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Bad compression test on 4 & 6 cylinders

Did you do a bad compression test on cylinders 4 and 6 and are just bragging about it?
Did you get a bad report from someone doing a compression test on cylinders 4 and 6? Relative to what?
What engine? An in-line 6 would be very different than a V8. How old/how many hours? What are the actual compression numbers for all cylinders? Are you trying to figure out what caused it? Are you trying to figure out how to fix it?
You didn’t provide much for anyone to go by with your post
 
Sorry about that I don't know a lot about this issue. I have a 2001 chapparal 230 ssx that I am trying to sell and have not had any issues. I had a buyer that was ready to buy and wanted a compression test done so the marina did it for them and it came back saying that. Its an inboard 5.7 Mercury. I'm just trying to understand what it means and what it might cost to fix. They did say the other cylinders had 90 lbs of pressure
 
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No one can accurately answer that unless you also do a leak down test. That will pinpoint what is causing the compression loss. Could be an intake or exhaust (more likely) valve or piston rings/cylinder wall.
Was the test done on a warmed up engine with the throttle valve wide open? If done on a cold engine, esp one that has not been run in some months, it may be inaccurate. Valves can stick a bit on engines that have been in storage for months. After they run long enough to warm up and plane out the boat, they will give a more accurate compression tests. It is common for marine inboards to have valve issues due to bad exhaust manifolds/elbows leaking water into the exhaust (which rusts exhaust valve stems) or if the engine was under propped and it never reached the specified max rpm which overheats the exhaust valves and causes low compression.
Cost to fix? if it is an accurate test and the compression is truly low, it could range from a valve job and cylinder re-fresh to a complete reman engine.
Just as a data point, I usually do a compression test on my 4.3 V6 every fall when I winterize it and I get 160-170 on all 6 cylinders.
 
Simply put, in order for an engine to run you need a process to happen repeatedly. In a 4 stroke engine without getting too complicated, in each cylinder you get suck, squeeze, bang, blow - coordinated with the other cylinders in the engine. Suck is where the gas and air mixture goes into the cylinder. Squeeze is the piston compressing that mixture. Bang is the burning of that mixture to push the piston down to force the crankshaft to rotate with force. Blow is clearing out the burnt exhaust gasses so the process can start all over again. The squeeze part of that process is what a compression test measures. When you have cylinders that don’t squeeze as much as others, it is low (or in really bad cases no) compression. Ideally, each cylinder should read roughly the same compression and the higher the better. Since we don’t know the compression reading of all the cylinders, it is hard to say what is the issue in your particular case. As noted above, a leak down test will help point to where you are losing compression. Without further examination, you are asking for a wild guess from people who are willing to try to help that can not be there to give an accurate answer. Low compression could be caused by the valves not sealing, the piston having a hole in it, the rings of the piston not sealing, cracks in the cylinder or cylinder head, or in a gasket that seals the cylinder head to the engine block among other things like someone not doing a good job performing the test. I’m not saying this is your issue since I did not personally inspect your engine, but when 2 cylinders that are next to each other have low compression, a leaky head gasket could be the cause letting the pressure from one cylinder bleed into its neighbor
 
According to the Factory manual The 5.7 for 2001 has a compression ration of 9.5:1

Simple math (9.5 times 14.7{ atmospheric pressure} suggests a theoretical value of roughly 140 PSI, real-world readings are usually higher because compression generates heat, which increases pressure beyond the basic volume ratio.

For an engine with a 9.5:1 compression ratio, you should typically expect a compression test reading between 170 and 210 PSI per cylinder.

On virtually any gas engine a compression reading under 120 PSI is considered a problem.

I would Redo the compression check before going further.

Start and warm up engine to at least 120*.

then Remove all spark plugs.

Redo Compression test on All cylinders and report back.

If the results are repeated from prior test then move to a leak down test to try to diagnose the issue more specifically.

How many hours of operation are on this engine?

It is impossible to determine your issues without more data.

A 5.7 may be a bit under powered for a 23 ft boat and that would lead to excessive load over time and cause early demise....
 
cmurrell said:
Sorry about that I don't know a lot about this issue. I have a 2001 chapparal 230 ssx that I am trying to sell and have not had any issues. I had a buyer that was ready to buy and wanted a compression test done so the marina did it for them and it came back saying that. Its an inboard 5.7 Mercury. I'm just trying to understand what it means and what it might cost to fix. They did say the other cylinders had 90 lbs of pressure
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as others have written 90 psi is considered a dead cylinder(s). So from buyers perspective this means they will have to put a new engine in. Some fixes for this aren't terrible cost wise- two adjacent cylinders have low compression is often a blown head gasket. Can also be from hung up or cracked valves, worn rings, detonation that hammers a hole in a piston etc. Too many to list with out more info and more testing (leakdown test).

Either way it will not be a cheap fix. head gasklet can be done in boat assuming the block deck is ok.

this is why it is a good reason to do a compression test and water test pre purchase
 
The problem with these kinds of situations is that you don't know for sure what it will involve until the heads are taken off. Yes you can do a leak down test and scope the suspect cylinders with an endoscope camera and even remove the exhaust on that side, you'll be able to see the condition of the exhaust valves then but as far as repair, until you see what the heads look like and the block deck you really don't know what you're dealing with. If the block deck looks good and isn't heat slotted, or warped, I'd take the heads to a machine shop and have them gone through.
I rolled the dice on mine and got lucky (compression wasn't low but had a bit of water in 2 cyls and sticking valves, leaky head gasket). Block deck looked good when I cleaned it up but the heads had cracks in the exhaust valve seat area of the center cylinders of the V6. So I installed a set of reman heads, new exhaust etc. Still running OK 8 seasons later, so it's worth a look. Back then the total parts cost was about $2000 including a 2 piece exhaust conversion (that was 1200 all by itself, the rest was the heads, gaskets, new head bolts, etc).
Started with this:
4.3 starboard cyl head removal.jpg
 
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