your egr could affect vacuum if the diaphram is ruptured/sticking or the EGR selenoid is stuck open.
timing should advance with the est wire plugged in and code 42 reset. at idle it should be around 20 degrees. when you give it gas it should advance further.
the ECM controls spark advance with the est system. to properly control ignition the ecm needs to know
cranshaft position
engine speed
engine load(manifold vacuum)
atmospheric pressure
engine coolant temperature.
tbi distributor 101:
the four post connector to the distributor:
pin A: reference ground low, may be ground to distributor, it makes sure the ground circuit between ecm and module. if open may cause poor performance.
pin B: bypass. at about 400 rpms the ecm applies 5 volts dc to this circuit to switch spark timing control from the module to ecm. an open or grounded bypass circuit will set code 42 and the engine will run at base timing, plus a small amount of advance built into the module.
pin C: distributor reference high. this provides the ecm with rpm and crankshaft position info.
pin D: EST. this circuit triggers the module. The ecm does not know what the actual timing is but it does know when it gets the reference signal. it then advances or retards the spark from that point. therefore if base timing is set incorrectly the engine spark curve will be incorrect.
results of incorrect est operation:
low map output voltage: more spark advance
cold engine: more spark advance
hot engine :less spark advance
high map output voltage: less spark advance
detonation can be caused by high map output or high resistance in the coolant temp sensor circuit.
poor performance can be caused by high map output or low resistance in the coolant temp sensor circuit.
DIAGRAMS HERE:
http://www.fullsizechevy.com/forums/...3&d=1206741675
http://www.fullsizechevy.com/forums/...4&d=1220576357
PICKUP COIL AND COIL TEST:
http://www.autozone.com/az/cds/en_us...rInfoPages.htm