Mercruiser 5.7 ran hot, loss of power

[Dentar]

Hi all!

Have a mercruiser 5.7 with vortec heads + alpha drive, engine fresh water cooled. Had a plasticbag stuck onto the drive resulting in the engine overheating.. peaked 240 on the meter once i noticed it..

turn off engine, found plastic bag, let engine cool off and it starts without a trouble.
however, on idle the temp gets to about 180 (i could lay hands on risers + knees, left one looking at the engine is a little warmer) , while revving or going on plane the temp rise to 200, which is where i turned of once again. (left side knee burns my hand now, left side riser does not. right side is hot but i can keep my hand on knee + riser)

dont take the above temps as exact values, ive had a feeling for a long time that my temp meter isnt exaclt correct (showing abit to high)

the boat usually runs 27 knots at 3200rpm, now it runs 23 knots at 3400rpm.. (WOT before this was 42 knots at 4600 rpm, havnt tried WOT after overheated)

damagecontrol:
oil looks good, no signs of water

compression right side 10.2 bar on all 4 cylinders, left side from front -> rear: 9 + 8 + 8.7 + 10, hence not terrible but def lower than the other side..

checked my fresh water. nothing have vanished (sucked into engine etc) but the fresh water is literally black, could a head gasket leak into the fresh water cooling without sucking water into the cylinder?

or could is be gasket leaking in the knee which would lead to less circulating (fumes get in there, resulting in temp rising?)

I know i should check the impeller. sadly my harbor doesn't have a ramp nearby and i have to be towed for over an hour to get the boat on a ramp. while engine is idling i can feel raw water getting sucked into the drive however.

just want some inputs if overheating could be caused by anything other than impeller. i have easy access to the engine so i rather tear it down while its still in the water.

 

[Bondo]

just want some inputs if overheating could be caused by anything other than impeller.

Ayuh,...... Welcome Aboard,...... The impeller is the Most Likely point of yer problem, hands down,......

Run without water flow because of the bag, No doubt smoked it,.....

 

[Scott Danforth]

the port side numbers are low, way too low in my opinion

if you have 10.2 bar on all the starboard cylinders, the least you should have is 9.6 on any one cylinder, you have 3 lower than that.

you most likely smoked the impeller like Bondo mentioned

you also burned the rubber exhaust flappers and the rubber exhaust hoses. they have a max temp of 250F.. anything more than 30 seconds without cooling water is a guarantee as 1100 degree exhaust is much warmer than the normal 220 degree exhaust/water combo that is normally flowing

you probably toasted the head gasket too (or warped the head or collapsed some pistons). do a leak down test.

 

[Dentar]

Alright, think I just going to hand the boat over to a mechanic and have them fix it to whatever it may cost.. whole summer has been nothing but trouble and season ends where i live within 2-4 weeks anyway..

engine only have 160 hours on it..

thanks for replys anyway!

 

[QBhoy]

Flappers melted and thrown themselves down the exhaust restricting flow, at a minimum. Impeller check needed or at least find out why it’s overheated too. Best of luck.