http://www.aa1car.com/library/map_sensors.htm" onclick="window.open(this.href);return false;
http://www.tirereview.com/Article/98309 ... nosis.aspx" onclick="window.open(this.href);return false;A low intake vacuum reading or excessive backpressure in the exhaust system can trick the MAP sensor into indicating there is a load on the engine. This may result in a rich fuel condition.
There were others that contributed too but I cant seem to find them in my bookmarks, including a couple that referred to an excessive calculated engine load value (derived mostly from the MAP reading) causing the ECU to retard the timing. (as well as richening the mixture)Speed Density System Diagnostics
Speed-density systems found in older vehicles are designed to provide a calculated, rather than direct-reading, value for engine airflow. Speed density calculations generally include data from the throttle position, manifold absolute pressure (MAP) and engine speed sensors.
Keep in mind that the MAP value will begin to equal the atmospheric pressure value as the throttle opens and load increases on the engine. The PCM responds to the increased MAP value by richening the air/fuel ratio. If the exhaust is restricted, the MAP value will equal atmospheric value as soon as the throttle is opened and the engine fails to accelerate. The engine generally tending to run rich and lose power are symptoms of exhaust restriction in a speed-density system.
I suspect the retarded timing is the largest contributor to loss of power that I've been seeing, and I have commented before on the apparent tendency for the ECU to apply a 20 degree "torque reduction" timing retard even during normal driving conditions, (when it shouldn't) so it fits in with that observation. Now I think I know WHY the timing was being retarded.