2016 DF 150 topping out at around 3600 rpms...

[supagruv]

Hello! We have a new-to-us 2016 Suzuki DF150 in great condition... but our first sea trial on our boat we noticed it just wouldn't go past 3600 rpms. Sometimes it would even be less, like 3100 rpms... but if I worked the throttle lever back and forth a few times it would then go up to 3500 give or take... I thought maybe it was the new prop (we bought the motor without a prop) which is a 14x21 4-blade prop... and I thought maybe the pitch is too aggressive... but after more research it seems like it's most likely a rev limiter (limp mode) issue.

I ordered the Suzuki troubleshooting software (SDS) but it's going to be a week until it gets here and I'm really hoping to figure the issue out sooner if possible. I have an identical parts motor from the same year that I've been pulling parts from to test things but so far haven't had any luck.

Since then I've done various things (as outlined below) while on muffs here in our carport.

More details:

• ~1800 hours on the motor
• All new Suzuki analog tachometer - no check engine lights flashing or remain on except the initial startup where all lights go on then off
• No beeps except the initial startup beep
• All wiring is new along with 2 new Interstate batteries
• Newer spark plugs (only ran a handful of times so far) and are still nice and clean
• New thermostat
• New water pump/impeller
• Boat has very minimal electronics in general (it's a work/fishing boat and only has lights/radio/bilge wired so far)
• Motor is in really good shape, clean, very little corrosion, etc
• Primer bulb is hard and fuel flow is solid (disconnected fuel lines in several places to ensure flow while using primer bulb)
• Also ran motor on external gas can with same results
• I pulled out one of the temperature sensors and it looked like new - no corrosion etc
• I swapped coils from the parts motor with no change
• I swapped the VST/fuel pump from the parts motor with no change

It's possible things could be resolved through my troubleshooting but we won't know until we put it in the water... but it seems to me to be still rev limiting. The video below shows the engine running and then me steadily increasing the throttle until it's at about 3600 rpms... at which time I'm holding the throttle level steady but the engine is SURGING big-time. Again, no check engine or oil/temp lights on at all.

I've read some posts where it sounds like the neutral safety switch could be the issue... but it clicks when I engage neutral. I do have an extra one from the parts motor I could try using... will report back.

I've read some other posts where someone replaced their cylinder temp sensor and the exhaust temp sensor and that fixed things. I've tested these with a multimeter but so far they seem to be outputting correct values (at a stop anyway). Without the SDS it feels like shooting in the dark though...

Any ideas would be greatly appreciated. Thanks!

[Moonlighter]

Ok well first up, the engines ecu will prevent the engine being revved past about 3000rpm in neutral, so what we saw in the video is perfectly normal and totally as expected. This is a protection feature.

So, if its doing the exact same thing when its on the water in gear and running, then there is a good chance the problem is the neutral throttle switch in the engine’s gear linkages. This switch sends a signal to the ecu that tells it if the engine is in gear. If that signal is not received then the ecu will limit revs to about 3000rpm, it does this by cutting spark, advance and fuel.

So when that happens the engine runs very much the same as a a race car does when the pit lane limiter is activated. Eg it runs BAD.

So the switch could have failed, or be loose or not adjusted correctly. If you had the SDS software you would see the engine being in neutral even when its put into gear.

https://www.boats.net/product/suzuki...ab911eff00bfc3

[Moonlighter]

Note: this switch is on the engine, starboard side, gear linkages.

Its NOT the switch on the throttle binnacle that prevents the engine being started in gear.

[supagruv]

Thanks Moonlighter, definitely appreciate the input. You're right, once it's in gear the limiter doesn't kick in. We're going to go put it in the water and see if some of my recent changes fixed things. Will report back soon!

[redlowrey]

Your engine has a code for the neutral switch, do the same test in gear you will know straight away if it has failed, take the prop off or be careful.

[supagruv]

Ok so that was definitely the rev limiter kicking in because we were in neutral. Once back out on the water it went up past 5500 rpms which was awesome... but after about 10 minutes of varying rpms it then was limited again to around 3k rpms without any beeping or dash lights on. Tell-tale is cool and engine is not running rough or missing. At this point it seems like some of the recent maintenance I did helped but most likely a sensor is not reading right once the motor gets warm. Anyway, I guess we'll know more once the SDS arrives. Thanks for the input everyone, much appreciated.

fiberform-rear1.jpg

[artdf175]

I would check your wiring very carefully... I experienced exactly the same symptoms..
measure the voltage on the white wire that feeds the power to your key switch
make sure you load test your battery as well
My engine was throwing all sorts of alarms when there was nothing wrong and the RPM's would go from over 5,000 to about 4,000
as you may know the ECM is sensitive to voltage and any fluctuations can create any number of false errors.

Art

[mlee76]

Your neutral throttle switch may be intermittent. Put it on a continuity tester and see if it reliably switches.

[supagruv]

Thanks for the input everyone. Finally got the SDS going yesterday and ran the error codes. Mostly what I thought... gradient overheat and IAT sensor failure (at least once because I tried to run it without that sensor plugged in). Most likely impeller/water pump/thermostat issue... even though it has a new impeller and I inspected the thermostat recently and it looked like new. Anyway, I'll be dropping the lower unit and testing the thermostat in hot water to see how it opens. I have a parts motor so might try swapping thermostat and IAC sensor.

Here's the screenshot of the error list in case you guys have any insight.

errorlist.jpg

[supagruv]

errorlist.jpg

[Moonlighter]

SDS will show the neutral throttle switch (and therefore gear) position as it is activated.

You should be able to quickly ascertain if its working or not with SDS while the engine is running and then put in gear.

[supagruv]

The neutral switch is definitely working. Verified that via SDS.

it does show two “failures” in the last two hours we used the boat in the ocean. It just says “Code of failure time 23” and the hours at 1848. Does this mean error code 2-3 which is IAT sensor failure? Or am I misunderstanding the error description?

thanks in advance!

[supagruv]

So I have an update... the SDS software won't work if the motor is running unfortunately (maybe a bad cable? the replacement is en route). In the meantime I did another full clean and inspection of the VST and fuel pump, all fuel lines, etc. Then we rented a fuel pressure test kit and hooked that up and took a sea trial to see how it would do under load. It went great for the longest time yet, about 5-10 minutes... then finally started bogging down and wouldn't go past 4k rpms. I looked at the pressure gauge and it was down to 20 psi. We took a break, swapped the ECU with one from our parts motor then tried again. Went another 5-10 minutes and same thing. Basically it drops down to 2k rpms and bogs down some more.

At this point we're thinking it's either the fuel pump or injectors. We sent the parts motor injectors out to get untrasonically cleaned and ordered a fuel pump. Will report back when we have more progress.

Thanks again for all that have chimed in with input.

[redlowrey]

It sounds like your low pressure fuel pump is not keeping supply up to the vst after a bit of time, did you start off with around 40 psi of fuel pressure and gradually drop down, or did it drop quick. I doubt very much if it has anything to do with the injectors, they would all have to stick open at the same time to drop the fuel pressure down that far and there would be a big cloud of white smoke at the back of the engine.
The engine would flood and you would have a lot of trouble starting it. You said in your first post you changed out the vst, so I would not think you would replicate the same problem.

At least now you know you are chasing fuel pressure, the only thing that controls that is supply into the vst, high pressure pump, pressure reg, power and grounds, what you should of done when the fuel pressure dropped was to squeeze the primer continuously and see if you could start filling the vst and see if the fuel pressure started to rise.

[redlowrey]

I can’t read anywhere that you have changed the high pressure fuel filter or low pressure. Air in the fuel line will drop the rail pressure, where did you run the external fuel line from, ? You could have a small air leak on the suction side and tricking you when holding pressure with the primer.