Motor sputters at ~30mph

[jackmanjls]

I have a F90TLR, 2006.

==== > Problem
At 25mph the motor runs flawlessly at 30mph it starts to periodically sputter.

====> Here's what I've done to gather data on the issue.

I take the boat out and run at 25mph (~4krpm) for about 20-30 mins...it runs flawlessly.

I then throttle up to 30mph (~4.6krpm) and it runs fine for about 2-3 mins before the rpm drops by about 500 or so.

I throttle back and idle for a min or so. I then throttle up to 25mph and it runs flawlessly. If I take back to 30mph it repeats the same as described above, ie, sputters.

====> My thoughts
It seems that there's a gas restriction or gas issue(s) in general when >25mph.

At 25mph there's enough gas supplied that the issue doesn't occur (could be right on the failure threshold) but at 30 there's a gas deficiency that causes the issue,
throttling back allows the gas to replenish itself sufficiently to avoid the issue.

I've looked at the gas from the water separator and VST and there's no indication of water...so I rule that out.

Plugs are brand new, <5hrs of operation. I checked coil and spark plug wires according the the SM...they were in spec.

I had the fuel injectors cleaned. There at 10yrs old, could be they are the original FI's. I watched when they were being cleaned and all performed identically.

Could it be VST pressure on the fuel rail? Bad injector? Pickup screen in the fuel tank? One of the power-heads inline gas filters partially plugged?

====> Next step?

I'm looking for forum feedback for suggestions on the next step.

[rejesterd]

Spray some starting fluid around the intake manifold at idle. If the idle speed changes, I would replace the intake manifold gasket and the throttle body gasket. Then see where you're at.

If you don't find an air leak, make sure the throttle plate isn't bent, and make sure it opens smoothly when you open the throttle from the remote control.

If all that is good, I would empty the vapor separator and see if there's any gunk in there. If anything other than fresh looking fuel comes out, I would get the rebuild kit for the VST. There's a tiny filter in there that can get clogged and cause performance issues like that.

Also search your service manual for the troubleshooting section. The steps I'm giving you are right out of my F50's manual. But they may be different for yours.

[rejesterd]

Also I forgot to mention.. when you check for the air leak, raise the idle speed slightly to just under 1000 rpm. Then plug up the ISC valve with your thumb while you spray. Otherwise, the spray might go in through the ISC which invalidates the test. Also don't spray near the air intake silencer for the same reason.

[pstephens46]

Have you tried pumping the primer ball when this problem occurs? To rule out a low pressure pump failure? Low pressure pump fills the VST.

[fairdeal]

Could it be VST pressure on the fuel rail?

IMO most likely you are loosing rail pressure - although why is TBD.

Unfortunately on that model its not easy to rig a gauge to the rail to confirm
- Yamaha sells an adapter for that,
but personally I wouldn't spring for it just for one problem on one engine.

Assuming its fuel starvation,
either the HP filter is plugged
(the 3 1/2" long "tiny" filter mentioned in the post above)

or something is preventing the low pressure system from keeping the VST full.

That could be restricting filters (there's an inline screen between the lift pump and the VST, btw)
a weak LP pump
crimped / pinched hose somewhere
among others....

have you tried the old 'pumping the primer bulb' trick when it starts to dies on you?