Mercruiser 350 MAG MPI 2004 - Overheating when above idle

[Kochisp]

Boat is a Bayliner 2855 2004 with a Mercruiser 350 MAG MPI w/ Bravo3 outdrive RAW water cooled. I’ve owned the boat one year, I’m new to boating and new to marine engines. I’ve been working with a few mechanics and looking to get a better understanding of what’s going on with my engine before sending it off to be hauled for service. Hoping there are other diagnostics I can run on my own to help triage the issue before spending $$ on someone doing their own trial and error.

Problem started on the first run of the season about 5-10 min after getting up on plane temperature creeped up above 175 and then alarm sounded at 200 degrees and quickly brought the boat to Idle, at idle the alarm turned off almost immediately and temperature creeped down to about 170 at idle. Any RPM past 2000 would slowly get the boat to overheat.

Replaced thermostat - No help
Replaced impeller - This made the problem worse, overheating at idle now.

Under closer inspection I can see the water intake hose (red hose) which should be a hard hose from what I understand is noticeably deformed and compressed especially while running the boat. When the engine is not running the hose still looks compressed, when running it squeezes even tighter and I believe the new impeller is squeezing it even more than the old impeller which explains why I was able to run between 1000 and 2000 RPM with the old impeller. New impeller would have stronger suction.

Pulled the intake hose from the water pump to check water flow, verified water flow is very good and provides good pressure through the drive when lowered down to the level of the water pump.

With intake hose pulled off the water pump we ran a 5/8” hose from the dock clamped down under pressure and had no problems pushing water back through the drive. 5/8” hose ran with good water pressure ran without any issue back through the intake hose.

Considering water flow through the drive does not seem to be a problem this makes me think the intake hose itself is too soft to be effective, possibly from age? Adding a picture of this hose to this post showing how deformed it is even when the boat is not running.

Any suggestions on things that should be looked into or replaced to further diagnose? I don’t have a trailer so if the boat has to be hauled it will go directly to the mechanic.

It looks like the connection at the transom is very tight and hidden under the power steering cooler somewhere, how difficult is replacing the intake hose once the boat is hauled?

One thought I had was to cut the intake hose as it turns behind the engine and replace the forward ~3ft of the hose with a coupling + ~3ft of brand new hard hose to see if things improve. This seems manageable while the boat is still in the water and other cooling systems I’ve seen online have at least one coupling in the intake line.

Thank you.

 

[alldodge]

The hose is your problem, that should be exhaust hard walled wet hose and its soft walled like a garden hose.

Should be 1.5 inch ID
https://www.iboats.com/shop/trident-seaflex-hardwall-im1228509.html

 

[Kochisp]

The hose is your problem, that should be exhaust hard walled wet hose and its soft walled like a garden hose.

Should be 1.5 inch ID
https://www.iboats.com/shop/trident-seaflex-hardwall-im1228509.html

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Thanks for the info, I think the direct replacement for the intake hose is Water Intake Hose - 18-3603 - Sierra but I’m not sure and also not sure if it’s a hard hose as it’s not mentioned in the description. Is the trident hose a direct replacement too or is it just a generic fit?

 

[alldodge]

NO :nono: , that is the intake water hose between the bell housing and the transom assembly

You need hard walled wet hose in the type I listed in previous post and it "MUST" be 1.25 (SORRY said 1.5 previous) again 1.25 or 1 1/4 inch ID hose. The stuff cost but you have to have that type hose.

 

[Scott Danforth]

based on the pictures, I would put all new hoses on the motor. who know what else the PO frankensteined

 

[Kochisp]

NO :nono: , that is the intake water hose between the bell housing and the transom assembly

You need hard walled wet hose in the type I listed in previous post and it "MUST" be 1.25 (SORRY said 1.5 previous) again 1.25 or 1 1/4 inch ID hose. The stuff cost but you have to have that type hose.

Thanks for your help, I took a good look at the issue, went to West Marine to see the difference in hoses when you actually have a hard hose and feel really good about this fixing the problem.

I’m thinking I can probably get this repair done while the boat is in the water, was thinking to plug up the inlet holes on the outdrive with a pair of muffs that are plugged up to prevent water from coming in while changing out the hose. Is this a ridiculous thought?

 

[alldodge]

Is this a ridiculous thought?

Maybe, the lower the boat sits in the water, the more pressure. If you have a good high output bilge pump would be the first. Get everything out of the way as possible.

I would remove the pump end first and hold it up high, this should stop water coming in. Tie it out of the way so you can get the new hose installed on the pump. Have clamp and hose measured to fit then remove the transom end.

There does also appear the other red hose is standard heater hose, and if so it to should be replaced

Good luck

 

[Scott Danforth]

I would pull the boat

 

[kenny nunez]

If you ever have the boat hauled, it would be a good time to have a through hull sea **** fitted going through a marine sea water strainer and plumbed directly to the sea water pump suction. It is also possible to tie in the drive water outlet hose to the incoming side of the sea water strainer.
On the port side below the power steering pump there is a plastic distribution fitting that should be removed to see it there is any restrictions in it.