NEED HELP 5.3 rough idle/miss

[ND-1965C10]

2003 Trailblazer EXT 5.3 160k

Recently found a hesitation/miss/rough idle happening. Low rpm is when noticed and really only when not on the gas except if I hold the brake and gas, it gets really rough. About 15k ago replaced the plugs, fuel pressure regulator, fuel filter, maf, upstream o2 sensors, and knock sensors.

Thoughts are on the injectors or coils?

I did not change the wires as when reviewing resistance the old all matched the new except one new was bad so returned them and stuck with the old. Could be I need to go back down that road too.

No engine codes.

 

[mrrsm]

There are two possible problems to start the diagnosis:

(1) The Throttle Body needs a thorough Off The Vehicle cleaning.

(2) Pull off the Fuel Rail Fuel Pressure Regulator Vacuum Hose... If any Fuel leaks out....Replace the FPR.

 

[Mooseman]

Are you getting any codes? Engine misfires usually set off a p030x code.

 

[ND-1965C10]

No codes and cleaned the throttle body about 15k I also changed the fuel pressure regulator 15k ago. Will recheck both.

 

[Mooseman]

If you have a bluetooth OBD interface and torque, you should have a look at misfires. There could be a few but not enough to trigger a code. Also check the fuel trims.

You could also check for vacuum leaks using carb cleaner but fuel trims should show if that's the case if it's dumping extra fuel to compensate for a leak.

 

[BRomanJr]

I had a similar problem, 2004 Envoy 5.3 slight miss, roughness at idle for a long time. I did the front O2 sensors and that fixed about 95% of it. Drove for another year or so (second vehicle, not DD). Every time I drove it, the roughness bothered me. A few months ago my brother bought a 2006 V8 Trailblazer that was so smooth, you couldn't tell it was running.

At this point I started checking coils, the 2004 and 2006 are different, but were similar enough that I could have swapped them bracket and all to test, but I chose to buy (4) new Delphi coils. I changed them on one side and it seemed to help, then bought (4) more, replaced them and now it is 99% smooth. I decided to call it good, 216 K miles and all.

 

[littleblazer]

Make sure all the plugs are tight. Our sierra was doing that a few months after its tune up (15kish) and the cause was one of the plugs loosened up. Simple enough check. Smelled a little rich and had a slight miss at idle and under load.