WOT RPM Too Low - major issue? cause?

[artg]

07 obv with 5.7 indmar assault 325.
prop is a 13.7x17.5 mvc hc oj prop which i understand is stock diameter and pitch so iiuc the low rpm is not a prop pitch issue and ‘correcting’ the low wot rpm issue by lowering prop would mask a potentially real and serious issue.
Top speed is about ~39 and top rpm is ~4510-4560 (alone or loaded).
spec for my engine is 4600-5200.
per engine manual wot should be on higher end, and others with same boat say they get top end rpm and closer to 45mph.
My concern is not with the speed or performance.

My question is if it’s symptomatic of a real issue, then i do want to track it down. it sounds serious per manual and others to be below minimum and so far from upper range of wot. my concern is the underlying cause - if the engine isn’t performing as it should due to other issues and consequently could cause (long term and or catastrophic) damage.

my speed matches gps (pp stargazer) so it’s not off by 5 mph. i also have tach and pp rpm so i think it takes gauge error out of the mix bc they both get the same signal and give me same rpm.

I revvd in neutral and it would cut out past high 2s/3s so it was like up and cut out, rev up and cut out. was weird.

Any inouts/suggestions?
i realize to track down everything - i could eother go diagnostic first or throw parts first. basically i would prolly start with parts replacement, i would clean flame arrestor, change out fuel filter (is ther just one or low and high pressure?), spark plugs, rotor and cap and wires, and for diagnostics i could check fuel pressure and compression test. before i go down the path of chasing that or paying a fortune to do it - is there any chance nothing is wrong or not serious enough to bother? and any other obvious/easy things to check or replace? really just looking to enjoy our boat every season and get it baselined in a good spot, don’t want to overthink it but getting concerned based on everything i’ve read on the topic since i’m off by hundreds of rpm seems like something is off maybe a single cylinder not firing or all not firing at full potential…


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[Holdmybeer]

So to sum up what you described....anytime you hit a RPM during a rev and it cuts out it is either computer lockout or lack of fuel.

ECM lockout isn't limited to that low of RPM since most GM based engines are good to 6000-ish.

Fuel pressure loss is my guess but figuring out why is the hard part. Fuel Filter, fuel pressure regulator (top of sending unit on Fuel tank), fuel pump are the first 3 things. Injectors could need cleaned or have a lazy 1 or 2 that is causing misfires. Again, this sounds like a fuel issue since under 3000 rpm the boat is running fine. If there is a port on the fuel rail, you can hook up a gauge and check pressure. You should be around 60psi.

[artg]

So to sum up what you described....anytime you hit a RPM during a rev and it cuts out it is either computer lockout or lack of fuel.

ECM lockout isn't limited to that low of RPM since most GM based engines are good to 6000-ish.

to be clear the cut out above 2-3k rpm was only when in neutral. When under load i have 0 issues other than wot below the minimum. So is there any reason fuel would cut out in neutral but jot under load? And then in general - why am i maxing at under 4600rpm when propped at correct diameter/pitch, where’s that few hundred rpm gone?…. and is below minimum wot rpm serious enough to care. the neutral cutout was a test i saw someone suggest to see if i could get to top rpm in neutral just jot under load and to my surprise, it cut out…. and would rev up cut out, rev up cutout…


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[Holdmybeer]

Rev up...cut out....in neutral could be ecm controlled. This would make since to protect the engine.

If everything is running good and no sputters or misses, check your throttle cable. You could have extra slack in the cable.

[artg]

Rev up...cut out....in neutral could be ecm controlled. This would make since to protect the engine.

If everything is running good and no sputters or misses, check your throttle cable. You could have extra slack in the cable.

i have efi so there’s a linkage but no cable right? i can see the linkages moving but don’t know how to confirm the linkage is moving as far as it can when throttle is as far forward as i can physically move it. it has adjustment turnbuckles… need to check service manual for specs unless there’s an easy/obvious trick i don’t know to confirm.


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[Holdmybeer]

The indmar manuals are online at the Indmar website.
You could always mark them in current position, then adjust to get more rpm.

If it messes anything up you have marks to put it back to where it was.

[brenpire]

Hi, I'm having a similar issue. let me know if i should create a separate thread or if i can tag on to this one.

When experiencing sputters or misses, where is the best place to start troubleshooting?

I have an indmar 325 assault with a sputter or miss above 3000rpm under load. It also seems to loose RPM after reaching WOT under load and when in neutral. It does not sputter in neutral, but does loose RPM when the the accelerator in the full position.

So far: changed the fuel filter, tried injector cleaner but the issue continues.

I added PP GPS a few year back, could that be sending mixed signals to the ECM?

[Holdmybeer]

Hi, I'm having a similar issue. let me know if i should create a separate thread or if i can tag on to this one.

When experiencing sputters or misses, where is the best place to start troubleshooting?

I have an indmar 325 assault with a sputter or miss above 3000rpm under load. It also seems to loose RPM after reaching WOT under load and when in neutral. It does not sputter in neutral, but does loose RPM when the the accelerator in the full position.

So far: changed the fuel filter, tried injector cleaner but the issue continues.

I added PP GPS a few year back, could that be sending mixed signals to the ECM?

OP has a different issue based on description. The boat is running great but won't reach max RPM by manual. I lastly suggested throttle cable linkage or TPS (throttle position sensor). Things could just be out of calibration and not allowing the last few hundred RPM's.

You are experiencing miss and firing issues above 3000rpm. I would suspect mis-fire under load from poor spark components. Easiest way to tell is to pull plugs and keep them organized so you know which 1 is from the removed cylinder. All should be a light to medium dark brown and might have a skight wet or oil look to them but not coated. White is bad (lean condition) and black is rich, oil blow-by, or bad gas causing soot. Also check the distributor cap and rotor. Moisture can cause cross-fires and soot will act as an insulator. Finally the coil. A weak coil won't charge or discharge fast enough will miss.

Check plug wires and harness wires for rub marks. Also while running, what is you oil pressure and engine temp reading. A faulty sensor will cause limp mode and shut the engine down to 3000rpm limit.

[HFarr]

OP has a different issue based on description. The boat is running great but won't reach max RPM by manual. I lastly suggested throttle cable linkage or TPS (throttle position sensor). Things could just be out of calibration and not allowing the last few hundred RPM's.

You are experiencing miss and firing issues above 3000rpm. I would suspect mis-fire under load from poor spark components. Easiest way to tell is to pull plugs and keep them organized so you know which 1 is from the removed cylinder. All should be a light to medium dark brown and might have a skight wet or oil look to them but not coated. White is bad (lean condition) and black is rich, oil blow-by, or bad gas causing soot. Also check the distributor cap and rotor. Moisture can cause cross-fires and soot will act as an insulator. Finally the coil. A weak coil won't charge or discharge fast enough will miss.

Check plug wires and harness wires for rub marks. Also while running, what is you oil pressure and engine temp reading. A faulty sensor will cause limp mode and shut the engine down to 3000rpm limit.

That just about covered it all!

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[VA LSV]

Hi, I'm having a similar issue. let me know if i should create a separate thread or if i can tag on to this one.

When experiencing sputters or misses, where is the best place to start troubleshooting?

I have an indmar 325 assault with a sputter or miss above 3000rpm under load. It also seems to loose RPM after reaching WOT under load and when in neutral. It does not sputter in neutral, but does loose RPM when the the accelerator in the full position.

So far: changed the fuel filter, tried injector cleaner but the issue continues.

I added PP GPS a few year back, could that be sending mixed signals to the ECM?

I had the same issue with my boat and a fresh set of spark plugs had it running like new.