Code 51 (when caused by the TPS) is specific to the IDL switch inside the unit. This switch tells the ECU when the throttle is closed so it knows better how to operate the IAC (Idle Air Control) valve. If the IDL switch is not closing when the throttle is the ECU will just pump the IAC to keep the engine running.... so your idle will be inconsistent. You cannot set timing in this situation... you must have no codes (not entirely true, but a good practice), the engine must be up to normal operating temp and the diagnostic jumper must be in place. Do know that a code 51 will also present if you press the gas while in diag mode or turn on the A/C.
What's wrong with the timing?
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