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79 marquis with OMC I/O 305 V8 Overheating

J

jdtm84

Member
Seems to be getting plenty of water, removed the thermostat all together, still pegging 240. I pulled the fresh water hose off and it shot a solid stream of water, checked the thermostat housing for blockage seems clean. Without the thermostat I should never get to operating temp let alone over heat right? After about 3 minutes of wide open running on the lake it pegged out again but when idled it cools off on its own. Any suggestions before I chop it up and mount a radiator and electric fan?
 
Next stop the risers / elbows pull them off bet they are clogged solid. And look for flappers, they maybe lodged in the pipe after the riser.

Remove them and leave them out.
 
Like chief says.The risers/manifolds.How old are they and salt or fresh water(profile?)
You need the thermostat in there.It allows the excess water to go through the manifolds right.
 
jdtm84 said:
After about 3 minutes of wide open running on the lake it pegged out again but when idled it cools off on its own. Any suggestions before I chop it up and mount a radiator and electric fan?
Click to expand...

try greasing the swivel bearing on the outdrive - grease fitting is inside the hole in the cavity front side of drive just below exhaust hole.

a radiator big enough would just about sink the boat
 
getting a lot of water out exhaust but will check risers when running on hose the manifolds stay cool all the way back beyond the rubber couplers wouldn't that say the water is flowing? I don't know, ran it on a radiator today still got hot but took a little longer but was only idling and pumping water through the manifolds used a real small radiator may keep it inline for additional cooling and winterizing. Witch way does the water flow on this through the water pump first and out the intake or reverse of that, radiator got warm but not hot at 190 degrees thinking the water pump is not circulating, still no thermostat. (running fresh water)
 
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I want to be as crystal clear as i can in this reply.

With no t-stat the temp gauge should not move one bit at all even at wot.

You know how i know that ? Cause i don't have a t-stat on my motor.

That means your clogged somewhere.

So you say you got good water to the t-stat and that means all and every exchanger is clean and clear and good water running thru them.

Also a car or truck rad. has no place on a boat.

You circulating pump could have been changed out and like you someone thought they can install a car pump.

Only problem is the car or truck pump does not have brass fins.

I still think your flappers are down and clogging you or the risers are clogged.

But your original idea has merit, you will mount the rad. on the bow and have a fan blowing thru it and maybe it will keep the motor cool.

Just keep it a closed system.

Keep us posted how it works out for you.

Oh, you have tow insurance ? Make sure you keep their number on your person or wrote down on the helm somewhere.
 
Flappers are in place and working properly, does not seem to be getting water to the center cylinders. Hooking it back up to lake water system, the water pump seems to be OK, is not a car or truck water pump every water pump I could get for a 305 car or truck engine would require modification to the front motor mount to fit. Not getting water in the oil so thinking the head gaskets are good, any ideas what I could do to clean the cooling ports without extensive tear down?
 
OK yeah got that been a mechanic for more then 10 years, never seen this, water will sit on the front and back of intake but boils off the center what might cause this other than blocked water ports anyone else ever had this problem? Thinking I might just go and pull the intake replace the gaskets and clean the ports.
 
like most all Chevrolet V6's and V8's of that era, the center of the intake manifold is heated by exhaust (crossover)
it helps the carburetor atomize gasoline by warming the bowl and carb base.
you have the option to install restrictors -little tin squares that come with most intake manifold gasket sets.
I honestly don't remember whether OMC says to block or leave open.
I boat in some cold climates, so I always break the squares off to open the crossover.

if you haven't greased the swivel housing, go grease it and then go enjoy your boat - your impeller is probably sucking exhaust
 
where does it need greased I greased all the fitting I could, the only one I didn't grease was the one inside on the lower unit where it bolts to the upper unit because I didn't have a grease gun when I bolted it back together. Where is the grease fitting for the thing your talking about.
 
OK problem fixed
was not the flappers
was not the on engine water pump
was not the water ports in the intake or the exhaust manifolds
was not the swivel housing grease
was none of these things any one mentioned
to recap I had good water flow to the engine at idle speed, removed the thermostat and still had the problem, only got hot at top end WOT.
here's what I did,
Pulled the out drive back apart, pulled the water pump, greased the lower unit and replaced the impeller, it was weather checked and was pushing good water at idle, however at higher RPMs the fins would fold and loose pressure causing a loss of flow to the engine. Played on the lake for more than 2 hours tonight still no thermostat engine runs great and stays cold, thanks for all the ideas and next time this problem surfaces we will all know what to say to get it fixed.
 
jdtm84 said:
was not the swivel housing grease
....
greased the lower unit
fixed.
Click to expand...
:)
this is why a factory service manual is such a good investment. For around 20 minutes worth of gas, one can be had on eBay.
It would tell stringer owners how that swivel bearing grease fitting on the lower unit needs grease twice per season.

The symptom of neglect of that fitting is high rpm overheating.

get a thermostat back in that engine, you're doing it no favours by depleting the proper circulation of water in the block and heads.
 
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get a thermostat back in that engine, you're doing it no favours by depleting the proper circulation of water in the block and heads.
Click to expand...

Not to mention the poor running qualities of a constantly cold engine. This guy was a mechanic for ten years? Bicycle mechanic maybe?
 
jdtm84 said:
OK problem fixed
was not the flappers
was not the on engine water pump
was not the water ports in the intake or the exhaust manifolds
was not the swivel housing grease
was none of these things any one mentioned
to recap I had good water flow to the engine at idle speed, removed the thermostat and still had the problem, only got hot at top end WOT.
here's what I did,
Pulled the out drive back apart, pulled the water pump, greased the lower unit and replaced the impeller, it was weather checked and was pushing good water at idle, however at higher RPMs the fins would fold and loose pressure causing a loss of flow to the engine. Played on the lake for more than 2 hours tonight still no thermostat engine runs great and stays cold, thanks for all the ideas and next time this problem surfaces we will all know what to say to get it fixed.
Click to expand...


Did you replace the impeller with stock impeller? We had a shop replace ours and the exact issue still happens. Fine at low rpms and pegs out at WOT.
 
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