Compression Test values - one is lower than the others

[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). The 7 others are 175 (and one is 180). Cylinder 6 is the 140.

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…

 

[makomark]

Not much detail to go on...If its a GM engine, it passes there compression test guidelines...without doing further tests, one can only speculate on the cause for the reduced compression. If you are gonna "rebuild the engine" then there's no need to "put in a new crate motor"....

My suggestion is either trust the mechanic or cut him loose...second guessing him is only gonna cause problems down the road...

 

[jimn]

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). The 7 others are 175 (and one is 180). Cylinder 6 is the 140.

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…

I found a video with this boat model- it had an Indmar Raptor engine. Is yours the 350HP version?

Did you or the seller do a full marine survey? If not, ask if he'd be willing to split the cost- for him, it provides definitive info in case you back out and for you, it gives you more info about the condition and this should include connecting diagnostic equipment to the ECM so they can check for overheats. When I was trained by Mastercraft for Indmar injected engines, the ECM stored overheat info for 300 Key On cycles and loss of compression can come from overheating, so you'll also want to squeeze the exhaust hoses to find out if they're firm, or soft. They shouldn't be soft. If they are, look into the exhaust from the transom and check for scorching of the rubber.

The compression test- was the engine hot, or cold? Was this done wet, or dry? Should be done dry- remove the fuel pump relay to do this.

A small diameter scope camera is about $35 and it connects to a computer or smart phone- use one to look into the cylinders, to see if they're scored. If the one with low compression is scored, find out why.

Look for any obvious signs of trouble- if you see a new freeze plug (casting plug), ask WHEN it froze, not if. If you see a new head gasket, ask WHEN it overheated, not if. I'm mentioning this even though you posted that it was rebuilt in 2016- if the cylinders were scored and not honed, this could be an indication. Who rebuilt it and what was the extent of the rebuild? The owner should have this info.