1997 DT 115 Poor High RPM Performance

[zhogar00]

Hello everyone, any help or ideas on solving this issue is appreciated. I have a 1997 Suzuki DT115S (Fuel Injected) on a Tidecraft Wildfire 120. I haven't had hardly any trouble with this engine until recently.


The issue that I am having is that when giving the engine full throttle it will climb in RPM up to around 5000 rpm then after a couple seconds it will surge and lose RPM until around 4100 rpm. The engine starts and performs fine up until this higher RPM.

If I reattempt full throttle after it has already surged and dropped RPM it will dog down like it is starving for something.

• Issue is evident at all engine temperatures.
• The engine will run fine at 4100 rpm or lower.
• Engine starts and runs fine until WOT is attempted.

Parts changed trying to solve issue:

1. New Fuel pump diaphragm
2. New Fuel strainer (pre-diaphragm pump)
3. New fuel lines from tank to engine.
4. New inline fuel primer bulb
5. New inline fuel filter.
6. New fuel tank anti-siphon valve.

Things investigated trying to solve issue:

1. Took apart the VST to inspect needle valve assembly, no issues found except that the small cone filter is missing. I think someone has been in the VST before and rather than replacing it they removed it. I will be ordering a replacement. Overall the VST was very clean and I blew out all the ports to make sure no debris was in it.
2. Took apart the diaphragm pump, no issues were found. I checked the plastic check valves inside and they look to be fine (no chips or missing pieces). Diaphragm had no apparent issues but was replaced anyways.
3. Inspected all fuel lines looking for kinks, no issues found.
4. Electric fuel pump turns on fine, I undid the hose and the electric fuel pump when priming flows alot of fuel. (hard to quantify I know).

If anyone has any ideas on where else to look please let me know, thank you.

[Solarman]

I can suggest a couple possibilities. The gear counting coil could be breaking-down at the higher rpms. The throttle valve sensor could be acting up at WOT (breaking down above 4100 rpms). And the fuel pressure regulator could be failing at WOT?

All of these items (above) can be tested per the service manual. It is also possible that your plug/coil wires might be breaking down at higher rpms?

If someone removed the small cone filter, then replacing the fuel rail pressure regulator could be your answer? But I would test the other items listed first.

Also, if someone has been in the VST, it is possible they did NOT adjust the needle valve float properly. If set too low, the motor would act like it is running out of fuel at WOT.

Good luck, post back on what you find out.

[zhogar00]

Solarman,

Thank you for your response. I apologize for the delay been away for a while. I have ordered the cone filter and the service manual off Boats.net. Once I get these and read how to check the Gear Counting Coil, Throttle Valve Sensor, and the fuel pressure regulator. I will respond back with the findings.

Thanks again for your help.

[Solarman]

Sounds good. Let us know when able.

[zhogar00]

Solarman,

Just a quick update, I received the filter and the manual. I have checked the Counting Coil and the Throttle Valve Sensor. Found my pressure gauge is not functioning correctly, and need to replace in order to test the Fuel Pressure Regulator.

Counting Coil Testing resistance check:
Manual calls for 170-250 ohm resistance, I measured 197 on mine.

Throttle Valve sensor resistance check:
Manual calls for 0-5±1 kiloohm resistance while going from neutral to full throttle. I measured, neutral @1.388, WOT @4.330.

I am working on getting a new gauge so I can test the regulator.

Thank you!

[Solarman]

Try checking the TVS in ********g positions all the way to WOT and then backing back down to lowest idle position. There should not be any blank areas that don't read. Your's should go in steps/stages (3-5 different readings.). The manual should tell you the different readings, and different amount of throttle valve opening.

If all is good in these areas, then check out your high pressure pump by the book, it might be getting weak?

The plug wires and/or pressure regulator still could be the problem.

Good luck, post back if you find out anything.

[zhogar00]

Here is the summary of where I am at now.


Counting Coil Testing resistance check:

Manual calls for 170-250 ohm resistance. I measured 197 on mine.


Throttle Valve sensor resistance check:

Manual calls for 0-5±1 kilo ohm resistance while going from neutral to full throttle. I measured neutral @1.388, WOT @4.330.


Throttle Valve Voltage Chart Check:

I see that in order to test per the TVS voltage chart I will need to purchase the “4-pin connect test cord” . I can't reach the connectors with alligator clips.


Electric Fuel Pressure regulator Check:

Manual calls for regulator operating pressure of 34.1-38.4 psi. I measured 35 psi.


Electric Fuel Pump Pressure Check:

Manual calls for approx. 36.3 psi or more. I measured 37 psi.


Electric Fuel pump Residual Pressure Check:

Manual calls for 28.4 psi or more. I measured 32 psi.


Clear Fuel Hose Test:

Placed a clear fuel hose between the electric fuel pump outlet and the fuel rail. No air was present in the hose while idling.

Let me know your thoughts, thanks again for your help!

[Solarman]

Sorry for the delay, been busy.

It looks like you've covered most areas of concern. Those numbers look good. Did you ever do a compression test on each cylinder?

Might try a new set of plugs? Or an old set that you know were good before changing? I have had new plugs that failed right out of the box.

Should test ignition coils, both sides of each coil. See if to specs.

Then if the clear fuel hose is still on, take for a spin with hood off at WOT? Just to see if any air at WOT?

Also, might add a piece of clear fuel hose from low pressure pump to VST, to see if any air there at WOT?

Just a few more suggestions. If all these areas above check out good, then it might be time to inspect the reed valves?

Good luck, post back on what you find out.

[zhogar00]

Well hindsight is 20/20.

I should have included this information into the original post. Last fall the boat was running fine. Over the winter I had the lower unit reworked and while I was at the prop shop I got a new prop to help with planing when my wife and 2 dogs were on board.

The prop that I have always had on the boat is a Stainless Raker 13.5X22 (runs great when the boat is light, but not when heavy). The prop shop made the recommendation for an aluminum Wheeler 12.75X21. When spring came is when the issues arrived.

Today I took the boat out with the Wheeler prop on and the low RPM issue was there (as expected). I swapped the prop to the Raker and engine ran great to 5600rpm. I came back to the boat launch and swapped the Wheeler back on to confirm and the low RPM issue showed back up. It was the prop all along.

I am aware that the "blade geometry" between the Wheeler and Raker props are very different and I understand that blade geometry can play a large role in how a boat behaves. But I am struggling to grasp how the Wheelers less diameter and less pitch resulted in a larger "load" on the engine compared to the Raker.

I say hindsight is 20/20 because we all know that when troubleshooting we need to look back and ask ourselves "what changed?". I did not think the new prop could have caused this issues do to it being a less aggressive prop. But it definitely a variable that I should have investigated none the less.

[Solarman]

Yes, props and height of motor play an extremely important role in getting the performance of the motor.

Thanks for letting us know what you found.
Happy boating!

[Moonlighter]

To prop dia, pitch, and rake ……. Add blade surface area as another variable to how a prop performs. Can make a big difference too.