Fluctuating rpm at idle then shuts off.

2016 250 ProXS. 380 hrs

Had an issue show up this weekend after a run. Came off plane and engine started shaking & missing.
Shut it down, restarted and it fluctuates from 600-800 to +-1000 rpm back and forth until it shuts off.
I was able to power thru the idle problem area and she ran normal past the idle mark. Got back to ramp with no issue but as soon as I came off plane, same idle problem.

Let it sit overnight with battery off, started it up on hose next day, same issue.
I drained the VST and changed fuel filter. everything was fairly clean.
Pulled the plugs, everything looked normal other than some carbon and #1 was a little drier than the rest. Cleaned and checked gap, re-installed.
Pulled fuel rails (air and fuel injectors cleaned last year) Air injectors were carboned up, cleaned those.
Put back together and no change.

Boat is always run on 90 no ethanol and quickkleen mixture.

fuel pressure test is needed sir

90 octane is not needed, actually can be detrimental over a long period.

Need air and fuel pressures checked at 5/10/15 seconds after shutdown.

Sorry for the delay, got married over the weekend and been pretty busy.

Only had 100 psi gauge available, at right above 1000 rpm (to keep it running), fuel pressure pegs it, as well as air.
Fuel pressure drops pretty quick at the intervals after shutdown.
New 140 psi gauge should be here 5/15. I will recheck then.

I checked and replaced tracker valve just in case. (Still no change)

I run 90 octane due to it being the only option non ethanol around here. (Ran the old Racing 2.5's for years which has me avoiding ethanol at all costs)

drops how far

Quote:

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Originally Posted by Bullet3837

Sorry for the delay, got married over the weekend and been pretty busy.

Only had 100 psi gauge available, at right above 1000 rpm (to keep it running), fuel pressure pegs it, as well as air.
Fuel pressure drops pretty quick at the intervals after shutdown.
New 140 psi gauge should be here 5/15. I will recheck then.

I checked and replaced tracker valve just in case. (Still no change)

I run 90 octane due to it being the only option non ethanol around here. (Ran the old Racing 2.5's for years which has me avoiding ethanol at all costs)

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Air pressure should never peg a 100 PSI gauge

+1... get an accurate gauge, focus on correct AIR PRESSURE as the first priority (as fuel pressure is dependent on air pressure).

High Traffic, Top-Tier (National Name Brands) Fuel, in the correct Octane rating is the first thing to be looking for.

The LAST criteria is "lowest ethanol content available, never to exceed 10%".

Key on fuel pressure goes to 45 then drops to 20 when pump cuts off.
Running, while it’s idling bad, fuel pressure is fluctuating.
While throttle held at 1000 or above its normal at 103-105 or so.
Pressure drops 1lb per 5 second intervals.
Air pressure is normal around low 90’s when held at around 1000 rpm or above.

No fuel in air side.

Marathon fuel is always used.

Quote:

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Originally Posted by Bullet3837

Key on fuel pressure goes to 45 then drops to 20 when pump cuts off.
Running, while it’s idling bad, fuel pressure is fluctuating.
While throttle held at 1000 or above its normal at 103-105 or so.
Pressure drops 1lb per 5 second intervals.
Air pressure is normal around low 90’s when held at around 1000 rpm or above.

No fuel in air side.

Marathon fuel is always used.

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got to have exact pressure readings, 1% or better accuracy 105 is far from normall 109 is correct if air pressure is 94 as it should be

Fixed.
Took to mechanic, found a chipped reed in compressor.

:thumb up2: Replacing the reed plate in the compressor?