Mercruiser 350 mag mpi OVERHEATING

[iluxa]

Can somebody please help. My engine overheat alarm keeps going off. It only goes off if I have the throttle above 3400 RPM. Anything less, the temp stays normal. I have replaced the impeller, however when I took the old impeller out, it did not have any wear on it. I will be replacing the thermostat tomorrow, but did anyone else have the same problem? If so, what was the cause of the overheating? ANY HELP IS GREATLY APPRECIATED!!!

 

[MExcelsior]

Iluxa, you are doing the proper order for checked parts. The most obvious is the impeller. Now that it is replaced, the thermostat is your next obvious check. Remember, there may be a truckload of reasons. Let us know after your thermostat....

 

[KnotEasy]

Yep, seen it before. When under load it overheats. At idle it does not. That would be manifold's and risers usually. Check them while you are at it.
You will find your thermostat is working fine.

 

[jason78]

Are you in Salt or freshwater? Adding some location info can prove helpful at times.

 

[iluxa]

Fresh water. Delaware river. Risers were replaced about 2 years ago. Replaced the impeller yesterday and replacing the stat today. Will go for a test ride right after. Is there anywhere a line could get clogged? After all, the Delaware isn't the cleanest river.

 

[Lazy Daze]

I would suggest starting with a few searches for similar threads - you will find plenty of them that will help guide you and give good info.

Sure, there's certainly places where stuff can get clogged - starting with the inlet hose on the inner transom plate, although that is usually due to salt buildup which obviously isn't your problem. Just follow the hoses and disassemble/check/reassemble everything along the way. This is a "freebie" but it does take your time to do so. There's no way any of us can give you an exact location. However, starting at (and inside) the t-stat housing is a good place since that's the first "smaller" area.

Risers are only part of the equation - what about manifolds? If I had to guess, that would be mine, too.

 

[iluxa]

does it look like i need to replace the thermostat housing?

 

[TARANTULA]

Is that sea weed in the bottom? No you can clean that with diluted acid

 

[iluxa]

Hey guys, here is an update since my last post. I replaced the thermostat with a new 160 degree one. Cleaned the thermostat housing out. I took a shop vac and attempted to suck out any debris that could've been in the two lines at connect to the thermostat housing. Took the boat out on the water for tests. Got the same result. It seems like if I am alone on the boat, it will take a lot longer to overheat vs having four people on it. Once on plane, at 4,000 RPM it takes about 7 minutes for it to throw the overheat alarm which kick in at 186 degrees I believe. Brought an IR thermometer with me. Alarm goes off, I lift the hatch, and it seems like its overheating before the thermostat housing. The T-housing was 195 degrees and the lines before it were also in the higher range. The risers were around 175 degrees. At this point, while in the water, I decided to remove the thermostat completely. After doing so, it made absolutely no difference. Still overheated. Being frustrated and disappointed, I dropped the boat off at the shop. Couple days go by, I get a call from the marina. They state that they are 90% sure that I have a bad water pump. I told them, when I was changing the impeller, the pump did not seem damaged or scored. Regardless I told them to change it. More time goes by, I get a call saying the pump was changed, however it’s still overheating and they suspect it could be the risers or manifolds. Couple days later I get a call AGAIN, mechanic says they pulled the risers, they are not clogged, put on new gaskets. At this point, they want to take off the outdrive to see if a problem is occurring from there. My question is, does it seem like thats the right thing to do? If so, what are the issues I can have there that can cause my overheating?

 

[iluxa]

BUMP. Any help is greatly appreciated!

 

[dwna1a]

To me it seems to be the next step. You've started at the engine side of the issue, replaced the impeller and checked the temps. So the next step I would make is the source of you cool water supply. In your case the lower unit where the engine draws in it's water supply. I wish you luck, please keep us informed

 

[kevin27]

Don't pull the drive yet. Have them pull the water inlet elbow off in the inner transome and look at the transome passage tube to see if it's closed down. This is very common more so on salt water boats though. This is what to look for.

 

[BigJoeB]

Check you exhaust flaps if you have that system, the problem may be restriction on the way out not coming in, just a thought

 

[CliffA]

don't forget about the water circulation pump on the engine.....i had to replace mine when the engine started overheating at higher RPM's.....at idle it held the temp OK but it could not keep up with the demand for circulating more cooling water at higher engine RPM's..........

there were no visible problems with the pump externally but when i removed the pump i could see where the vanes inside the pump had worn thus reducing its circulating capacity.....

cliff