Chasing a rough idle?

2009TahoeLT said:

Ok so my 2009 Tahoe 5.3 has been running a bit rough/choppy. It has had codes previously but have since not come back. Small and large evap leak, and misfire codes. Problem is it’s idling at about 575-600. When it idles over 600 it runs nice. When it drops into the 500s is when it runs rough but doesn’t show any misfires. Now if I rev it to 1200-1600 it misfires horrendously. But doesn’t throw a code. It also sputters when I come to a slow stop. I’ve replaced all plugs, wires. Replaced intake manifold gasket. Replaced purge valve. Fuel pump was replaced. Cleaned maf. Intake manifold pressure reads 69-70kpa. Long term fuel trim bank 1 is -11.719 and bank 2 is -10.156.
Short term fuel trim bank 1 and 2 is all over the place. Bank 1 is anywhere from -3.960 to 1.556 and bank 2 is -1.568 to 4.688. I’ve done a throttle body relearn, crank relearn, fuel trim relearn. I don’t know if any of those numbers are in a good range honestly. There’s also 2 coils that seem to be clicking loud and you can feel it. Also injectors seem to be louder than normal also. Any ideas? My buddy mentioned timing chain as a possibility but I’m not sure. I have 250k on this 5.3. Got it with 242k. Idk what has been done previously. Any insight would be amazing! Thanks in advance

Click to expand...

2009TahoeLT said:

Ok so my 2009 Tahoe 5.3 has been running a bit rough/choppy. It has had codes previously but have since not come back. Small and large evap leak, and misfire codes. Problem is it’s idling at about 575-600. When it idles over 600 it runs nice. When it drops into the 500s is when it runs rough but doesn’t show any misfires. Now if I rev it to 1200-1600 it misfires horrendously. But doesn’t throw a code. It also sputters when I come to a slow stop. I’ve replaced all plugs, wires. Replaced intake manifold gasket. Replaced purge valve. Fuel pump was replaced. Cleaned maf. Intake manifold pressure reads 69-70kpa. Long term fuel trim bank 1 is -11.719 and bank 2 is -10.156.
Short term fuel trim bank 1 and 2 is all over the place. Bank 1 is anywhere from -3.960 to 1.556 and bank 2 is -1.568 to 4.688. I’ve done a throttle body relearn, crank relearn, fuel trim relearn. I don’t know if any of those numbers are in a good range honestly. There’s also 2 coils that seem to be clicking loud and you can feel it. Also injectors seem to be louder than normal also. Any ideas? My buddy mentioned timing chain as a possibility but I’m not sure. I have 250k on this 5.3. Got it with 242k. Idk what has been done previously. Any insight would be amazing! Thanks in advance

Click to expand...

Doubeleive said:

Have you done a back pressure test?

Click to expand...

2009TahoeLT said:

I have not. How would I go about that one?

Click to expand...

j91z28d1 said:

trims removing 10% fuel from each side is a signs it's NOt happy.

can your scanner read alcohol percentage?


how does one do a fuel trim reset? I've seen it posted but don't know how this commonly done?

Click to expand...

Fless said:

Good catch; I forget about the alcohol content issue with some of these. It might think the fuel has more alcohol than it does, and is adding fuel, but then the O2 sensors tell it to take it away.

LT fuel trim reset is done with a scanner. STFT resets with ignition cycle.

Click to expand...

j91z28d1 said:

ahh, I've not noticed ltft reset on mine. so my thoughts is, you check alcohol %, if zero. reset ltft, unplug the front o2 and see how it runs on base fueling. if it idles smooth and doesn't break up at the 1200-1600rpm then at least use know the fueling is correct. Just gotta find out why the trims are chasing lean.

if it still runs badly, I'd say your coils making noise are worth checking out. pop the hood at night and look for arching. that clicking noise might be spark breaking out to a ground instead of firing the spark plug.

Click to expand...