4th gen slightly high idle problem

I recently noticed that my 4th gen 87 Celica is idling a bit higher than normal, around 900-1200. The idle also slightly bounces around 100 rpms at times. Ive also noticed that my gas mileage has been going down.

When I ran the diagnostic, I got a code 51. All of the vaccum lines were replaced 50K ago, and I just replaced the EGR tube, which was cracked. Fuel filter has been changed about 30K ago. I cleaned the air filter as well. I know its not an issue with the throttle body spring, becuase I put a second spring to help it shut.

Im not sure what the problem is, any sugesstions?? Manuels says TPS or Neutral Start switch, but Im not so sure i belive it.

Thanks.