System Diagnosis - Electronic Ignition

Symptom Diagnosis
The PCM uses information from the MAP and Engine Coolant Temperature (ECT) sensors in addition to the RPM in order to calculate the spark advance as follows:

^ A low MAP output voltage = More spark advance.
^ A cold engine = More spark advance.
^ A high MAP output voltage = Less spark advance.

A hot engine = Less spark advance. Therefore, a low MAP output or a high resistance in the coolant sensor circuit could cause the detonation. A high MAP output or a low resistance in the coolant sensor circuit could cause poor performance.