Seizing in first hour after top end rebuild - 2005 Sea Doo Sportster Le DI ROTAX 947, 2-stroke

[BoyzFunBoat]

My 2005 Sea Doo Sportster Le DI has about 175 hours. It's ROTAX 947, 2-stroke direct injection engine.

I'm the 3rd owner, it had 149 hours when I purchased it last year. I've always used Sea-Doo brand XP-S 2 Stroke Synthetic Oil. When I purchased it, I had a new impellor and wear ring installed and the top speed was 49 MPH (around 7,500 RPM). I was real happy about that! I should note that I ran this boat hard, often 95%-100% throttle for 10-15 minutes at a time, while pulling two kids on a tube. Gradually, week by week, the top RPM decreased and the top speed decreased to about 33 MPH. Also, towards the end of the season, it started cutting off when it was running at higher RPMs (while engine was hot), but on some outings, it wouldn't cut off at all. If I let it cool down for 3-5 minutes it would start right back up and usually run for 5-20 minutes. This cut-off, cool down, restart cycle was becoming more and more frequent, all the while the top speed and top RPM was decreasing. Eventually, it got to the point where after a cut off, the boat required much longer (5-15 minutes) to cool off in order to start.

I took it to a mechanic and the compression on one cylinder was only about 25 PSI. The other was about 135 PSI. He said I needed a new motor or at least a top end rebuild, so I went with the cheaper top end rebuild.

The first time in the water, I took it easy, gradually throttling up and down through the RPM range of 2,000 to 5,000 for the first 30 minutes. I got up on a plane a half dozen times and everything was running smoothly. The first attempt at a brief speed run, the motor cut-off at around 7,000 RPM. After letting everything cool for 2-3 minutes, I started up and did another 10-15 minutes of break-in. The second attempt at a speed run, the motor cut off again at 7,200 RPM. Again, I let the motor cool and returned to the landing keeping RPM below 5,000.

I immediately took the boat back to my mechanic (the very next day I picked it up) and he said first test would be a compression test. I had already lost compression on one cylinder, down to only 135 PSI. The other cylinder is still at 150 PSI. Mechanic says something is causing a cylinder to overheat and it's seizing, which is causing the motor to cut off. He removed the RAVE valve and did a visual inspection piston with the 135 PSI cylinder and said there were noticeable marks on the piston indicating a partial seizure.

He thinks it could be a leaking crankshaft gasket, but won't know until he spends a lot of labor hours to take the motor apart and inspect the crankshaft. He also said it could be something else, he just wasn't sure. His computer didn't show any errors and I never heard any alarms during the break-in. If I put in a rebuilt motor, and it fails under warranty, I'll still have to pay labor and freight, so I'm not doing that until I can determine the root cause of the issue.

I'm going to get a second opinion. Anyone have any suggestions on what's going on?

 

[IDoSeaDoo]

Unfortunately, you're experiencing what many owners of Seadoo's 947 DI motor underwent. It's likely a fueling issue. When the cylinder doesn't get enough fuel, it runs lean, which runs much hotter than normal. The aluminum piston expands inside the steel cylinder sleeve and starts to wear very quickly. If you research these issues, you'll come across people always saying "fix the problem that caused the initial failure". Your mechanic should have told you this as well. When it's cutting out, your piston is expanding so much, that it's actually slowing down the other piston, which is working full force to keep up the rpm.

You must make sure that you're getting proper fuel pressure while the engine is running. It's common for the fuel pump to fail on these and kill the top-end. Also, your problem could be a faulty fuel injector, but I don't know how common this is on these DIs.

It doesn't sound like a lubrication issue to me, as your other cylinder seems to be doing okay, and the oil system is shared between both.

 

[mikidymac]

Could be a number of things. First the break in should be 1/2 tank under 1/4 throttle with a 30 minute cool down after 15 minutes of running, 1/2 tank at 1/2 throttle, 1/2 tank at 3/4 throttle final 1/2 of the second tank you can go full throttle but never hold it there. You jumped the gun running it wide open.

1. Next you need to make sure your oil system is working correctly and using only full synthetic API-TC oil.

2. Your engine should have been checked for air leaks when the top end was replaced in my opinion.

3. I would never suggest a top end on any 951 with 175 hours, the crank is the weak link on these engines.

4. What pistons were used? THe OEM pistons on a DI are special as they were specially treated for the hot lean burn of a DI. Most of the aftermarket DI pistons do not have this treatment and have melted down prematurely.

5. Perfect compression of a DI is 135 psi not 150 and they should be dead even.

As IDoSeaDoo said I would be looking at the oil system for broken hoses, wrong oil and a lean condition from a possible injector or low fuel pressure.

 

[wrench2ride]

what cylinder is down on compression? I have seen more than a few go down on the mag piston and found it was because of a lack of lubrication, typcialy because of someone mixing different brands of oil, the oil ends up reacting with each other and gets really thick like jelly, remove the inline oil filter and cut it apart, if you find it is like jelly inside the filter you have found your cause of both failures. the filter ends up allowing a bit of oil through but not enough to lubricate properly.

if you mechanic suspects a leaking crankcase then he can do a leak down test to verify, it will take time to do it but not hours to do and it doesn't need to be torn down to do it, at any rate if you do find it to be a lubrication issue I would want to pull the engine down to inspect the crankshaft bearings and air pump

 

[BoyzFunBoat]

what cylinder is down on compression?

The front, PTO cylinder, but i was way off on my initial post. It wasn't 135 PSI, it was 65 PSI.

It's likely a fueling issue.

It was a fuel issue. I took it to another mechanic for a second opinion, and he said there was 5-6 gallons (Yes, GALLONS!) of water in the fuel tank. He said it was caused by a faulty check valve in the fuel vent.

He's recommending flushing the tank, cleaning it, re-fueling, pressurizing it along with a rebuilt engine from SBT. Does this all sound plausible and reasonable?

 

[mikidymac]

The front cylinder is the MAG, the rear is the PTO.

5 gallons of water in the fuel tank would be a huge issue and I am not even sure how it would run since the water would be on the bottom being pumped directly into the engine. IF you go with a new engine make sure you test fuel pressure and it is exactly 107 psi when running or you will have another toasted engine.

 

[FlyboyX]

yikes!