04 AR230 Yamaha Jetboat Compression Issues

[BubbaMechanic]

Hello everyone...
I have a 2004 AR230 Jet Boat with one of the engines that has compression issues. It all started with engine running rough then eventually found fuel in the oil and fuel / oil mix in the intake box. And you all know that is probably from a bad ECU or stuck injector. I pulled and cleaned and checked all the injectors and they seem to be working fine. Long story short, because i was concerned about engine damage and thin oil, I pulled the motor out and did a compression test on it. The first two cylinders came up to 140psi and the last two came up to 180psi. My questions are:
1 - What should the compression be in these engines?
2 - What does that difference in compression indicate? Blown head gasket? Dirty or leaking valves? or piston rings not sealing / ie damaged cylinder walls from thinned oil?
3 - Any ideas on what I should look for / do next?

I already did an oil change (there was no existing oil I could suck out of the tank at the time - could have been because I didn't have the boat level at the time) and threw the oil filter away so I can't really go back and see if there were shavings in the oil filter. Now that I have the engine out I could find the crankcase drain plug and drain the oil there, or I could drop the pan entirely.

I have heard a ton of negativity about a SBT core replacement combination crankcase / valve head assembly - if that is where i am headed due to the compression issues. So wondering what the thought is there.

Learning a lot...so bear with me and any comments, answers, or advice on the above questions are appreciated.

Thanks!

 

[Bottom Bracket]

Roll it around a couple of times,
Leak down check will answer your questions.

How to do a leakdown test | Mobil™ Motor Oils

Before you tear down the engine, pinpoint where problems are by using these three steps to do an engine leakdown test.

www.mobil.com

"Hearing problems
Beyond getting an overall picture of engine condition, the engine leakdown test is an excellent way to pinpoint where problems are before tearing down the engine. Listening for where the air is escaping by ear can isolate the problem

• Intake valve : Air whistling out of the intake, carburetor or throttle body indicates a leak at the intake valve.
• Exhaust valve : Air heard hissing out of the tailpipe, turbocharger or exhaust manifold means an exhaust valve leak.
• Piston rings : Whistling or hissing out of the PCV valve, oil filler cap hole or dipstick tube means the air is pushing past the rings. Suspect ring or cylinder wall wear.
• Head gasket : Air bubbles in engine coolant seen at the radiator filler cap could mean air escaping into the coolant past the head gasket.
• Cracked cylinder head : Bubbles in coolant or coolant being pushed up out of the radiator neck can also indicate cracks in the cylinder head or cylinder walls."

 

[tijo]

Hello everyone...
I have a 2004 AR230 Jet Boat with one of the engines that has compression issues. It all started with engine running rough then eventually found fuel in the oil and fuel / oil mix in the intake box. And you all know that is probably from a bad ECU or stuck injector. I pulled and cleaned and checked all the injectors and they seem to be working fine. Long story short, because i was concerned about engine damage and thin oil, I pulled the motor out and did a compression test on it. The first two cylinders came up to 140psi and the last two came up to 180psi. My questions are:
1 - What should the compression be in these engines?
2 - What does that difference in compression indicate? Blown head gasket? Dirty or leaking valves? or piston rings not sealing / ie damaged cylinder walls from thinned oil?
3 - Any ideas on what I should look for / do next?

I already did an oil change (there was no existing oil I could suck out of the tank at the time - could have been because I didn't have the boat level at the time) and threw the oil filter away so I can't really go back and see if there were shavings in the oil filter. Now that I have the engine out I could find the crankcase drain plug and drain the oil there, or I could drop the pan entirely.

I have heard a ton of negativity about a SBT core replacement combination crankcase / valve head assembly - if that is where i am headed due to the compression issues. So wondering what the thought is there.

Learning a lot...so bear with me and any comments, answers, or advice on the above questions are appreciated.

Thanks!

do you find your problem i have same problem

 

[BubbaMechanic]

do you find your problem i have same problem

Hi, I did. It was a blown head gasket and scored cylinders. So I ended up putting in a remanufactured block / head assembly from SBC. Still putting it all back together. Lots of work and I'm replacing other parts on the way just to do it right.

 

[Brad_Ct]

Hi, I did. It was a blown head gasket and scored cylinders. So I ended up putting in a remanufactured block / head assembly from SBC. Still putting it all back together. Lots of work and I'm replacing other parts on the way just to do it right.

Do yourself a favor and install a mechanical oil pressure gauge while you have it apart. You will need a 1/8” npt tee that will go where the original oil pressure switch goes. I put one of their rebuilds in and kept getting oil pressure alarms, their tech. support said I needed to do this to verify that it was in fact low oil pressure. https://www.amazon.com/dp/B07FG1K7XZ?psc=1&ref=ppx_yo2ov_dt_b_product_details

 

[BubbaMechanic]

That's an awesome idea! Thank you very much for taking the time to write me that response!