picca00

i have fresh water cooling on my 96 454/502 mpi. i need help trying to figure something out. when i run at over 2500 rpms my overflow bottle fills up fast and overflows alot. i have a thru hull fitting for my raw water pickup. the engine temp always sits around 160-170 even though the starboard riser feels a little hotter than the other one. im thinking that i have a small crack in the heat exchanger allowing the sea water to enter the fresh water and since its under pressure (over 2500 rpms and about 30mph) its pushing up into the overflow. my oil and plugs were fine, when i looked 2 trips ago. is this possible? what else could it be? i didnt bleed the thermostat housing, could that be? and what is proper procedure to use this bleeder valve?
thanks guys
jay
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blue thunder

When I had that happen it was a cracked head, but a blown head gasket could act this way too. Get a coolant system pressure tester from autozone and see what you have.

picca00

what about the rubber gaskets and end caps on the heat exchanger? mine had a small crack that used to leak under pressure but stopped and now it seems like its going to the overflow. could water be cross flowing into the separate chambers?
if i had a cracked head or blown gasket wouldnt my oil have water in it? also the boat runs like a top just has alot of overflow.....

picca00

anybody got any ideas? am i on the right track?

blue thunder

If it had a crack in the combustion chamber like under a seat, it would blow cylinder pressure into the water jackets. This would manifest itself by blowing cooland out the reservoir/overflow.

If you are actually gaining coolant level then I would lean toward the heat exchanger leaking like you are thinking. You could pressure check that fairly easy as well.

picca00

thanks for clearing it up, yeah i have alot of overflow, pissing out at over 2500 rpms. like i said the temp stays right below 170-175 under all loads basically a little cooler at speed. im going to get new gaskets, o-rings, and cover plates for the sides of the exchanger, since all mine are shot. the leak make sense to me because when i fill it on top it comes out the raw water pick up! it should be 2 closed systems so well see after i replace the pieces and pressure check.
also i opened the bleeder valve on the thermostat while running and up to temp and i got alot of air and then just bubbling water, no steady stream...is this normal? or is it because i have a problem with the heat exchanger?
thanks ...jay

picca00

ok well i pressure tested the system today and im not sure of the results. first i left the raw water intake open filled the system and pressure checked it, the water was leaking out the intake like before just a little faster now. i closed the intake,refilled and pressure checked again, this time it was coming out the port side exhaust tip... while pumping i could hear some air in the thermostat and it was bleed just prior to that. so now im back to square one. do i have a leak in the exchanger or is my thermostat stuck open? any help would be great im trying to get this done right now.... thanks jay