Service Engine Soon Light and Rough Idle (Below 1200 rpms)
My 1995 Chevrolet Blazer 4.3 Vortec has had this problem for the past couple of months. I have replaced the fuel pump, fuel filter, spider fuel injection system (CMFI), rotor button, distributor cap, wires, plugs, PCV vent, checked for vacuum leaks, checked for upper intake manifold leak, installed internal fuel lines, replaced the MAP sensors, and the problem still exists.
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Check engine light on? Have it scanned and if it is a p0420 code, that will a catalytic converter code. That will make it run rough and not run well. Usually the catalytic converter will be stopped up. If no engine light on, unclear of what it could be but I hope you get the issue resolved. Thanks and Jesus loves you John 3:16-17 (KJV)
Sounds like a dirty air filter is putting the mass airflow sensor out of range. The throttle position sensor is subject to physical wear, the age of your truck warrants replacing it. The coolant temp sensor controls the Idle Air Solenoid which determines idle speed ( cold, high idle, 1200 rpms open loop mode for the ECU) and (hot, low idle, 800 rpms, closed loop mode for the ECU) If the truck does not have a Mass airflow sensor, check the Manifold absolute pressure sensor (MAP). Perform an OBDII scan for stored fault history to avoid buying parts you don't need. Do the cheapest fixes first. Go ahead and change the fuel filter also. There is a lot of water in todays fuels
IAC idler air control valve bumps up the RPM and doesnot let the engine to die, how ever if its not the IAC then Ide be looking at the TEMP sensor which migh have a fair line crack or may be the MAF mass air flow sensor might be bad, seems like the engine is not geting air, check the air filter first.
Inspect/replace air filter.
Fill the gas tank to above 3/4 and add Lucas FUELSYSTEM CLEANER.
Ran on the highway for 10+ miles.
Monitor is the rough idle condition is getting worst on below 1/4 tank.
Change fuel filter and check fuel pressure.
Most failing/weak fuel pump will not work well below 1/4 tank mark.
It will fail in a HOT DAY.
check your Throttle Positioning sensor that should fix your rough idle the TPS controls the breather on the front of the engine its affixed to the right hand side of the throttle body just infront of the spyder on the V6 Vortec 4.3L Models