Mercury 40 hp 3 cylinder 4 stroke EFI 2009

by Bill Zweig
(Walla Walla, WA, USA)

When I cruise at a fixed rpm, say 2600 rpm, for about an hour the motor starts to gradually slow down then surges and will keep doing that until I move the throttle even just a little bit. Then it will go back to normal for another hour then do it again. At any time I can go to full throttle and its normal all the way to 6000 rpm. I'm on the 3rd throttle position sensor, TPS. I've been fighting this problem for about 6 years. It runs normal except for this quirk. When I replace the TPS I can get 200 to 300 hours before the problem comes back again.

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