7.4 closed cooling system ran hot.....

[sunseekerpaul]

long story short version, changed the exhaust manifold gasket on the starboard side of the port engine --- was misinformed on the gaskets for the riser / elbow and used block off gaskets on the top of the manifold and the riser. WRONG. Started boat ran for 15 minutes at the dock ran fine temps good (idling). Took the boat out for a ride and seemed fine for a while (45 mins) noticed temp was hot on the gauge (no alarm sounded) we turned off the engine, waited a short time and raised the hatch and found the exhaust manifold got so hot the paint melted off. We waited several hours before trying anything else but decided to start heading back to the marina with one engine. While we we moving slowly, we looked at the heat exchanger and found it was empty and decided to fill it with lake water, filled and started the engine and ran all the way back to the marina and docked with out any visible problems. Pulled off exhaust manifold and had it pressure tested and of course failed, pulled spark plugs from the starboard side and they were dry and looked ok, pulled the spark plugs from the port side and on 2 cylinders water came out--- this is the confusing part. We cranked the engine over several times and changed the engine oil which had a lot of water in there. My next step is to do a compression test, if that is ok then put it back together and fill with water then do a pressure test on the cooling system. I'm afraid to ask but how could the water have gotten to the other side of the engine??? I think that there is a crossover in the intake manifold that allows coolant to flow to help with warming up the old carbureted engines ....... sorry for the length here but trying to give as much detail as possible. (1991 454 closed cooling fresh water 470 hours) Kinda got a big mess for a few dollar gasket.

 

[Bt Doctur]

While we we moving slowly, we looked at the heat exchanger and found it was empty and decided to fill it with lake water,
That would indicate a overheat of the engine and possabily blown head gaskets depending on how hot the engine was.

 

[Bt Doctur]

unless you have a half system that does not use AF to cool the manifolds. How many hoses are on the manifold/riser
If a riser fails it usually dumps water into the 2 center cylinders

 

[sunseekerpaul]

I have 1 hose on the elbow , 1 hose on the 3"riser and 1 hose on the bottom of the manifold. Still thinking about compression testing and then pressure testing the cooling system..... your thoughts on that?

 

[Bt Doctur]

then you have a full system that requires a blockoff gasket between the riser extention and the elbow. In the pic called stainless steel separator plate. coolant enters the bottom of the manifold and flows out the extention back to the heat exchanger. water outlet from the heat exchanger goes to the elbow fitting and out the exaust

 

[Bt Doctur]

 

[sunseekerpaul]

yes thanks I know that now........but still need to troubleshoot this engine.......

 

[Scott Danforth]

you do not fill the engine and then do a pressure test. you pressure test with a completely empty system to look for leaks

with a full system, you do need the block-off gaskets between the 3" riser and the elbows, you do not have a block-off between the manifold and riser.

if your manifold got hot enough to melt paint, there was no coolant in it at all and the temperature exceeded 300 F.

 

[sunseekerpaul]

thanks Scott

 

[tpenfield]

I have twin 7.4's with aftermarket closed cooling ( a Full system)

Anyway, it sounds like you need to take better care of your things

Based on what you described, it sounds like you put the block-off gasket in place where there the the regular restrictor gasket there before ? Three hoses going to the exhaust would indicate a Full system, not a half system.

Got any pictures of the cooling system hoses, etc. to post?

Water on the port bank of cylinders might indicate that the gasket needs replacing too.

 

[Lou C]

What is confusing here is it appears that the OP has a full system with the manifolds on the closed loop as well, and therefore he should have used the block off plate, but only between the extension and the riser, if the return hose was connected to the extension. Sounds like there is a manifold, riser extension and riser. If block off plates were used on both the riser extension and the riser, and the extension has a hose hooked up to it, then that would have stopped antifreeze from getting returning from the manifold to the heat exchanger it seems. So it appears that he should have used a regular gasket between the manifold and 3" riser (extension) and the block off plate between the 3" riser and the exhaust riser.

 

[tpenfield]

Any updates on this? My thinking, as Lou stated, is that the one side of the cooling system was blocked off by using the block-off gaskets on both top and bottom of the 3" riser. (should have been only the top)

Also wondering if there was pure water or coolant/water that was found in the cylinders and oil? Also could have been the gaskets that were just replaced, but that water seems to be on the wrong side of the engine for that :noidea: