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454 mercruiser overheating

D

Darrell Orr

New member
i have an older 454 with rebuilt carb and a closed heating system with heat exchanger. It is overheating to 200 deg. I put a thru hull pick in, new impeller, new water pump, stat is 160 deg. Slowly runs up to 200 deg and then I idle back down. Any thoughts on what else could be wrong. Thanks D
 
Several possible issues,
Air in incoming water stream,
Lack of watet flow out of elbows and or exhaust manifolds
Wrong routing of water hoses
Have you confirmed engine is at 200*. A faulty temp sending unit could be giving you wrong actual temp.

How about more info? Where do you use the boat? Salt or fresh water?
Actual year of boat/ engine...
Alpha or bravo outdrive??????
 
in addition to what Jack listed, if its old enough, not enough belt tension and scoring inside the pump - in other words, a new impeller may not be adequate.....
 
Did this engine ever run right for you? What type of hull inlet? Raw water hose ID? If you are running a 454 exhaust thru the drive ( which one there are a number of DP's) likely too restrictive an exhaust flow.
 
Last edited:
sandkicker said:
Did this engine ever run right for you? What type of hull inlet? Raw water hose ID? If you are running a 454 exhaust thru the drive ( which one there are a number of DP's) likely too restrictive an exhaust flow.
Click to expand...

Thanks for your reply. I have a thru hull water pick up but the exhaust goes through the drive. Used to work fine. Last night I pulled the power steering oil cooler to see if there was any restriction there...it's clear....same as the heat exchanger...clear. However when running the top of the heat exchanger (antifreeze section) gets very hot and so do the hoses to the manifold and circ pump...which I replaced as well....so I know the pump is good. Is there any way to flush the antifreeze system....or is there a product to clean it out without having to pull manifolds etc. I'm running out of ideas...ugh. Thanks again
 
sandkicker said:
Did this engine ever run right for you? What type of hull inlet? Raw water hose ID? If you are running a 454 exhaust thru the drive ( which one there are a number of DP's) likely too restrictive an exhaust flow.
Click to expand...

Thanks for your reply. I have a thru hull water pick up but the exhaust goes through the drive. Used to work fine. Last night I pulled the power steering oil cooler to see if there was any restriction there...it's clear....same as the heat exchanger...clear. However when running the top of the heat exchanger (antifreeze section) gets very hot and so do the hoses to the manifold and circ pump...which I replaced as well....so I know the pump is good. Is there any way to flush the antifreeze system....or is there a product to clean it out without having to pull manifolds etc. I'm running out of ideas...ugh. Thanks again
 
sandkicker said:
Did this engine ever run right for you? What type of hull inlet? Raw water hose ID? If you are running a 454 exhaust thru the drive ( which one there are a number of DP's) likely too restrictive an exhaust flow.
Click to expand...

What type hull pickup??? pipe ID??? Hose ID??? Did it ever run Ok with the hull pickup?
Re: OK withexhaust thru drive... yep... runs OK... I changed my 5.7L to thru transom w mufflers instread of thru my Volvo drive and picked up 450 RPM...Engine will run fine until the exhaust valves die. There is a MERC specified maximum back pressure for that engine... check it. ( drill 0.32 hole in the thick exhaust hose near elbow, insert spit lubed basketball inflation needle, measure pressure... Mine was waaaaay above max factory back pressure. 5.7L engine spec's @ 4" minimum dia exhaust... Hole thru transom waaaay smaller than that despite fancy wide mouth thru cav plate...
 
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