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FJR Motorcycle Forums
Technical & Mechanical Problems
Engine only runs with vacuum line disconnected
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<blockquote data-quote="torch" data-source="post: 1447619" data-attributes="member: 1295"><p>Throttle pressure sensor??? Intake air pressure sensor maybe? </p><p></p><p>Interesting difference in AP value between my 03 and his 04. Maybe Yamaha re-wrote some code so things are reported differently in 04. Did you D/C the sensor and see what happens to the value by any chance? </p><p></p><p>Let us assume that your sensors are working properly, based on that comparison. Odd that it's throwing those codes though. Any chance your buddy would let you put your ECU in his bike, just to verify that it's not the ECU itself causing havoc?</p><p></p><p>If in fact the sensors work reliably and the ECU is good and there is no intermittent faults in the wiring, then the logical conclusion is that the sensors are reporting symptoms of a mechanical fault. So back to the basics: fuel, spark, compression, timing.</p><p></p><p>An unlit propane torch can help diagnose fuel issues. If it runs better when inserted into the intake, that suggests things are lean. It can also be used to locate vacuum leaks by moving the torch tip around the boots and throttle bodies -- if there's a leak, it will suck in propane and the idle will change speed and/or quality.</p><p></p><p>The fact that your bike does draw vacuum when cranking over suggests there's no significant vacuum leaks and that there is at least some compression. But it wouldn't hurt to do a compression test just to verify. I would also have a look at those new plugs. It's possible that they got fouled during all the fiddling, and require replacement again. </p><p></p><p>It might be helpful to throw a set of carb sticks on to check actual vacuum during cranking/running. Low vacuum could be a symptom of a timing chain issue -- possibly jumped a tooth? Easy enough to check, but requires removing the cam cover and the right side crank cover to see if all the dots line up properly.</p></blockquote><p></p>
[QUOTE="torch, post: 1447619, member: 1295"] Throttle pressure sensor??? Intake air pressure sensor maybe? Interesting difference in AP value between my 03 and his 04. Maybe Yamaha re-wrote some code so things are reported differently in 04. Did you D/C the sensor and see what happens to the value by any chance? Let us assume that your sensors are working properly, based on that comparison. Odd that it's throwing those codes though. Any chance your buddy would let you put your ECU in his bike, just to verify that it's not the ECU itself causing havoc? If in fact the sensors work reliably and the ECU is good and there is no intermittent faults in the wiring, then the logical conclusion is that the sensors are reporting symptoms of a mechanical fault. So back to the basics: fuel, spark, compression, timing. An unlit propane torch can help diagnose fuel issues. If it runs better when inserted into the intake, that suggests things are lean. It can also be used to locate vacuum leaks by moving the torch tip around the boots and throttle bodies -- if there's a leak, it will suck in propane and the idle will change speed and/or quality. The fact that your bike does draw vacuum when cranking over suggests there's no significant vacuum leaks and that there is at least some compression. But it wouldn't hurt to do a compression test just to verify. I would also have a look at those new plugs. It's possible that they got fouled during all the fiddling, and require replacement again. It might be helpful to throw a set of carb sticks on to check actual vacuum during cranking/running. Low vacuum could be a symptom of a timing chain issue -- possibly jumped a tooth? Easy enough to check, but requires removing the cam cover and the right side crank cover to see if all the dots line up properly. [/QUOTE]
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FJR Motorcycle Forums
Technical & Mechanical Problems
Engine only runs with vacuum line disconnected
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