Compression test results - one cylinder low

[Quocto]

Hi guys—in the process of buying a new boat. It’s a 2006 MB Sports B52 team edition. Agreed price: $28k. I think it’s a steal.

then my mechanic did a compression test and one cylinder is much lower than the others (but still at 140 psi, which is ok by itself). He told me it being 20% lower than the others is a red flag. I’m reading some reviews that yes, that indicates a problem. Others have said it doesn’t necessarily mean a problem.

some more info: he hasn’t used it all season due to injury. Took it out on a test drive week before test and it ran smooth, no issues, started up immediately, didn’t run hot. Engine was rebuilt in 2016 at 140 hours due to overheating. Not sure what exactly was done. Will be getting more info.

My mech said he would pass on it bc this reading suggest we’d need to rebuild the engine and put in a new crate motor, which is $4500 plus labor. I’ve read that these issues can be fixed with a “short block” replacement? Or some have suggested fixing the head gasket? Or pistons? Mech said these repairs require taking out the engine completely, which is a lot of work.

Anyways, hoping you guys have some advice or ideas. Any thoughts? Would you pass or negotiate a repair that isn’t the whole crate motor? New to boating. Only at it for 3 years…

Thanks!

[SilentSeven]

It's concerning. Did your mechanic do any additional diagnosis as to the cause of the low compression in cylinder 6?

Broadly, it can one of three issues - problem with cylinder wall/rings; valve sealing issue, head gasket issue. Rings are the most serious and require block work - big $. Headgasket or valve seal problem requires head removal which typically can be done without removing the block. This can be DIY or moderate $.

There are a few secondary tests that can help ID the problem. To check for ring/bore issues, put a tablespoon or so of engine oil in the bore vie the plug hole and re-run the compression test. If the compression rises, it's bore related. If it stays unchanged, it's gasket/valve train. A leakdown test can also be helpful to check cylinder health and can help indicate where the compression leak is occurring.

If the data suggests it's head related (valve or headgasket), I'd consider the boat. If it's ring/bore related, I'd likely pass.

[SilentSeven]

Also...just a few terminology items:

- A long block replacement is where the engine is removed and a complete new engine is installed. This would be the crate motor.

- A short block replacement is where the engine is removed and just the bottom half of the engine is replaced. For many motors you can order a rebuilt short block or have your current block rebuilt. This is done when there is cylinder, piston or crankshaft damage. The existing cylinder heads are re-used. The existing cylinder heads may be refurbished during this process.

- A head gasket replacement is when the sealing gasket between the block and the cylinder head is replaced. This is done when block and head are OK and the leak is due to gasket failure. The reason why the head gasket failed may be the result of other problems. For example, you mention this boat has a history of heating. An overheated engine can warp block or head components leading to head gasket failure.

[Quocto]

Thanks so much. This was very helpful. My mechanic seemed to suggest it could only be fixed with the long block replacement even though he said he wasn’t sure specifically what it could be so I’ll ask him about what you’ve mentioned. Thank you!

[SilentSeven]

Glad it was helpful.

Mechanics will bias towards a complete replacement (ie / long block-crate motor) as it reduces their risk. Attempting to remediate the problem with just fixing one part is always risky unless there is a perfect diagnosis. For problems like this, you may not get a complete diagnosis until you actually tear down the engine. I think if the data indicates it's head gasket or a leaking valve (sometimes referred to a 'burned' valve), you have to have a candid conversation with your guy about the risk that there may be additional damage and who owns that risk (it will be you most times).

This is why these decisions are tricky and your mechanic recommended a new engine. If you just want a clear assurance you'll not have cost overages or ongoing repairs, then a new motor is the best route. If you want to attempt a lower cost repair and it's not cylinder damage, then look at the other options.

There's a third choice - if the engine runs fine and the # 6 spark plug shows no sign of oiling/fouling (another item to ask your mechanic)...just run it and see what happens. Buy the boat at a negotiated discount, put some cash in reserve and have the compression checked every hundreds hours or so.

Good luck with the decision.....