Engine Speed Drops When Trim is Operated

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
bobct1
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Engine Speed Drops When Trim is Operated

Postby bobct1 » Wed Jul 01, 2020 11:29 pm

[On a 2011 Mercury 90-HP EFI FOURSTROKE fuel injected engine] the engine speed decreases when the engine trim is operated. I think this is a new problem.

Watch the video clips below [if you want to see and hear how the engine speed decreases when the TRIM is operated]. [In the presentations the engine speed drop will be heard to be] about 300-RPM.

https://www.youtube.com/watch?v=fkGOslY_jBo

https://www.youtube.com/watch?v=FY2qQ5k31X8

jimh
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Re: Engine Speed Drops When Trim is Operated

Postby jimh » Thu Jul 02, 2020 9:14 am

I have not watched your two recorded audio and visual presentations. I rely on your narrative to inform the nature of the problem.

Engine speed often decreases if an increase in load occurs but throttle setting is maintained constant. For the engine speed to decrease by 300-RPM when the power trim is operated suggests that there has been an increase in the load on the engine during the operation of the power trim.

The power trim is operated by a hydraulic pump. The hydraulic pump is operated by an electric motor. The power for the electric motor is supplied by the boat battery and the engine charging system.

A reasonable inference for the drop in engine speed is the engine load has increased due to the alternator trying to supply electrical power to run the electric motor that operates the trim pump.

To investigate this problem, monitor the battery voltage at the battery terminals while the trim pump is operated. If the battery voltage sags significantly underload, the battery may not be fully charged or the battery may have an internal resistance that is higher than normal.

However, generally the highest electrical load on the battery is the engine starter motor. If the battery can turn over the engine starter motor, then the battery should have enough capacity to run the motor on the hydraulic trim pump.

In any case, the problem is likely in the electrical system and could be external to the engine. Check the primary battery cable wiring between the battery terminals and the engine.

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Phil T
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Re: Engine Speed Drops When Trim is Operated

Postby Phil T » Thu Jul 02, 2020 12:23 pm

Conducting this test on the muffs is not an accurate method to evaluate if there is a problem.

When at idle, selecting the trim will result in an increased electrical load, it is not surprising the idle falls. In an automobile, when you select the AC, often the engine idle increases to account for the increased electrical draw to run the compressor.

Launch the boat, get up on plane, adjust the trim and see if there is a material concern. I would not spend any effort diagnosing until completion of the water test.

Report back.
Member since 2003
1992 Outrage 17, 1992 Evinrude 115

biggiefl
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Re: Engine Speed Drops When Trim is Operated

Postby biggiefl » Thu Jul 02, 2020 2:13 pm

Ditto on what Phil said.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall. :roll:

jimh
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Re: Engine Speed Drops When Trim is Operated

Postby jimh » Fri Jul 03, 2020 11:18 am

NICK, PHIL—very good observation re the idle speed and no load on engine. Under those conditions adding any electrical load will be a significant added load on the freely running engine.