5.7 mercury overheating

[dtoufar]

Hey every one,
i have a 2 year old 5.7 Mercury MPI motor with Alpha One out drive exclusively running in salt water.
Two weeks ago it run great all day, last weekend it started overheating.
I suspected a faulty thermostat, so on the water i pulled it out and tested if i can make a day without a thermostat. (i read now that it is not a good idea to do so)
I was able to run up to 1800 rpm and staying at 180 degree engine temp. We trolled and fished without a thermostat like that for about 10 hours.

Next day i replaced the impeller ( old one was still in mint conditions with no sign of wear) and the thermostat. (tested the new thermostat in boiling water for function)

i thought my issue would be gone but just to make sure i checked on the nearby lake today with the result that the engine still overheats.
Engine runs great in my driveway with no signs of overheating.
Water flow seems to be normal at exhaust!
At idle up to 1500 rpm engine stays in good temperature range. (under load on the lake)
Little above 1500 rpm temperature climbs up to 200 when i shut it off.

After every use i flush the engine with Salt Off. and after water cooled off i drain the water out of the engine. ( boat is in my driveway)
What should be the next step to look at? I am a bit lost here at the moment with fishing in full swing.

Any help will be much appreciated.

Thank you very much

Daniel

 

[tpenfield]

I see that you joined iBoats 6 years ago, but this is your first post. :welcome:

I assume that you do not keep the boat in salt water, but trailer it to the ocean each use (right?) When you say "2 years old" you mean it is a 2015 model engine (not just new to you)?

I would check the intake hose that lead from the transom assembly in the engine bay. Follow that hose as it leads up to the thermostat housing. The intake hose should go to one or more 'oil coolers' . Take the hose off at the entry side of those coolers and see if you have got sea weed or other marine life that is trapped and blocking the water flow.

 

[Bt Doctur]

You may need to replace the water pump base,
I was able to run up to 1800 rpm and staying at 180 degree engine temp
WAY too hot without a T-stat, your not getting enough water in or not getting water out

 

[Scott Danforth]

either you have a blockage on the inlet side or a blockage in the manifolds/risers limiting the water out. do you flush your motor after use in salt?

common raw water inlet restrictions are debris in the power steering cooler line
common raw water outlet restrictions are rust in the exhaust manifold and risers

you can easily test the inlet side

with boat in the water, remove the line from the transom assembly to your thermostat housing. with the engine running at 1000 RPM, you should be able to fill a 5 gallon bucket with that line in 15-18 seconds

 

[dtoufar]

thank you guys very much for the quick responds.
Here some answers...
I bought a brand new crate engine 2 years ago. (engine still under warranty but all mercury dealers are busy for at least another 3 weeks, can't wait that long)
After every use in the ocean i flush the engine at the harbor with "Salt Off". and after i park the boat in my driveway and she cooled down i let the water run out of the engine.

great advise, i will take the inlet hose of today and see what i can find. the 5 gallon bucket in 15 to 18 seconds idea does it also work in my driveway with the ears and a garden hose connected to it ??

I report back later

 

[Scott Danforth]

does it also work in my driveway with the ears and a garden hose connected to it ??

NO. you have to test the system in the water. the garden hose could be forcing water past your RWP in the leg, so you must test in the water. you can test while the boat is still on the trailer, just in the water.

 

[dtoufar]

sorry for getting back so late, busy busy weekend!
water intake is in my opinion fine, study, strong i really believe its all good.
Will do the 5 gallon/ 18 second test tomorrow in the lake... just to prove it.
Over the weekend i pulled the risers and manifolds, they are two years old looked in good conditions but did have lots of rusted rocks in it.
The connection on the bottom where you plug the hose in was plugged to i would guess 80%. Everything is big and hard to clog but that particular part of the manifold is not. Water streams in and right away needs to do a right or left turn thru a i guess half inch or less area to make it into the manifold - sorry but seems like a very poor bad design to me. Correct me if i am wrong ...
My theorie is: Since i always drain my engine when i get back to my driveway all the rust rocks and what ever are getting forces right to that 1/2 inch area and cloaking the intake to my manifold.
Now the good and bad news.
Good news is that i am visually getting more water to come out of my exhaust, bad news is - she idles at 600 rpm and sounds ruff up to 800 rpm.
I did changed the spark plugs in the event and did unplugged the wires to the spark plug before i pulled the manifolds.
I have it all back together and ready to through her into the lake tomorrow. Hope the idle issue will resolve it self and she will not overheat again.
Positive thinking i know probably not.
Anyone sees something or does have a idea please let me know, fishing is in full swing with 200 lb tuna just of the coast and i really need her to work again next weekend.
thanks for reading

Daniel