4.3 Mercruiser Overheat Issues/ Are my Head Gaskets still Good!!!??

Just picked up a project boat 89 4.3 Mercruiser, the original owner ran the boat and it overheated, he stated the port side manifold was really hot, starboard seemed normal, he noticed upper starboard side 3/4-1" hose was collapsed, don't know if that means anything, he shut it down and tried to start it and the motor was tight from being hot,struggling to turn over, which is pretty normal for a hot motor, so he hung out and fished for an hour, hit the key, motor started right up, he drove it in and pulled it. He stated to me he was gonna install a new set of manifolds and risers, but he was also worried abt. a blown head gasket on the port (hot) side. So I have the exhaust all apart, do I need to pull the heads, should I do a compression check? If I need to pull the heads to do gaskets, should I have them checked for straightness etc. I don't want to get too crazy, I figure if the machine shop starts telling me do a valve job, seals, mill the heads, its gonna start climbing money wise, but I also don't want to throw them back on with fresh gaskets and have issues, So I guess what I'm asking is when these motors / manifold risers overheat, do they usually take out the head gaskets with them, and if they do do they usually damage the heads. Thanks in advance to anyone who's already gone thru this small nightmare.

You do not mention which style cooling system...... Open (raw water) or a closed cooling system (heat exchanger).

Since the circ pump supplies coolant to an area that will divide flow throughout the engine block, it's unlikely that one cylinder head became hotter than the other.
Conversely, it's very likely that on the "spent" sea water side, that one exhuast manifold became hotter than the other due to un-qual sea water flow.
It does not take all that much restriction to create enough un-equal flow that may cause an issue like this.


For a RW Cooled engine:
You'll want to follow the sea water path from point of entry, completely through the system.
Everything..... an oil cooler (if equipped), the T stat and T stat housing, the way that the T stat housing splits and diverts the spent sea water on to the exhaust system, etc..... including the exhaust manifold/riser transfer ports.

For a Closed Cooling System engine:
Again, you'll want to follow the sea water path from point of entry, completely through the system.
Check oil cooler (if equipped), and the way that the Heat Exchanger splits and diverts the spent sea water to the exhaust system.

Look at the exhaust manifold sea water supply hoses for interior delamination.

Many over-heating issues are a result of something very simple that has been over-looked or neglected.

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I got a chance to look it over this morning a little and I do see the collapsed upper starboard hose and he mentioned if he had changed that it probably wouldn't have happened, he said it was even more collapsed when the motor overheated. I have the port side manifold off that he claimed was the hotter one and where the riser bolts to the manifold there is a small crack in the center port, they don't look overly clogged, but who knows what it looks like inside. It turns over nicely with a breaker bar, and the oil doesn't look really milky, but it is overfull by abt. a quart if that means anything other than he might have overfilled it?

...where the riser bolts to the manifold there is a small crack in the center port...

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There may be water in the oil pan. Pump it out asap. Pull the spark plugs and crank it a few seconds to see if water comes out the plug holes. If you don't see water right away place a cupful of the old oil in a tall slender jar to see if water settles to the bottom.

Yes get the old oil out NOW!!! Then do a comp test. Replace the hoses and manifolds.Replce the impeller FIRST!!! One manifold gets hotter as water flows to the least resistance. If your pump is weak, it might only pump enough water to cool one side.

You have yet to say whether this 89 4.3 Mercruiser is Raw Water Cooled, or equipped with Closed Cooling system.

We are not experts at tooth extraction........ but we will resort to this if need be!






It may shed some light on the over-heating issue!


Since this is a 89 4.3 Mercruiser, it is likely an A drive. The A drive sea water pump is in the drive.
With the sea water pump in the drive, there are no suction hoses, which means that there are no hoses that would typically collapse?

I think that we need a bit more information.




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Its an open raw water system, the one upper, left side hose is collapsed, its abt. 3/4 - 1" heater hose size and I think it goes into the thermostat housing, I hear what you're saying, but it is collapsed, if not from suction, probably from old age,I'm gonna change out all the hoses. OK so , drain oil, I'll put in a glass jar, see if there is water, check cylinders for water and do a comp. test and post my findings. If and I hope not, there is water in the oil, what does that tell me, also what if in cylinders. Would any of this be a tell tale sign of head gaskets? Or could this water also have come from manifold leak, just trying to differentiate causes and rule out head gaskets, how many of you guys have seen 4.3 overheat and blow head gaskets vs. not and just bolting on manifolds and everything being good after that ? Also wondering if the overheat condition caused that crack in the manifold. Hate to pull the heads, but will if I have to, also would hate to put it all back together and then have to pull the heads ugh LOL decisions decisions!

Typically the cylinder heads would need to become hot enough to warp. When these warp, the gaskets will usually fail.

If the exhaust manifolds are cracked internally, water can enter a cylinder or two, then it's pulled into the cylinder, and eventually down past the rings. This takes a bit of time.
However, the damage may not be limited to head gaskets only.

Do the compression test like suggested. You may also need to do a leak-down test.
Start here, or you may be chasing your tail for quite some time.

It sounds like you'll want to at least replace the unknown condition exhaust system.
These are wear items, and it's common protocol to replace these after XXX hours anyway.


Was the purchase price good enough to warrent an engine replacement?

# 1 boat owner proclamation:

I, the boat owner, fully understand that boats are never investments, and are almost always expenses only!


Write this on the chalkboard 100 times, and you'll be OK!



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So I guess the only way to tell for sure if the gaskets are bad, is pull the heads, or put it back together, start it and see if it leaks, although if the gasket leaks in the right spot you can pick that up with a compression test, I got it for $800 so I have some room, and I really like the hull, I can get the manifold riser kit for $450, also I've seen rebuilt long blocks for $ 1200 which is pretty cheap, but I hate to go that route as this was the 2nd motor in this boat with not many hours on it, so I was hoping to get away with manifold risers. I'll do the compression, oil test like suggested and go from there I guess.

mtnxtreme said:

1.... So I guess the only way to tell for sure if the gaskets are bad, is pull the heads, or put it back together, start it and see if it leaks,

2.... although if the gasket leaks in the right spot you can pick that up with a compression test,

3... I got it for $800 so I have some room, and I really like the hull, I can get the manifold riser kit for $450, also I've seen rebuilt long blocks for $ 1200 which is pretty cheap, but I hate to go that route as this was the 2nd motor in this boat with not many hours on it,


4.... so I was hoping to get away with manifold risers. I'll do the compression, oil test like suggested and go from there I guess.

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1.... I think that you are missing the point of the testing.


2.... NO.... this would hopefully be picked up by performing a leak-down test.


3... Well, depending on the boat, that's certainly a minimal initial cost. (notice that I didn't say "investment"! )


4... There are no free get out of jail cards when it comes to boats and boating.


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Something else to stress about...dry rot. It's an 89 hull so when you decide it needs an engine take the next step before buying an engine and sound the transom, floor and stringers. A damaged floor is easy to replace. Once water gets into the floatation foam...which it will...the rot begins quietly and spreads like a quiet cancer to the stringers and the transom before you know it. It is a major project to take on but it can be done. BTDT

I've done quite a few compression tests, but never the leak down ( if I remember correctly your basically watching to see how long a cylinder holds said pressure ? ), can you give me a quick rundown, and also what kind of compression should I be getting, then I'll post what I get from the compression and leak down and maybe you can help me decipher it. Thanks

RicardoMarine said:

1.... I think that you are missing the point of the testing.


2.... NO.... this would hopefully be picked up by performing a leak-down test.


3... Well, depending on the boat, that's certainly a minimal initial cost. (notice that I didn't say "investment"! )


4... There are no free get out of jail cards when it comes to boats and boating.


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OK she leaked right down then found water in 2 cylinders, has a slight knock when turning over, so I can imagine what it sounds like running, she's toast !! Already found a low hour marine motor, so onward. Thanks guys!