I rebuilt my 4.3 TBI GMC S15 Blazer engine 18k miles ago. It has been running great until two weeks ago. The CAT melted down and I had 30 psi of back pressure. The engine ran very ruff. I had the cat and muffler replaced with new. The engine still is running very ruff and rich. The distributor is a MSD with new cap and rotor. The spark plugs were just replaced. The ignition leads are Magnecor and are new. I replaced the fuel filter and measure 12 psi fuel pressure steady at the filter. I tried a different MAP sensor, Ignition control module, a different TBI and the engine still is running very ruff and rich. I have checked the temp sensor with a scanner and the ambient temp is correct and the sensor is responding normal. The MAP sensor voltage is correct. The throttle position voltage is correct. The timing is bang on at 0 degrees with the base timing (wire under the dash disconnected). The O2 sensor measures correct as well. The EGR valve was just replaced also. I believe I am down to a bad ECM.

Any thoughts or ideas or would be appreciated.

Thanks in advance,