When a Mercury EFI outboard or SportJet begins to stumble at idle, hesitate under acceleration, or lose top-end power, one of the first components to check is the Throttle Position Sensor (TPS).
 
This small sensor sends a precise voltage signal to the ECU that tells it exactly how far the throttle plate is open. If that signal is even slightly out of specification, the ECU will miscalculate fuel delivery, leading to rough running, sluggish throttle response, or wasted fuel.
 
The Buckshot Racing #77 Mercury EFI TPS Test & Adjust Harness, replacing the Mercury OEM Part Number 91-816085,  is purpose-built to diagnose and correct TPS settings accurately and repeatably.
 
By plugging directly into the EFI control system and TPS connector, the harness allows you to connect a standard digital multimeter and watch the TPS voltage in real time. This live feedback lets you adjust the sensor back to precise factory specifications, restoring smooth idle, crisp acceleration, and optimal fuel economy.
 
This tool is a direct replacement for the Mercury Marine SportJet Quicksilver Test Harness, OEM part numbers 816085 and 91-816085. The factory version carries a manufacturer’s suggested retail price (MSRP) of $88.92, the Buckshot Racing #77 delivers equal performance with the bonus of clear, detailed step-by-step instructions.
 
Compatibility is broad, covering all Mercury Marine V6 2-stroke EFI outboards and SportJets equipped with front-mounted throttle body TPS connectors. This includes the popular 2.5-liter EFI series—such as the 135 HP, 150 HP, 175 HP, XRI, Mariner Super Magnum, Pro Max, 150 HP, 200 HP, and 225 HP EFI models—as well as the larger 3.0-liter EFI series like the 225 EFI, 250 EFI, 300 Pro Max, and 300X models.
 
It is also suitable for SportJet applications, including the 105, 140, 210, and 240 models.
 
The only exceptions are Mercury EFI engines that use the horn-style intake systems (found on race 2.5-liter and 2.4 Liter Bridgeport models) and later DFI/Optimax and Verado platforms, which use different TPS connectors and integrated sensor systems.
 
Instructions
 
First, the TPS is disconnected from the EFI harness, and the Buckshot harness is inserted between the two.
 
With your multimeter set to read low DC voltage, the TAN/BLACK head temperature leads are disconnected from the port cylinder head, and the ignition key is turned to the “ON” position without starting the engine.
 
The TPS mounting screws are then loosened, the sensor rotated fully clockwise to the closed position, and the voltage measured.
 
For most models, the correct closed-throttle setting is between 0.240 and 0.260 volts. If the reading is outside that range, the TPS is rotated slightly until the voltage falls within spec.
 
The mounting screws are then tightened to 20 in-lb (2.0 N·m).
 
After the idle setting is correct, the throttle is slowly moved from idle to wide open while watching the voltage rise smoothly to a maximum of about 4.16 volts.
 
Any sudden jumps or drops indicate a worn TPS that may need replacement. Once the readings check out, the harness is removed, the TPS is reconnected directly to the EFI harness, and the TAN/BLACK temperature leads are restored.
 
Whether you’re maintaining a nimble bass boat powered by a 2.5L EFI or tuning a 3.0 Liter, this harness ensures your TPS and ECU are in perfect sync.
 
Contact Mike Hill at +1-714-697-1716 or email mike@buckshotracing77.com for technical support.