Originally Posted by
1project2many
If the calibration is going very lean during deceleration then it's likely the manifold has no fuel left by the time you press the throttle. This is a tough condition to detect and to tune around with 7747 / $42. The challenge is covering the difference from very low kPa MAP to high MAP without causing flooding during transitions from idle range kPa to high MAP. What is the range of MAP transition logged by the scantool when the problem occurs? I'm thinking above 60 kPa change? You may also want to adjust the lean decel so it is not quite as lean. In a carburetor where changing the pump shot is not as easy we might richen the lean stop, change a spring under a primary rod, or change taper on primary rods if those options are available.
You may also want to think about IAC operation. IIRC $42 does not have a way to differentiate between idle and deceleration when VSS is disconnected. Closing throttle to decelerate when VSS is not present signals to ecm that engine should be at idle. ECM may consider idle speed to be above threshold and will close IAC to attempt to bring engine speed into line. Stepping on throttle (Again, IIRC, study Rob Rauscher's IAC info) trips ecm to open IAC for "throttle cracker." There are time delay values that could be manipulated to prevent or increase rpm response time in order to reduce airflow changes while throttle is changing. Are you using a manual transmission calibration? Comparison between manual and auto trans cals may suggest which direction to go.
7427 is truly much more capable. 7427 is better able to handle a wider range of conditions imo. 7427 code is built with the knowledge learned while calibration earlier ecm's.
Bookmarks