Uneven Idle is getting progressively worse but cannot find the cause (Fuel Pump / Injectors?)

[SNES Avalanche]

Hey all,

I drive a 2008 Chevrolet Avalanche (Body has 150k Miles Engine has 40k Miles...AFM sure is great) 6.0 L L76 V8 (gasoline) with a GM 4L70 transmission. I have an issue where the truck's idle has been getting progressively worse over the last few months.

Issue: When at a stop the idle fluctuates between 450-600. It never stalls out, but you can feel the entire truck vibrate when it dips. Initially it was happening every 4-5 seconds but now it's happening closer to every 1-2 seconds. (Note: It does not happen during acceleration or at speed only at idle).

Notes:
There is no misfire being detected by any scanner on any of the cylinders.
The "Stumbles" do not trigger a misfire and they are not triggering any MILs.
There's no hesitation when I press the accelerator.

---

I have replaced the following parts in an attempt to fix the issue:
1. Driver side Motor Mount
2. Plugs and Wires
3. Cleaned the Throttle Body
4. Cleaned the MAF Sensor
5. Replaced Air intake filter
6. Repaired Exhaust Leaks at the Donuts, at the exhaust Manifold and had a new flex exhaust welded on as the original was leaking
7. Changed to AC Delco oil and have been using the ACDelco UPF48R as instructed in the TSB
8. Replaced Oil pressure Sensor and Filter
9. Intake Manifold Gasket on both sides - Noticed something during this and need input

---

Next Step(s) help required:
When I replaced the Intake Manifold Gasket, I noticed a lot more fuel deposits than I expected all around the injectors. It was caked on to the intake / old gasket.

I hooked up my fuel pressure gauge and found the following:
Key moved to Run (but not cranked) Fuel Pressure jumps to 50PSI
At idle Fuel Pressure at Rail is 42.51 PSI
Leak down test (truck off) lost 10 PSI over 6 minutes falling from 50PSI down to 40 PSI
Long term fuel % Trim Bank 1: -.78%
Long term fuel % trim Bank 2: +3.91%

Based on the fuel information is my problem the injectors or the fuel pump or am I barking up the wrong tree?

 

[swathdiver]

Hey all,

I drive a 2008 Chevrolet Avalanche (Body has 150k Miles Engine has 40k Miles...AFM sure is great) 6.0 L L76 V8 (gasoline) with a GM 4L60-E transmission. I have an issue where the truck's idle has been getting progressively worse over the last few months.

Issue: When at a stop the idle fluctuates between 450-600. It never stalls out, but you can feel the entire truck vibrate when it dips. Initially it was happening every 4-5 seconds but now it's happening closer to every 1-2 seconds. (Note: It does not happen during acceleration or at speed only at idle).

Notes:
There is no misfire being detected by any scanner on any of the cylinders.
The "Stumbles" do not trigger a misfire and they are not triggering any MILs.
There's no hesitation when I press the accelerator.

---

I have replaced the following parts in an attempt to fix the issue:
1. Driver side Motor Mount
2. Plugs and Wires
3. Cleaned the Throttle Body
4. Cleaned the MAF Sensor
5. Replaced Air intake filter
6. Repaired Exhaust Leaks at the Donuts, at the exhaust Manifold and had a new flex exhaust welded on as the original was leaking
7. Changed to AC Delco oil and have been using the ACDelco UPF48R as instructed in the TSB
8. Replaced Oil pressure Sensor and Filter
9. Intake Manifold Gasket on both sides - Noticed something during this and need input

---

Next Step(s) help required:
When I replaced the Intake Manifold Gasket, I noticed a lot more fuel deposits than I expected all around the injectors. It was caked on to the intake / old gasket.

I hooked up my fuel pressure gauge and found the following:
Key moved to Run (but not cranked) Fuel Pressure jumps to 50PSI
At idle Fuel Pressure at Rail is 42.51 PSI
Leak down test (truck off) lost 10 PSI over 6 minutes falling from 50PSI down to 40 PSI
Long term fuel % Trim Bank 1: -.78%
Long term fuel % trim Bank 2: +3.91%

Based on the fuel information is my problem the injectors or the fuel pump or am I barking up the wrong tree?

Mine picked up a bad idle and otherwise runs just fine immediately after changing the plugs and wires. Used GM OE 748UU wires and 41-162 plugs. Did you use the same plugs?

Fuel pressure ought to be 43.5 psi, your trims are telling you something is wrong but I do not know what. Well, I would however change all of the o-rings on the injectors. Can you read your Fuel Pump trims?

Did your truck originally come with the 6.0? If so, it would have been paired with the 4L70 Supermatic, RPO M70.

 

[SNES Avalanche]

Mine picked up a bad idle and otherwise runs just fine immediately after changing the plugs and wires. Used GM OE 748UU wires and 41-162 plugs. Did you use the same plugs?

Fuel pressure ought to be 43.5 psi, your trims are telling you something is wrong but I do not know what. Well, I would however change all of the o-rings on the injectors. Can you read your Fuel Pump trims?

Did your truck originally come with the 6.0? If so, it would have been paired with the 4L70 Supermatic, RPO M70.

Hey Diver!

I used ACDelco Iridium Spark Plug 41-162 and GM Wires.

(Truck came with the 6.0 so you're correct on the transmission!)

I do not have a Tech2 so I do not believe I can read fuel pump trims (unless it's under a name I do not recognize)?

Would you recommend that I replace just the O rings or the injectors as well?

 

[swathdiver]

Hey Diver!

I used ACDelco Iridium Spark Plug 41-162 and GM Wires.

(Truck came with the 6.0 so you're correct on the transmission!)

I do not have a Tech2 so I do not believe I can read fuel pump trims (unless it's under a name I do not recognize)?

Would you recommend that I replace just the O rings or the injectors as well?

There may be a way to get the right PIDs @kbuskill to see the fuel pump trims but that is not my wheelhouse.

I would definitely replace the o-rings before the injectors and then check for leaks. This might be my source of woe too, I'll have to look more closely at them on mine.

We have the same plugs, and so does @George B and we both have a funny idle since we changed them to that number.

 

[petethepug]

This is off a 05, not an 07-13, but you get the idea. These vapor / charcoal canisters have hit their life expectancy. If yours is going shaky, shaky, shaky, like Mr. Mayhem it will play havoc with your fuel flow.

 

[SNES Avalanche]

This is off a 05, not an 07-13, but you get the idea. These vapor / charcoal canisters have hit their life expectancy. If yours is going shaky, shaky, shaky, like Mr. Mayhem it will play havoc with your fuel flow.

I will give this a shot after I take a pass at the Fuel Injectors / Fuel Injector O-Rings.

I really with the truck would through a MIL/CEL so I can at least pull a code.

 

[Mickey_7106]

You want your Long term fuel trims to be as close to 0% as posible but no more than 10% +/-. Check bank 2 o2 sensor voltage maybe.

I use torque pro and recently checking a similar issue on my old e450 ford rv

 

[SNES Avalanche]

You want your Long term fuel trims to be as close to 0% as posible but no more than 10% +/-. Check bank 2 o2 sensor voltage maybe.

I use torque pro and recently checking a similar issue on my old e450 ford rv

I will check them! I replaced all 4 O2 sensors 10 or so months ago.

 

[bdbull]

I will give this a shot after I take a pass at the Fuel Injectors / Fuel Injector O-Rings.

I really with the truck would through a MIL/CEL so I can at least pull a code.

Really interested to hear the update on this. I have a 2010 Avalanche with the same issue (I have a thread about it on here somewhere). The first issue I found was the plugs had been worn out after 20k miles. I replaced them and the stumble went away, but I can feel it coming back now after only 1k miles so there's definitely something else going on too. I recently replaced the valve cover with the updated part number and installed a catch can. Only got about 150 miles on those changes so not too much data to go on yet.

 

[Ruscat]

You want your Long term fuel trims to be as close to 0% as posible but no more than 10% +/-. Check bank 2 o2 sensor voltage maybe.

I use torque pro and recently checking a similar issue on my old e450 ford rv

View attachment 364205

View attachment 364207

What obd2 reader did you
use with Torque pro. I want to get Thor set up for my 2011 6.2 yukon