You back off the throttle coming into the dock and the engine of your GP1800 or GP1800R quits. It restarts, idles for a while, then does it again. This applies to the 2017 and newer GP1800 and GP1800R, HO and SVHO, which have the 1.8 litre four-cylinder with an electronic throttle. The 1.9 litre GP HO from 2024 is a larger engine, but the checking order below works for it as well.
I found no single cause that owners agree on for this symptom, so what follows is a check order and not a diagnosis.
What I would check first
- Read the fault memory before touching anything. Write down every entry, including ones that look unrelated.
- Look at the injector connectors and the coil connectors. A plug that is slightly loose can drop one cylinder out for a moment, and a throttle that snaps shut is the kind of event that would show it.
- Inspect the intake hose between the air filter box and the throttle body, and its clamps. Air getting in after the throttle upsets the idle.
- Start the engine with a hose connected for water supply, or on the water, and watch the live values as it idles.
- Test each coil and each injector individually, then the fuel pump.
- Only then relearn the idle.
What the numbers tell you
The GP1800R has four cylinders, so the component test covers four coils, four injectors and the pump, nine in all. A coil or injector that fails its test alone is a clean answer. If all nine pass and the engine still stalls, the cause is probably outside the component list.
The live values to watch are engine speed, throttle opening, intake pressure and battery voltage. The throttle sensors come in pairs (TPS1 and TPS2, APS1 and APS2), and both signals in a pair should move together. If one of them jumps when you release the lever, think contact problem first. Intake pressure that won't settle to a steady value at idle is worth a second look at the intake hose, and codes 29 or 63 would back that suspicion. Code numbers can differ between control unit generations and sources, so trust the description in the app.
The fuel pump test only shows that the pump runs. It says nothing about how much it delivers, so a weak pump can pass. When you work on anything in the fuel system, do it with good ventilation and no ignition sources nearby.
Idle relearn and what it won't fix
The app can teach the idle and the throttle valve. That is meant for situations such as after cleaning the throttle body or after the battery was disconnected, when the stored values no longer match. It doesn't compensate for a loose plug or an air leak, so doing it last keeps the earlier results clean: if you relearn first and the stall disappears for a day, you don't know which step helped.
The M-JET Diagnostic supports the GP1800 and GP1800R in HO and SVHO form as well as the 1.9 litre GP HO, and it runs the individual coil and injector tests and the relearn routines from the phone. The throttle valve routine is described on the teaching the throttle valve page, and the model specifics are on the GP1800R diagnostics page.
The app can't see compression loss or the condition of the spark plugs. If every test is clean and the engine still stalls at idle, the investigation moves to mechanical checks.
