5.7 Merc. overheats at high speed


Old 08-09-07, 05:18 PM
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Join Date: Jan 2003
Posts: 56
Exclamation 5.7 Merc. overheats at high speed

buddy has 1994 Silverton with twin 5.7 merc cruisers...putzing around under 2000 rpm engines are fine..but when getting down on it at above 3000 port side overheats...he did, new impeller, thermostat, water pump, intake hose,thought it might be collapsing, even switched out sensors with the other and still the same..any thoughts on what to try next? had the drive off and all is clear,no clogs of sorts too...
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Old 08-09-07, 07:17 PM
Join Date: May 2007
Location: VA
Posts: 16
Sounds like youíve checked everything on the intake side so itís time to check the exhaust side. All that cooling water that the engine sucks in must exit through the exhaust manifolds and risers. Itís not uncommon for one or both of the risers to become blocked by rust, scale or some other obstruction. If the water canít get out it wonít cool the engine. A partial blockage will allow enough flow to cool the engine at low speed but not at high RPMs.

Tell your friend to pull off the risers and check for a blockage. Take a close look at the manifold to riser water passages and at the discharge ports located at the end of the risers where the rubber exhaust hose clamps.

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