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2008 SN EX343 Spontaneous Engine Revving


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I am out of ideas on this problem and would appreciate some advice! The symptom is that when putting the boat in gear (forward or reverse) the engine will intermittently rev UP all by itself and settle down as well... enough to knock someone off the platform! It seems to have gotten worse as now when it revs up the driver needs to pull the throttle back to stop it. Fuel being used is the same I have used for a long time and from the same gas station. Other than this issue the engine/boat runs fine. I haven't been able to correlate to engine or ambient temperature, whether it is the first or second set, etc..


Here is what I have done to no avail:

- replaced both fuel filters (drained FCC)

- Set timing to 8 (latest spec as far as I can tell)

- checked fuel pressure - rock solid at 60psi

- Checked for DTC codes - none

- replaced distributor cap and rotor

- replaced the potentiometer/throttle position sender

- replaced the throttle cable

- verified the problem is there even if ZO is off

- tested with several settings of throttle cable to potentiometer "home" position


Anyone have any advice on how I can correct this problem????





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  • Baller

Have you checked the Idle Air Control? It controls idle speed and can cause the issue you're having.


Both the position sensors for measuring throttle lever position and the position of the butterfly in the throttle body have redundancy. It's very unlikely for either of those to cause the problem you're having and they will both set DTCs if anything is out or range or the redundant sensors readings don't match.

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  • Baller

I may be confusing the pre-drive by wire throttle body and the post-drive by wire throttle body. I've worked on both. On the pre-drive by wire throttle body there is a separate IAC so the computer can electronically adjust idle and the main butterfly is controlled by the mechanical throttle cable.


On the Post-drive by wire throttle body the main butterfly is controlled by a motor and is therefore likely used to control idle speed. I'll have to take a peek at the EX343 in my boat later this week.


In either case, what I said about the position sensors both on the throttle body and the one connected to the throttle handle is correct. They both have dual potentiometers and set DTCs if the two reading don't match or are out of range. The sensor for the throttle handle position also has an idle validation switch that indicated the throttle lever is in the idle position. A DTC is also set if the IVS reading is inconsistent with the other 2 potentiometer readings.


Your problem may be a problem with the motor on the throttle body, but it could also be other EFI sensors so I would start testing stuff instead of replacing parts since some of these parts are pretty expensive.



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This is a post-drive by wire so there isn't an IAC. I wish I would get a few DTCs! Not having any is making this hard. I appreciate the help because at this point I am baffled.


I agree with testing parts as you say but except for the throttle potentiometer which was really sloppy and suspicious, all the rest of the stuff was needed maintenance anyway.


However, I am having some new symptoms that are starting to worry me about this being the ECM. Today, I had a couple of starts where the engine would rev up an extra 500 or so RPMs and back to normal idle and repeat this several times 3-4 seconds apart before settling down to a smooth idle. Two other times I would have it at, say, 2500 RPMs and slow down to take it out of gear but the engine would just keep on rev'ing. I had to shut it off to make it stop. When started again it was normal.


I may have to break down and take it in to get a Diacom connected to check this out in more detail.

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