to the elbows is too hot, then it will tend to make them overheat.

I switched my 338 to outboards- you can check out my thread of the conversion
https://forums.iboats.com/threads/splashed-i-o-sterndrive-conversion-to-outboards.773453/
 
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tpenfield

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Post a picture of your engines . . . Not sure I'm finding the correct one, based on the serial number.
 
Lou C

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2 years in salt water normally won’t clog elbows at least not in my experience with 2 different style exhaust systems on the same engine. That is a thing after 5-6 seasons usually. However that cat system may be set up to alarm at a conservative level of elevated temp and the only way to know is to take it apart & inspect.
 
tpenfield

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I've always felt that the cooling of the exhaust elbow (riser) was a weakness in this design. It seems like once it gets to about 200-210˚F it wants Guardian mode. I think the elbow gets more easily restricted than the old-style elbows.

@vasilios at some point in your ownership were you able to run WOT for longer duration? Also post some pictures of your boat. Are you a member on the FB 328/338 owners group?
 
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