Hello,
2002 LeMans with 15M computer. A hot engine surges at small throttle openings just off idle up to at least 4,000 rpm. Also when hot the engine will ping accelerating hard from 4500 rpm to 5500 or higher in 4th gear. I'm trying to understand how these symptoms are related but need someone to double check my thinking.
The 15M computer has a preloaded fuel mixture and spark advance map so fuel delivery and timing are not adjustable independently - correct? The start or "bottom" of the fuel map is closed throttle and as the throttle opens the computer increases the fuel proportional to throttle opening - correct. While the start or "bottom" of the ignition advance curve is 0 rpm and probably reaches maximum advance by about 2,500 rpm - correct? (was not able to find a timing curve on the forum)
If the above are correct, then setting the TPS too far one way would move the starting point further up the fuel map so that the computer would provide more fuel than a correct TPS setting (computer thinks the throttle is open further than it is) while setting the TPS too far the other way would provide less fuel than a correct TPS setting (computer thinks the throttle is open less than it is).
Because timing is predetermined by the ignition map, pinging under hard acceleration indicates the mixture is too lean at large throttle openings - correct? So surging at low throttle openings would likewise be due to lean mixture. Does this logic make sense? (provided all the sensors and computer are working correctly)
Thank you!