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Unequal Compression - 1996 Johnson 115

S

sbrady46

New member
I am trying to sell my boat that has a 1996 Johnson V4 115. During inspection by a potential buyer, we performed a compression test. The results: one cylinder 125 psi, the other three 100 psi. I have never had an issue with the engine in the 4 years that I have owned the boat. Runs, idles, trolls, and runs at 45-5000 RPM at WOT.
Is there anything significant about the unequal compression? Thanks any information would be helpful.
 
Yes there is if it is a crossflow model.--------Remove bypass covers on one of the low cylinders and have a look at the pistons / rings.--Broken rings is very common.--
 
Did you run the engine a bit before taking the compression? If not, I'd suggest you do so.

Were all of the spark plugs removed so that the engine cranked at the same speed on all cylinders? Again, if not, do so.

Actually the compression with the higher horsepower models was lowered at some point around the middle 1980's or so. I forget the exact year. However, the compression since then has always varied a few psi from bank to bank... the starboard bank usually reading around 95 psi with the port bank reading about 90 psi.

What I find unusual in your case is that one cylinder is reading excessively high like it has a abnormal amount of fuel/oil within it.

Standing in back of your engine and facing the spark plugs, the cylinders are marked as:

2......1

4......3

Which cylinder is reading high?
 
joereeves said:
Did you run the engine a bit before taking the compression? If not, I'd suggest you do so.

Were all of the spark plugs removed so that the engine cranked at the same speed on all cylinders? Again, if not, do so.

Actually the compression with the higher horsepower models was lowered at some point around the middle 1980's or so. I forget the exact year. However, the compression since then has always varied a few psi from bank to bank... the starboard bank usually reading around 95 psi with the port bank reading about 90 psi.

What I find unusual in your case is that one cylinder is reading excessively high like it has a abnormal amount of fuel/oil within it.

Standing in back of your engine and facing the spark plugs, the cylinders are marked as:

2......1

4......3

Which cylinder is reading high?
Click to expand...

Thank you for the input and assistance.
Yes all spark plugs were removed.
The compression was checked cold and then after the engine was run for at least 30 minutes.
Cylinder # 2 is the one with the high compression, 125 PSI versus 100 for all the others.
 
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