the 2018 Polaris RZR 1000 endless drama

[HelmetHair]

my bro like buy crappy cars and bikes and repair them. This time he grabbed this beat up 2018 Polaris RZR 1000, totally dead and backfiring.

I helped him a little. We stripped it, painted the rusty chassis, then rebuilt the engine with a new piston and cylinder head. Still no start. A new Moose Utility fuel pump finally got it running!
But then, after only 18 miles, it'd stutter and hit a check engine light when losing traction (though it'd clear up if he let off the gas). Now, it's totally dead again, showing a "throttle stuck" code.

I swapped the throttle cable (it was trashed), adjusted the TPS perfectly (0.465V), and the injector sprays fine. Still won't start, just backfires.
It feels just like when the old fuel pump was shot, even though this one's practically new. What do you think we should check next?

 

[eclipse10]

How do you mean by backfire? Like is it popping too loudly from the exhaust?

 

[HighGearHunter]

Take a moment to check out the wiring around the throttle and fuel systems. It's super important to make sure everything's in order there. Oh, and don't forget to give the ECU a quick test for any error codes or faults. You never know what might pop up, and it's better to catch those issues early on