04 vmax 250 ox66 issues. Looking for advice

[NJfishinGuy]

My 2004 vmax ox66 250 is acting up. I did a tune up this spring. Plugs, Filters(including VST) and cleaned the o2. Injectors weee services 2 years ago along with new reeds.

A few times now the engine decided to just cut power when I’m running wot. Or almost wot. I’ll be gaining speed and then bam it just cuts and the boat glides to a almost stop. It’s not shutting off just goes to an idle basically regardless of throttle input. After about 5 seconds of being at almost idle it’ll decide to just open up and go again. Sometimes it’ll do it again once I get back up to speed and sometimes it won’t. It’s random. Other then this is runs perfect. If I keep it under 5k rpm it doesn’t seem to act up.

Any ideas.

[Chris85]

low pressure fuel pumps are a possible suspect

[javelin90]

Might be a long shot. Did you replace the fuel primer bulb? Maybe bad relay.

[javelin90]

Try using a portable tank and see if it happens. If it does but happen, start troubleshooting fuel line at motor up to the gas tank.

[Amistad Tackle]

Likely the fuel pump relay.

[scotty34]

What reeds did you put in ?

[OverTimeAgain]

Hey I know this!

Probably low pressure fuel pumps.

You can run around forever while below 4500rpms?

You can test them by gently loosening the two bolts that attach them to the block and squeezing your primer bulb, if gas leaks out on the engine side of your pumps you should replace them.


Likely the low pressure pumps. The diaphragms get tears in them and they won't be able to fill up your vst tank fast enough to deal with the increased fuel demand of higher rpms. At lower rpm ranges they will be able to keep the vst tank full enough for the high pressure pump to keep rail pressure.



They are around $50each off Simyamaha, and easy to replace yourself. You will want to replace all 3 and date them. I find myself replacing mine every few months, generally my first symptom is the engine having taking a few tries to get it started during the day after it's been running a while.

[201pro]

It may not be your problem, but you really can’t clean the O2 sensor.
Just better to replace it with the NTK21006 sensor.

[NJfishinGuy]

Hey I completely forgot to check back here like an idiot. Thanks for the responses. I have replaced the anti siphon and my fuel lines from the tank to the filter on the engine including a new primer bulb. Went out and it still acts up. I have a good feeling it’s the low pressure pumps which I’m going to attack next since they are probably way over due anyway to be rebuilt. I’ll check back in after that and let you guys know the results.

[CJ1]

Do yourself a favor and eliminate them. If you are even somewhat mechanically inclined. a holley Red pump, good inline filter and a 5 pin relay with a fuse and the headache is gone. The blockoff plates are very easy to make. CJ

[NJfishinGuy]

Interesting idea. First time I’ve heard of someone doing that. The parts are already ordered so I’m gonna roll with it but if it fixes it and I have the issue again down the line maybe I’ll go this route.

[NJfishinGuy]

So update I replaced my anti-siphon valve and lines and it did not help. My next step I decided to go for the low pressure fuel pumps. When I am both of them from the block and pump the primer my middle fuel pump leaks fuel out of the back. Discovered a small tiny tear in the diaphragm in that pump.

[javelin90]

With the low pressure pumps you do not need to pump the primer bulb. I never do and no issues running.

[Texas Larry]

He was pumping the bulb to check for leaks.