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2006 Yam 250 HPDI
Guys I have a problem I know one of you can help with. Either one of you have had it or seen it. Start my boat in the morning motor jumps on plane and I will run about 65 mph with 5500 rpm. As the motor runs I continue lose Top end speed and RPM. So bad that by the mid morning and lunch I can barely get on plane.
i have done the following
1. Change out high pressure filters
2. Ran computer on it. Computer shows perfect fuel pressure even while the rpm are dropping
3. Ran with an external tank
4. Change spark ugs
5. Compression test is perfect on all cyclinders.
6. Tested all spark plug wires and they are all firing.
7. Replaced a fuel with a fresh tank of gas
8. Had the injectors tested and they all came back okay.
9. Change fuel filters
Just don’t know why a motor would just slowly lose power and not throw any error codes or messages
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I had a buddy with a sim situation … he ended up finding the problem was exhaust was leaking under the cowling and choking out the motor after running for a while. He took it to several shops and they could not figure it out. Not sure how he found it, but I guess the sealant used around the exhaust port wore out or got pushed in somehow and allowed the exhaust to fill the cowling. He cleans and used some sort of sealant in the problem area and problem fixed.. been running fine for couple years now. He does have a Mercury, so not sure if something like that can happen to a HPDI...
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Don’t think that is it. Ran the motor with the cowling off and still had the same results
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Had very similar problem. Replaced the vst pump and all is ok now.
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Was it showing up on computer as the problem or was the computer not showing a problem?
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I know you said all plugs were firing but I would check the resistance of the sparkplug boots. They are spec'ed for something like 5K-7K ohms. They simply unscrew from the wires. Remove each one and check the resistance across them. It only takes a few minutes. Resistance can change as temperature changes.
Although the high pressure fuel system checks OK, I would check the water separator filter top cap which can leak. Pump up your fuel bulb to pressurize the fuel system. Check the top cap for leaks. Under pressure, a leak will show a little fuel coming out. But when the engine is running, the water separator filter is under vacuum and a leak will introduce air into the system.
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I didnt hook the computer on. Just kinda threw parts and got lucky lol.