Hello,

I have a 2014 Chevy Suburban LTZ (5.3L) with 148,000 miles. I do all my own maintenance and keep her in as good of shape as I can. Recently she started to have a slightly rough idle. (Her first issue besides a cracked dashboard) Initially it was intermittent, sometimes idling perfectly smooth and sometimes the idle was noted to get a little low causing a slight shake. I could barely see the Rpm needle moving but it was noticeable. I figured it was possibly carbon build up or possibly dirty fuel injectors. I tried a concentrate fuel injector cleaner but there was no apparent change. I then did the following:

• Using an OBD2 reader I see a P0300 Random Misfire (No Check Engine Light)
• Short Term Fuel Trim on both Bank 1 & Bank 2 is approximately 0% to -2%
• Long Term Fuel Trim on both Bank 1 & Bank 2 is approximately -17% to -20% (Rich)
• Idle rpm varies from ~620 to 550
• Engine running fuel pressure = ~42/43 psi
• Replaced 8 new spark plugs (No change)
• Replaced 8 new spark plug wires (No change)
• Replaced 8 new ignition coils (No change)
• Cleaned MAF sensor (No change)
• Mass air flow rate from MAF sensor = 0.706 lb/min (Should ideally read around (5.3 grams per second at idle)
• Converting 0.706 lb/min to grams per second (x453.6 / 60) results in approximately 5.34 g/s.
• Unplugged MAF sensor to see if there was a change (Ran worse and threw MAF codes)

By now the slightly rough idle is getting worse, sometimes stumbling and sounding like it wants to die before the ECU catches it. I then did the following:

• Removed and cleaned the throttle body.
• Noted it as somewhat dirty, dirtier on the back side of the butterfly.
• Noted the intake appears slightly wet.
• Re-installed and did the idle re-learn procedure (No change)
• P0300 Random Misfire remains (No Check Engine Light)
• Long Term Fuel Trim on both Bank 1 & Bank 2 is approximately -17% to -20% (Rich)

By now the slightly rough idle is causing the engine to randomly die when slowing down to a stop (This happened twice)

I then

• Replaced 8 new fuel injectors
• Replaced the EVAP Purge valve
• Noted reduction on idle stumbling below 500 rpm, no engine shutting off noted while slowing down however slightly rough idle remains
• P0300 Random Misfire remains (No Check Engine Light)
• Long Term Fuel Trim on both Bank 1 & Bank 2 is approximately -17% to -20% (Rich)

I then

• Dropped the fuel tank and replaced the fuel pump / fuel pressure regulator assembly
• Fuel tank appeared very clean inside
• Used compressed air to blow out all the fuel and vent lines, no debris/charcoal came out of the lines (Only a small amount of fuel)
• Reinstalled and drove ~5 miles round trip to the gas station to fill up.
• Started right up, no leaks, fuel gauge works, no issues filling.
• Idle still feels slightly rough.
• P0300 Random Misfire remains (No Check Engine Light)
• Long Term Fuel Trim on both Bank 1 & Bank 2 is approximately -17% to -20% (Rich)

My next steps will be to inspect, clean or replace the O2 sensors.
What else am I missing? Why is this engine running rich on both fuel banks?

Other maintenance I have recently done:

• New brake pads and rotors
• New tires
• New front and rear air suspension
• New serpentine belt
• New battery
• New wipers

Thank you for any help or thoughts.