I just installed a rebuilt 951 block that was purchased from SBT into a 2002 GTX. The ski will not attempt to start at all. Has a little over 100psi on both cylinders, which is normal until it breaks in. Engine has spark, tested with spark plugs removed and grounded to the side of the block while cranking. It is getting fuel, tested by disconnecting fuel return line. I also sprayed starting fluid into the throat of the carbs and even directly into the cylinders and it still didn't attempt to start. Pulled the carbs. off and checked the reed valves and they look good as well. Hoping someone has had this issue before and can help. I have been rebuilding these for years and never seen one that has spark, fuel, and compression and not fire. Thanks in advance for the help.
Seadoo 951 Troubleshooting
- Thread starter DoubleD
- Start date
The gauge is good and was tested on a newly rebuilt 787 engine I just installed on another ski and showed just under 150 on both cylinders. I would think the engine would at least attempt to fire at 100. Tech support at SBT is telling me it is fine as well. I will follow up with SBT as well.
The rumors of low compression SBT engines are true.
It might fire out of the water at 100 psi but will probably not in the water with a load on it.
Something is wrong with that engine if it is only 100psi, doesn't matter what SBT says.
Remember, SBT isn't in business to do warranty repairs so they don't want it back if they can help it plus you are on the hook for shipping.
It might fire out of the water at 100 psi but will probably not in the water with a load on it.
Something is wrong with that engine if it is only 100psi, doesn't matter what SBT says.
Remember, SBT isn't in business to do warranty repairs so they don't want it back if they can help it plus you are on the hook for shipping.
JC-SUPERJET
Active Member
SBT lowers the Compression by machining the Cylinder Head Domes bigger/deeper so that the Engines last thru the Warranty Period at the expense of Performance.
For Combustion to occur, a Compression minimum must be met. I've read about 90 psi is the bare minimum.
Could swap in a known good Stock OEM Head, re-check compression to compare and try starting it.
May void the Warranty, but that is another option...
For Combustion to occur, a Compression minimum must be met. I've read about 90 psi is the bare minimum.
Could swap in a known good Stock OEM Head, re-check compression to compare and try starting it.
May void the Warranty, but that is another option...
You will void the warranty by doing that, they are really strict on that.
Not sure either.
If SBT is unwilling to warranty it because in some strange universe 100 psi is acceptable they you are stuck.
Like I said, at 100 psi there is a good chance it will never run.
You have spark, fuel and that only leaves compression.
Try a little premix instead of starting fluid.
So I received another block from SBT and got it installed and tried to start it last night with no luck. Compression on this block is around 130. It appears there is now an electrical issue that needs resolved. I believe I may have a bad MPEM and was wandering if anyone knows how to test them to verify they are bad. The engine will crank for a couple seconds and then stop for a split second and then start cranking again. The gauges all go blank like the battery was disconnected when the engine stops cranking, and then they come back on when it begins to crank again. The engine has spark and it looks good. I tried to disconnect the start/stop switch and jump the pins to see if the engine would crank that way and it did the same thing. All the fuses on the MPEM are good as well. Any thoughts or help would be appreciated?
When I jump the solenoid the starter turns over just fine. I will note that sometimes the starter cranks just fine when pushing the start/stop switch. When I got home today it cranked just fine and then backfired bad enough to blow the exhaust tube off in the back of the ski. I decided to test the stator and trigger when this happened. Stator was within, trigger ohm test was within spec but Vac test was a little out of spec. Manual says between the white and black/yellow wire of the harness adaptor it should be between 0.2 and 0.5 Vac. Mine was around 0.7. Not sure if that's enough to worry about or not, or if this might still be a computer issue.
artr
Well-Known Member
Since you stated the all the gauges shut off when it stops cranking and come back on when it resumes cranking I think it would be a worthwhile check of what appears to be the main ground and power connection for the MPEM on a schematic for a 2002 GTX: Connector 3 pin 24 and 26. It does not show exactly where the ground connection is made, my guess would be the engine block. I would check that ground connection is clean and tight. I would also open connector 3 to inspect all pins on both sides are clean and corrosion free, especially pin 24 and pin 26 . The 12V comes into the MPEM connector 3 from the rear electrical box so that connector should be checked also.
I checked the pins in the harness and everything looks good. Had 0.2 ohm on both pin 24 and 26 when testing it to the wire coming off the battery side of solenoid and also the block to the other pin. Is it safe to assume this is a computer issue, or the trigger barely being out of spec suspect? Any other ideas?
artr
Well-Known Member
Since replacing the MPEM is probably costly and time consuming (getting key reprogrammed if it is not supplied with MPEM) I would check the following before replacing it
Check the physical ground connection to the block by removing it and inspecting the mating surfaces. You measured a low resistance with a voltmeter which supplies a very small current to measure. If the connection is loose or corroded, the higher current to run the MPEM can be interrupted.
I would also put a voltmeter on the drive signal at the base of the start solenoid that comes from the MPEM to observe that the MPEM is dropping this signal when the starter gets interupted.
You could also disconnect the output of the regulator to see if a faulty regulator is causing strange behavior of the MPEM. The regulator should not affect anything during cranking but it should be easy enough to try and rule out.
Check the physical ground connection to the block by removing it and inspecting the mating surfaces. You measured a low resistance with a voltmeter which supplies a very small current to measure. If the connection is loose or corroded, the higher current to run the MPEM can be interrupted.
I would also put a voltmeter on the drive signal at the base of the start solenoid that comes from the MPEM to observe that the MPEM is dropping this signal when the starter gets interupted.
You could also disconnect the output of the regulator to see if a faulty regulator is causing strange behavior of the MPEM. The regulator should not affect anything during cranking but it should be easy enough to try and rule out.
