engine pings as if there is no timing advance when you accelerate from a stop.

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I had a new timing belt installed. After about 100 miles the engine started pinging upon acceleration from a stop as if it was out of timing. I returned it to the mechanic & he told me it was my ECM that controlled the advance. I had it tested from another mechanic & he said it was not the ECM. He believed it was a bad installation of the timing belt. I took it back to the 1st mechanic. He retimed the belt which did not fix the issue.