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1996 5.7L Searay making rapping noise under load

J

Jeff88

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This boat is new to me this season, so I don't have a lot of history on the motor. (S/N OF766732)
It was running very nicely, with no issues.
Last week it developed in issue, where when I accelerate to bring it up on plane, it starts a rapping sound, and seems to have less than full power. The motor will continue to accelerate, but the rapping will not diminish until I drop it back down to a lower RPM.
I have not yet replaced any components. All the wire conections looked good, plugs looked good, cap and rotor had some crust which I cleaned off. No change. I also used a spark tester and found good spark on all cylinders.
My car mechanic friend suggested that it sounded like a possible broken valve spring. I've pulled both valve covers and found no visible spring, rocker or valve damage.
Suggestions?
 
Have one doing the same thing, good to 2950 ,any more and it raps around #8 cylinder.Suspecting a cracked piston or ethanol damage.
 
When I pulled the plugs, I did notice that one plug (maybe number 5) was discolored. It wasn't what I would consider fouled (wet, etc.) but the ceramic stem had a rust coloring.
 
Thanks. I'll test the compression tonight. I did run the motor hot several days before the problem developed, due to weed buildup on the outdrive. A headgasket issue would be a likely result.
 
You got water in the oil ?

It continues to run hot ?

If not i don't think it's the head gasket.

Do the compression test, with all the plugs out, on a warm motor with the throttle down.

Keep track of what plug goes in what hole.

Spark Plugs

Post back we will go from there.
 
Finally got the chance to investigate further.
I have water in atleast one cylinder (number 5) and probably number 7 also.
I spun the motor with the plugs out on the left side, and had a splash of water come shooting out of cylinder 5.
Didn't bother to do the compression testing.
So I guess my next step will be to remove most of the motor periferals and pull the head.
Fotunately with this Searay, I have clear access to the motor.
Hopefully it is just a gasket failure.
 
I've not worked on this type of manifold before. Can you explain a bit about what I should look for, regarding the manifolds?
 
Chances are you have cracked heads. These heads are notorious for cracking. It would be better, and cheaper in the long run, to replace the engine than to replace the heads on a 1996 engine.
 
Pulled things apart tonight. The good news is - the head gasket had failed. Cylinder 5 was definitely burning water (the piston was very clean, compared to the others). The gasket material was gone between cylinders 5 and 3. It appeared that it had corroded away. The overheating incident must have expanded things enough for it to give out. Did not see any evidence of a crack in the head, block or piston.
Is there anything else I should check out or replace before puting it back together? THe other side of the motor had uniform, clean burning plugs, so I did not pull off the head.
 
Make sure to get a good marine grade head gasket.......In my opinion it is best to go to a Merc dealer with your serial number in hand and purchase OEM.

You will get two, Again in my opinion I would do both just as a course of preventitive maintanence......If it got hot or it is due to corrosion then the other side may not be far behind...........JMHO!
 
Yes, do both sides.
While the heads are off, check the casting numbers.
We never know what a previous owner or previous mechanic may have done.

There are several SBC head casting numbers that should be avoided.
Post the numbers, and/or do a search for SBC casting numbers.
The light castings will be listed if you dig deep enough.

As a general rule..... look for the amount of scalops at the lower areas of the cylinder casting.
If the casting offer only two scalloped areas, they are likely the heavy castings.
images


If they offer multiple scallops, they are likely the light castings, and should not be used in a Marine application.
images



NOTE: by 1996, this may use the Vortec castings. Easy to determine by casting number and/or by the absence of the four center intake manifold bolts.

But again..... not knowing what a previsous owner may have done:
A...... you must also consider the combustion chamber volume, so that this matches the piston selection in order to maintain the correct C/R.
B.... add to this the intake manifold bolt angle and center or perimeter valve cover fastener pattern.

Basically, what I'm suggesting, is to make sure that the correct cylinder heads are used for the piston selection and year model intake manifold.
If Vortec, you'll want to go back with Vortec..... should you need to replace one or both.



I take it that this is raw water cooled...... Yes/No?
If so, ditto the Marine gasket set.

If closed system cooled, a high grade automotive set works just fine!
There is no compromise in doing this.

.
 
Last edited:
It is raw water cooled.
I will do both gaskets. It's not much more work to pull the other side, now that the intake is off and everything is stripped down.
The company I work for has a large fabrication and machine shop, so I was planning on borrowing a good straight edge to check the head flatness, and also the materials to do a dye penetrant test on the head surface, to check for cracks.
 
Any other suggestions for other parts or gaskets, etc. I should replace or check out while I have things apart?
Unless I find a warped or cracked head, I'm planning on replacing just gaskets (heads, intake manifold, exhaust manifold).
 
When reinstalling the Intake manifold DO NOT USE THE Z GASKETS> Use a good ultra black,gold,grey (what ever color floats ur boat...lol) and
use ~ a 3/8" diameter bead along the front and rear.......

When installing the bolts for the intake many have asked what torque, My answer is to tighten in a circular fashion from center working your way out. Tighten till they are good and tight......done.


Intake gaskets are standard No marine there, Exhaust gaskets either marine or automotive will work.

May be more but I have other work to do right now so maybe some others can chime in...........
 
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