Mercury 25 HP 4 Stroke EFI Occasional Very High

PABayGuy

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Joined
Oct 4, 2021
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3
I have a 2006 Mercury 25 HP 4 Stroke EFI. The boat and motor sat for 7 years inside a garage after my uncles stroke. Of course, gas was in the lines the entire time. Before running it, I took it in for a complete servicing this summer. They replaced the high speed pump, plugs, and all other regular and consumable parts for a full tune. The mechanics stated it ran great in the tank and to take it and run it to see if anything else was out of whack.

Took it to the Potomac and ran it for about 4 hours. Hard and fast, slow, hole shots and the works. Ran great for about 2 hours. I was stopped to check the rest of the boat out for any issues. When I restarted the engine, it started at what is best described as a almost full throttle idle. The tiller throttle control was extremely hard, but tried to bump the throttle and of course it was like a moon shot. Ended up hitting the kill switch. Played around for a while starting and shutting it off until it sort of started at a slower idol, above normal, but still higher than normal.

it seemed to settle out and run correctly, as i had tiller control, and it settled to a low idle.... but mostly after I bumped it to go from a slow forward to idle.

A week later on another run, no issues at all.

A few weeks later on the Chesapeake, the issue flared up again. On the third day of running fresh gas with Stabil, after running across the bay at 2/3 power. We entered a channel and were casting the edges. Moving the boat at very slow ide or shutting down and then moving at a slow speed. Started it up and got the full throttle idle. Shutting down and restarting produced the same effect. Poor decision on my part.. but i tried the fast bump and ended up a boat length in the grass bed before shutting it down. % minutes later, after excavating the boat.... it started normally at that nice low idle.

We fished at low power in and out of gear for the next hour or so with no issues. Headed across the bay to the ramp. No issues. At the ramp dock, it resisted going to idle again and surged again.

This is making no sense. Obviously something fuel related, but what. I did call the service tech after the river incident. He had no idea.

The engine (tiller) also seems hard to get into idle from forward or reverse.

No lights or beeps / codes.

Thanks in advance.
 

vroom ZOOM

Chief Petty Officer
Joined
Aug 15, 2017
Messages
468
is it revving up only in neutral, or does it run the boat fast in gear as well? It will be hard, if not impossible to get out of gear if the rpms are above the rated idle rpms. Also, DO NOT put it in gear at anything above idle. You could instantly destroy the clutch dogs and gears in the lower unit. I am thinking there is a huge vacuum leak somewhere.
 

PABayGuy

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Oct 4, 2021
Messages
3
Rev's up in neutral. It will be at high speed at start-up. It might settle down if shut off. Usually take time between starts to return to a fine idle.

Someone suggested it was the Idle control sensor, but i am not seeing that part listed anywhere for this motor.

2006 OR149637

Thanks for the reply!
 

PABayGuy

Recruit
Joined
Oct 4, 2021
Messages
3
I will add - sometimes at slow speed, the motor will not respond to tiller control (to slow / idle). I have to give a quick twist of throttle and back to drop idle.

Same issue or another?
 

vroom ZOOM

Chief Petty Officer
Joined
Aug 15, 2017
Messages
468
Same issue. I had this exact same thing happen on my suzuki 200😲. Well, not as bad, but it wouldn't return to idle fully, would stay around 2000 rpm in gear, if I took it out of gear then it would smack the rev limiter no problem.

Two things could be going on:

1. throttle plate is hanging up. Easy to check. Move the throttle plate with your hand and see if it moves easily. Also, the throttle linkage could be not returning fully and causing the throttle to hang open.
2. TPS is messed up. TPS controls the timing, so if it is showing a more open throttle position, the engine gets more timing, and generally this makes it run faster and rougher. Test the TPS with a multimeter. Look it up on google.
In my case, I fixed the idle speed by adjusting the throttle linkage.
could be the IAC as well, not sure if your motor has one, ill check.
 
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