Posted by Owen Aaland on January 17, 2015 at 15:07:25 from (216.47.34.37):
In Reply to: Gmc 7000 posted by Zach Scherf on January 16, 2015 at 20:13:46:
Since the engine has been replaced was the original a 366 or a 427? Did this truck ever run correctly for you before having this problem? If it was originally 427 the ECM may be providing too much fuel for your engine displacement.
You have a fairly simple fuel system on that truck. Once the ignition switch is turned on the ECM looks for a tach signal to start the injectors pulsing. Fuel deliver is based on rpm, the throttle position setting, the coolant temperature, and the manifold vacuum signal from the map sensor. The coolant temperature trims the injector pulse width to give more fuel for cold starts. Starting timing is set by the distributor until the engine rpm exceeds 600 rpm at which point the ECM takes over spark timing. Once the ECM takes over timing, it advances the spark until the knock sensor picks up the spark knock. The ECM then uses this information for base timing control.The O2 sensor does not come into play until the engine enters closed loop. It is a trim device only. Being able to see the actual O2 reading and the injector pulse width can go a long way in chasing down too rich a mixture problem. If the O2 is reading full rich and the pulse width trim is maxed to the lean you are getting too much fuel from somewhere.
ECM problems are more likely to be wiring problems rather than a bad ECM. Since most of the ECM works on voltages of 0 - 5 volts a poor connection or a bit of corrosion in a connector can cause a lot of problems.
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