7.4 GS (1998) DP Idle Fluctuation, Specific RPM range

[Bottom Mapping]

Engine in title, my 1st ever MPI powerplant. Love it overall, but now in my third season with it, there is an annoying ~300 RPM continuous fluctuation in neutral, beginning when I try to come off-idle and hold 1000 rpm; it cycles between 1000 and 1300. Another notable characteristic is that pulling throttle to idle from the fluctuation range almost always results in a shut down, but it returns to steady idle with a bump of the starter.

It idles like silk, steady at 600 rpm. Judicious gentle increases can sometimes get a steady idle around 800-850, but often the 1000/1300 cycle is what I achieve, using two hands to carefully meter the throttle advance; increase more, and it settles down steady at 1600 or so, and incrementally up from there, no more fluctuation. Conditions are on the trailer in neutral, 3000' above sea level, about 80 degrees fahrenheit. I first noticed the fluctuation in the water at sea level, temps in the 70s. Identical symptoms as described above, but not evident in gear, although I can't say with certainty that I paused the throttle at the spot that might have induced the fluctuation to occur, under a light load. Although no troubleshooting has been pursued yet, the following actions were performed as part of usage prep and normal maintenance: fuel/water separator changed, MAF cleaned with carb spray, plenty of fresh fuel with the latest tank running Stabil storage blend. Flame arrestor cleaned and spark plugs change.

The boat runs well in all other aspects, including at WOT, pulling skiers and cruising at about 3200 rpm. At one point we had 13 people on board (rated max capacity) and she popped right up on plane with maybe 3/4 throttle, literally stunning friends who'd never experienced DP outdrives. Fuel consumption is normal. It starts with a simple turn of the key, hot or cold.

All-in-all, I don't have much to complain about. Days on the water are not impacted, but I'd like to resolve or understand what's going on, in case it's symptomatic of "trouble brewing." Any thoughts?

 

[tpenfield]

2 thoughts . . .

1) You really need the computer diagnostics to troubleshoot a fuel injected engine.

2) If I had to guess, I'd be looking at the throttle position sensor.

 

[Bottom Mapping]

2 thoughts . . .

1) You really need the computer diagnostics to troubleshoot a fuel injected engine.

2) If I had to guess, I'd be looking at the throttle position sensor.

Thanks. I have a ways to go prior to jumping into marine OBD. Any suggestions for a stand-alone tool or something IOS-friendly?

 

[alldodge]

What is the motor serial number?

 

[Bottom Mapping]

What is the motor serial number?

I'd never looked for the engine serial number before. There's no way I'm getting back to the block stamp, but the decal on the starboard exhaust manifold cover would be easy to read, if the factory had printed it in its entirety. It shows only the upper half, but I will take a stab at deciphering. Maybe: T01707501. If there's an error, one or both sevens could be twos. I'll attach pics showing what I can see, plus one of the ECM and its connector. Evidently, this is a OMC joint venture engine - it says Cobra in some places and OMC in others.