17 Scarab twin 250 Low Oil Pressure Issue

[cmckenn]

First off, jetboaters.net has been more help and more informative than any dealer I have found in my area. Shout out to @Luc Lafreniere for not only helping me with my last issue but obviously helping many others here.

Took Scarab 255 out Friday, ran all day and miles upriver with zero issues. Loaded the family up Easter Sunday and again, rode miles upriver with zero issues. Fast forward 2 weeks, launched the boat, made it past the No Wake zone, accelerated full speed for about 1 minute no problem. Stopped to make sure everyone was good to go across reservoir, accelerated LOW OIL PRESSURE, back off throttle warning goes away, Throttle Up, back on. Turn the boat off a few minutes, fire it back up, let idle for a minute, throttle up, LOW OIL PRESSURE. Engine sounded fine, no warnings at idle or no wake speed. Stopped, checked oil levels, all good. Fired back up, throttled up, again on and immediately back off when throttling down. At this point (I'm assuming because I tripped the warning multiple times, explained in more details below) Portside engine went into limp mode. We actually spent 6 more hours on the water this day, in Dock Mode creeping along the banks and beaching the boat for a fun filled family day, (minus any tubing or skiing). I did some research and my assumptions from automotive mechanics and I ordered the Candoo as well as a new oil pressure sensor. I have not received the oil pressure sensor yet but I did receive the Candoo so I plugged it in, checked the codes for LOW OIL PRESSURE, CLEARED them, checked all the sensor connections, all seemed ok. Hooked water hose up to starboard engine, let system run through checks, No Codes, No Check Engine Light. Increased RPM to 3500 for about 30 seconds, no warning, no codes, all seemed good. Decided to carry boat out to reservoir. Launched, eased out of no wake zone then throttled up and ran fast for about 1 minute. Stopped, said I think we're good to go, throttled up again and the same symptoms as above. This time I paid a little closer attention. The moment Starboard engine hits 2800(ish) RPMS oil light comes on, back off, warning goes off. This time I did not try it over and over to keep it out of Limp Mode but slowly went to sand bar in No Wake speed, anchored and enjoyed the day. I brought the Candoo and laptop on board and plugged it in before leaving. No Code at all for low oil pressure this time. (assuming because I did not set off the warning enough times like before) Headed back to dock, attempted throttle once, immediately Oil Pressure over 2800 RPM, runs fine all day under if under 2800. Got to dock, powered up on trailer (over 2800) no warning. Got home, hooked hose up, started Starboard engine, slowly raised throttle to 3500 RPM, let it run there for about 30 seconds.... Oddly, no warning, no Low Oil Pressure.

My original thought was oil pressure sensor (that is on the way) but in theory... if the sensor was bad would it not be bad all the time? Why is it only registering a fault above 2800 RPM, but can be driven for hours under that threshold with no problems? Furthermore, why out of the water (twice) does the engine perform above 2800 RPM with no issue. The only thing I was unable to test is both engines running at the same time out of water, but I don't see how one has anything to do with the other since they are completely separate systems. I don't believe at this point I have a "major" or internal problem, but I'm wondering where to start with my process of elimination. I'll change the sensor when it gets here, but given the above out of water test, I'll have to actually launch boat to determine if that is the fix.

Any advice will be greatly appreciated, and I assure everyone that I will follow up with the "Fix/Solution" once figured out.

 

[Luc Lafreniere]

Yeah, definitely doing everything you can here. I really do agree to start with the sensor. It's not the first time people have had bad ones. I can't explain the behavior though, I'm no expert. But sensors usually work on voltages or resistance. If it's faulty, there could be an offset causing it to trigger at certain levels. Start with the sensor.

Otherwise, double check your filter to make sure it's all good. We've seen things like collapsed filters and such.

And happy I and others are able to help.

 

[andrei_b]

What oil are you running and how long ago was it changed?

Definitely double-check the oil filter, as that's one possibility.

As for the difference between issues with running in the water vs out, I think the principle of load might apply. Just as free revving a car's engine in neutral doesn't apply much (if any) load on it, driving it down the road at those same RPMs certainly involves the load of moving the car up to speed. Perhaps it's not the RPM that is triggering the fault, but when the engine hits a certain load point while underway that triggers the fault.

It'll be interesting to see whether the new sensor resolves your issues, good luck!

 

[Luc Lafreniere]

I had similar thoughts even though I didn't write anything. Good comment.

 

[anmut]

While not a scarab owner, just a shade-tree mechanic wannabe, I tend to agree that it's sensor/voltage related. Most likely at 2800 RPMs the ECU is expecting X data back from your sensor and it's receiving that data out of range, causing it to throw the warning. By chance have you checked all your battery connections to make sure you don't have a loose ground somewhere?