GT 40- no full power

[minasianj]

Hello,
My 1994 GT 40 (fuel injection/EFI) is not getting full power at WOT. At WOT we are getting 4000 RPM rather than the usual 4600-4800. Listed below is a timeline of the problems development.


1) Initially at WOT the boat would top out at 4000 RPM and intermittently surge to 4800 full power. I added octane booster and a fuel system cleaner with no impact. Some days the surging was more prevalent than others, and at time the boat would run at full power for significant periods of time.


2) I replaced 8 spark plugs (.045 gap) and discovered one spark plug had a significant stem crack with carbon residue around the stem. Some individuals had previously mentioned they smelled fuel vapor during operation so this made sense. During the water test with 8 new spark plugs I received full power 4800 RPM at WOT with no surging. We thought the problem was solved, and maybe it was.


3) After the water test with 8 new spark plugs described above the low-pressure fuel pump ceased operating when the ignition was turned on. I replaced the relay and the pump has kicked on every time sense, but since the relay replacement the boat is back to 4000 rpm at WOT except now there is NO surging to full power, it tops out at 4000 and stays there.


4) performed a spark plug test and every plug had spark while cranking.


Any suggestions or ideas are appreciated.
Thanks,
Jackson Minasian

[DanielC]

If you learn nothing else from this post, learn this.
You MUST know what the fuel pressure is when you are having problems. There is no feedback to the computer on the fuel air ratio, and the computer just assumes the fuel pressure is correct.

In your other post there is a link to how to check your fuel pressure.
Do that. Help us, help you.

[minasianj]

Sorry for the delay. I ordered a fuel pressure tester and it only arrived this week. The fuel pressure at idle is 35 PSI and during a water test at WOT (4000 RPM) it was a constant 40 PSI. The the LPFP prime was 40 PSI.


However, at the dock in neutral at 4000 rpm it was 34 PSI and at WOT (to the neutral throttle stop) at the dock the engine reached 4800 RPM and FP was at 34 PSI and the RPM"s reached 4600.


Are there any obvious red flags?
Thanks for the help,
Jackson

[DanielC]

Those are pretty normal fuel pressures.

How does the engine sound? Does it sound smooth, or does it have an uneven sound?

As much as I hate just replacing parts to see what happens, It might help if you replace the plug wires, the cap and the rotor. There are ways to actually put a diagnostic scope on the engine, but paying a mechanic to do that test is probably more than just buying the new parts I listed

[minasianj]

I understand. The engine sounds great and I suspect the plug wires/rotor/coil are factory original so replacing them wouldn't be a waste.
Thanks for the help and I will report how things play out.
Jackson

[minasianj]

After replacing the cap, rotor, and plug wires I am still not getting full power at WOT. I suppose it is time to try a diagnostic scope.

[Rick]

There is a good chance that your low pressure fuel pump may be failing. That happened to me as it ran fine then the next time I went to start it it wouldn't start. The breaker was popped. The mechanic said it's pretty common to see a fuel pump fail intermittently at first then al together.

[slob02]

Long shot here.......had a similar problem with 97. I never would have guessed this one in a million years.
Long story short. I had a crack in my muffler and when i was wot, there was enough steam/mist going into the air intake that it was choking it out a little. I just happened to run the boat with the box up. I couldn't believe it. Worth a look.

[GCSuper]

I also had a similiar problem with a 96 model ski supreme. Turned out to be the knock sensor. It thought it was detecting detonation, so every time we would try full or close to full throttle, the knock sensor would retard the timing. We finally just unplugged it and ran it that way for several more years.

[minasianj]

Thats interesting about the muffler leak. I never would have thought of that. However I recently ran at WOT with the engine cover up so I don't think thats the case. Will a diagnostic detect a faulty knock sensor or do I have to unplug the knock sensor on the lake and give it a try? I ask because we are done for the year (Lake Tahoe) but if the diagnostic will detect it then maybe I could try it in the garage.

[minasianj]

Since the fall a diagnostic was performed and no codes came up. I also used a stethoscope to listen to each injector and they all sound as if they are operating normally. The anti-knock sensor that GCSuper suggests is a possibility but I won't be able to try it until the boat is in the water next month. Any other suggestions? Anything is appreciated,
Jackson

[CJ]

I'm sure you've already checked/replaced, but didn't see it mentioned in the thread - did you replace your fuel filter? We experienced some surging several years ago, but in the 3500 RPM range, and a new fuel filter did the trick..

[minasianj]

Yes the fuel filter has been replaced and unfortunately it did not solve the problem.

[Mamigacz]

Somebody else, will have to jump in on some of my questions.

What type of fuel injection is this? Throttlebody injection or tuned port injection?
Is timing advanced mechanically (in the distributor) or through the ECM?

[jedgell]

I had a similar issue a few years ago. Ran fine at idle and slow speeds. It wouldn't maintain 34mph or faster, but didn't run rough. Fuel pressure at idle was fine. Replaced the HP fuel pump and it's been perfect ever since. You want to be sure that's the problem though, it's expensive, if I remember correctly about $350.