Engine overheating on high RPMs

planeracer

Cadet
Joined
Aug 5, 2011
Messages
12
Hello,

I have a Volvo 350 5.7L engine on my boat. It is raw water cooled cooling system and I installed new manifolds , raisers, sea water impeller and 160 F thermostat on it. But, last weekend I found new problem with it, all running ok up to 2000rpm . Temperature around 160 F. As soon rpm passes 2000 temperature starting to going up, I never let it go to high,
Lowered rpm, and temperature went back to normal again. It seems not enough water from sea pump to cool engine
down. I read a lot on the net about that problem, but first think what i did, I lowered outdrive into big bucket to cover sea water inlet holes. BTW, I have 275 outdrive unit. Then started the engine, waited couple min. and pulled sea water pump hose from thermostat housing to see how much water impeller pumps water out, at idle it was ok, but water had lot of air bubbles in it, looks like hose sucks an air from somewhere and actually im not sure how much water must come out from that hose. I pushed rpm up to 2000 and amount of water changed a little but amount of air bubbles increased a lot. Outdrive is not completely in the water , only inlet holes. Now, Is airleak causes that problem? or I still have to go forward and check for impeller condition, or I have a bad thermostat? Please let me know what you think.
Thank you
 

planeracer

Cadet
Joined
Aug 5, 2011
Messages
12
Re: Engine overheating on high RPMs

Hi again,

I taped sea water inlet holes on outdrive and filled hoses with water. The only place i saw water drips was on outdrive itself. Just got that picture on the net. It will be easy to show where is water drips. How easy will be to fix that problem? Do I have to take outdrive a part?
Any Ideas?

Thx
 

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captmello

Captain
Joined
Jun 30, 2008
Messages
3,826
Re: Engine overheating on high RPMs

There is a bushing in that area that may be worn, To replace the bushing you need to pull the outdrive and dismantle the intermediate housing. Also the gasket below the water intake fitting may be torn/worn and leaking. I would replace the gasket under the intake first and see if that helps. to be done properly, you should pull the upper gear box for access. Also that intake fitting itself may be corroded as well.

Heres how to pull the upper gear box.

http://forums.iboats.com/mercruiser-i-o-inboard-engines-outdrives/outdrive-side-pin-removal-309095.html

Good luck, let us know what you find.
 

Silverbullet555

Chief Petty Officer
Joined
Mar 13, 2011
Messages
621
Re: Engine overheating on high RPMs

Did you replace the impeller? I know you said you were going to check it, but that would be a first replacement in my mind and then go from there.
 

planeracer

Cadet
Joined
Aug 5, 2011
Messages
12
Re: Engine overheating on high RPMs

Hi,
yes I did replace impeller, and leak from bushing. Replaced pump in and out hoses with clear ones and bubbles showed up on pump in hose. I already ordered a Volvo Steer Stem Repair Kit. Now time to take a outdrive a part.
I'm going to do it this weekend. Let see how it goes.
Thanks guys for reply.
 

planeracer

Cadet
Joined
Aug 5, 2011
Messages
12
Re: Engine overheating on high RPMs

Hello,

Today I took boat out to test after bushing and o rings been replaced. Engine temperature was about 150 F when engine rpm was about
1000. Then went to 2000rmp and temperature raised to 170F. At 3500 rpm it started to go up again went to 200-220F and it was going up and down at same rpm. it was jumping between 180 to 220F . I opened engine bay to check manifold temperatures , they was ok. Looks like water flow is ok. Before I took boat out, I tested on the trailer , it was no air bubbles on sea water pump out. Tested up to 3000 rpm. Now, what is next step?, do I have to check thermostat? Maybe not opens completely? I installed new couple month ago and it worked ok for 1 day it was holding 160F on any speed. Please let me know what you think.
Thank you
 

captmello

Captain
Joined
Jun 30, 2008
Messages
3,826
Re: Engine overheating on high RPMs

Please be more specific about which Orings and bushings you replaced.
 

planeracer

Cadet
Joined
Aug 5, 2011
Messages
12
Re: Engine overheating on high RPMs

I replaced hose connection, sealing between fork and hose connection, bushing, 2 white color washers, sealing rings around bearing and
and o ring between steering tube and lower gear housing. And all o-rings between lower unit and intermediate housing and upper gear box. I just pulled thermostat and put it in hot water . It started to open at 73 C and opened only 1/8 of inch and fully opened at 90 C.
I think I got a bad thermostat.
 

planeracer

Cadet
Joined
Aug 5, 2011
Messages
12
Re: Engine overheating on high RPMs

hi,
Finally I found a problem, I had cracked circulation pump impeller, it was not turning, I have know idea, how I was able to use that boat with out working pump. Probably sea water pump was doing a job for both.

Thanks
 
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