2017 octane sensor after rebuild

[Arcticwolverine]

I rebuilt my 2017 m8000 top end after I had a piston go bad from detonation and has real low compression. After the rebuilt it runs great but started throwing a octane sensor at real high rpms (never had this problem before) I took it back apart and the piston still looked great. Any ideas?

 

[Old & slow]

What grade of fuel are you using?

 

[Arcticwolverine]

I use 90 because that’s all they have up in Alaska, but I did add oreily octane booster and it did nothing

 

[jim]

Well, good news is your detonation sensor is working now....so that's good. If you had only one side go bad before, something is up. Maybe an air leak or going lean for some reason? Can't keep up with fueling at high RPM? And if still happening with new piston, you still have an undiagnosed issue.

 

[kiliki]

did you find out why it went down the first time? you can also add 100 LL avi fuel by 1 or 2 gallons to bring it up to see if it is the fuel.

 

[Arcticwolverine]

Well, good news is your detonation sensor is working now....so that's good. If you had only one side go bad before, something is up. Maybe an air leak or going lean for some reason? Can't keep up with fueling at high RPM? And if still happening with new piston, you still have an undiagnosed issue.

is there a way I can diagnose this?

 

[Arcticwolverine]

did you find out why it went down the first time? you can also add 100 LL avi fuel by 1 or 2 gallons to bring it up to see if it is the fuel.

no I did not, it did have over 3000 miles on it though. Is avi fuel better than adding octane booster?

 

[Old & slow]

Octane booster is not really any help in my opinion. Aviation fuel is the answer but you should not need it. Does the fuel have ethanol in it? You either have bad fuel or another problem. First what code is coming up? Is there any modifications to the engine or exhaust. Did you rebuild it with oem parts? What is the compression after the rebuild?

 

[Coldfinger]

Did you use OEM parts for the rebuild, including gaskets?

 

[Arcticwolverine]

Octane booster is not really any help in my opinion. Aviation fuel is the answer but you should not need it. Does the fuel have ethanol in it? You either have bad fuel or another problem. First what code is coming up? Is there any modifications to the engine or exhaust. Did you rebuild it with oem parts? What is the compression after the rebuild?

Okay so I used AV gas 1:10 and it ran great for and hour and then when I really opened her up in powder it popped once again. 135 psi compression

 

[Arcticwolverine]

Did you use OEM parts for the rebuild, including gaskets?

Everything OEM except the piston. That shouldn't make a difference though should it? I took it apart after it did it the first time and the piston still looked great so I don't think its piston slap

 

[Coldfinger]

I was just curious. I don’t know if certain aftermarket pistons would change the compression ratio.

I think I would siphon out all the fuel and add 2-3 gallons of av gas to see it that makes a diff.

 

[Old & slow]

Okay so I used AV gas 1:10 and it ran great for and hour and then when I really opened her up in powder it popped once again. 135 psi compression

What elevation are you at when you did the compression test? It sounds like you have a fuel problem. I would drain all the fuel out and start with some fresh non ethanol fuel if you can. When you say it popped did you loose a piston or did the knock sensor activate. What code if any is coming up?

 

[Arcticwolverine]

the best e have up here is 90 and i drained it, put in 1:10 AV gas and still no bueno. I replaced a piston this fall and its been an issue since. and now the OCTN code keeps pooping when I consistently ride at high RPMs

 

[kiliki]

can you post a post a pic of a spark plug from each side? did you look at the top of the piston to see if the new one was lean? might have a cracked carb boot.