My 525 boat did the same thing. TPS sensor

Just replacing the TPS sensor won't necessarily solve the problem. The intake butterfly valve needs to be adjusted to the get the proper voltage on the TPS sensor at idle.

Anyone have updates?

We swapped TPS and set voltage. IAC is new. Crank sensor checks ok. PCM swapped. Cam sensor swapped. Throttle linkage ok because it's disconnected.

One thing the diagnostic tester showed was that the timing was low-525's constantly adjust the timing, but it is less than 10 degrees where the other engine is 10-15.

Stumped but still have some parts swapping and voltage readings to do.

Anyone have a solution?

No solution ... I've got a 525 EFI that idles just fine it's when you apply throttle that the thing goes nuts. When I push the throttle, it starts surging really hard, and backfires through the intake It goes in the shop Monday to get its brain scanned. When it is doing the surge and backfire gig, it is throwing a consant alarm, so the computer should be logging it on mine. Have you had your computer on the scanner while it running?

My mechanic simply adjusted the idle on mine. I took it out for the first time yesterday and it ran flawlessly.

You cannot adjust the idle, it's PCM controlled. You can however adjust the voltage to the TPS at idle.

Just curios why you would rev it so high in neutral? My 525's alarm above 1500 at idle.