• If you currently own, previously owned or want to own an Avalanche, we welcome you to become a member today. Membership is FREE, register now!

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

Joined
Sep 23, 2020
Messages
92
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?
 
Sure sounds like bad injector(s) to me.
 
Is there any way to pinpoint which one or should I bite the bullet and replace all injectors / O Rings?
Do you see any misfire counts when looking at the live data (Mode $06) that could point you in the right direction? I've seen misfire counts that are low enough not to trigger a light.
 
Do you see any misfire counts when looking at the live data (Mode $06) that could point you in the right direction? I've seen misfire counts that are low enough not to trigger a light.
There's none after idling for 30 minutes / driving for an hour. There might be literally 1 a random cylinder after longer drives but I've never seen more than 2.
 
I based my guess on what you say you have already done and what you have not.

Let me just say what I think I would do without having specific codes pointing at specific injectors.

I'm not fond of firing off the expensive parts cannon without at least trying all available alternatives.

The very first thing I would do would be to buy a few cans of Berryman's B-12 Chemtool and start running this through the next several tanks of fuel.

I have used many fuel system cleaners over the years with good results, but I feel the Berryman's is the strongest I have used.

I poured some of that into a glass jar and tested how well it cleaned a part just dropped into the jar.

It did a much better job at cleaning than did some Seafoam or straight gas.

I would caution against letting it run down the paint of your truck when you pour it in the tank.

Run the Berryman's in several tanks and see if that will help unstick the bad fuel injector.

Or not.

My next step would be a little more tedious.

At the most, I would buy two new fuel injectors and since you can not pinpoint the bad one(s), I would methodically replace two injectors and work my way around as I begin to test for the culprit(s).

I seriously doubt all of your injectors are bad.

They could be, but I just cringe at the thought of dropping a ton of money into parts that may not be bad.

Things like spark plugs, I have no problem changing them all at once, but expensive parts like coils and injectors, I would rather see if I could locate the offender and save some money by continuing to get some use out of the good ones.

But, that's just how I roll.
 
Last edited:
I based my guess on what you say you have already done and what you have not.

Let me just say what I think I would do without having specific codes pointing at specific injectors.

I'm not fond of firing off the expensive parts cannon without at least trying all available alternatives.

The very first thing I would do would be to buy a few cans of Berryman's B-12 Chemtool and start running this through the next several tanks of fuel.

I have used many fuel system cleaners over the years with good results, but I feel the Berryman's is the strongest I have used.

I poured some of that into a glass jar and tested how well it cleaned a part just dropped into the jar.

It did a much better job at cleaning than did some Seafoam or straight gas.

I would caution against letting it run down the paint of your truck when you pour it in the tank.

Run the Berryman's in several tanks and see if that will help unstick the bad fuel injector.

Or not.

My next step would be a little more tedious.

At the most, I would buy two new fuel injectors and since you can not pinpoint the bad one(s), I would methodically replace two injectors and work my way around as I begin to test for the culprit(s).

I seriously doubt all of your injectors are bad.

They could be, but I just cringe at the thought of dropping a ton of money into parts that may not be bad.

Things like spark plugs, I have no problem changing them all at once, but expensive parts like coils and injectors, I would rather see if I could locate the offender and save some money by continuing to get some use out of the good ones.

But, that's just how I roll.
Taking your advice.

I'm going to start by cleaning the injectors / replacing the O-rings first and if any are in bad shape I'll replace them.
 
Be careful with new injectors aswell I purchased all 8 from Amazon & by time I changed them out couldn't return.
Two of 8 spraying constantly never shutting off, hydrolocked engine at idle b4 I figured out 2 bad injectors. Broke down and spent 515$ on 8 lifetime warranty injectors. So any future issues can swap free
 
Keep us posted !
I removed the fuel rail and replaced the o rings for all 8 injectors / cleaned them out.

The engine is still idling rough. The injectors looked okay for 150k miles.

I think my next steps are going to be replacing the passenger side motor mount / rechecking the driver side

8419A067-B795-46DC-9428-6E30B4D2E6C5.jpeg
 
I feel your frustration I did new parts couple thousand dollars worth & still having troubles with idle. Mine I'm thinking is mostly electrical as ecu reports low voltage from just about every sensor it talks to.
There may be an evap leak aswell ill troubleshoot with smoker if I can find somewhere can borrow one from.
Just ran new grounding points to engine frame and body. Hopefully helps about to add second battery to system if original having trouble holding voltage maybe x2 will correct somewhat.
Damn ecu adjusts idle fuel air everything trying to avoid throwing codes.
Good luck Mine is being difficult but will get sorted soon I hope.
 
I feel your frustration I did new parts couple thousand dollars worth & still having troubles with idle. Mine I'm thinking is mostly electrical as ecu reports low voltage from just about every sensor it talks to.
There may be an evap leak aswell ill troubleshoot with smoker if I can find somewhere can borrow one from.
Just ran new grounding points to engine frame and body. Hopefully helps about to add second battery to system if original having trouble holding voltage maybe x2 will correct somewhat.
Damn ecu adjusts idle fuel air everything trying to avoid throwing codes.
Good luck Mine is being difficult but will get sorted soon I hope.
Yea. I just really wish the truck would throw me some kind of code so I can trace a bit.
 
Yea. I just really wish the truck would throw me some kind of code so I can trace a bit.
Or it will do what mine does lol, it threw rich bank 1&2 fuel mix codes & sometimes evap leak.
My brother expensive reader shows almost 100 small fault codes mostly saying bad voltage signals to ecu from just about every sensor in the truck.
Really hoping my ecu isn't going out, I added second battery to see if keeps voltage steadier just incase battery not 100%? Plus I added additional grounding points frame/body/engine.
Made sure I didn't bypass the main grounding sensor to battery.
Cleared codes now trying drive cycle hoping I helped it.
I do still get to remove & re install better cat pipes 🤦‍♂️, had to pull when added larger transmission pan. But I hear exhaust leak passenger side more when cold less almost gone when hot.
Ordered new seals arrives tomorrow & I'll try fixing exhaust, weird it hasn't thrown code yet o2 sensors usually find exhaust leaks quickly sometimes without hearing sounds
 

Attachments

  • 16472905084851930987163747270287.jpg
    16472905084851930987163747270287.jpg
    176.9 KB · Views: 5
  • 16472905631642440070994813690498.jpg
    16472905631642440070994813690498.jpg
    148.9 KB · Views: 5
I have a 2007 Avalanche 5.3 and have been experiencing similar idle and low rpm issues in the past four months with no codes to help. Plus, I've had some no start and gas cap / pumping gas issues. I replaced the charcoal cannister, upper purge valve, lower solenoid purge valve. My mechanic cleaned the cannister beads out of the lines, and checked the fuel pressure which was ok. I then put on a Range cylinder deactivation disabler and ran some 3M injector cleaner with 93 Shell fuel. Still, the problems continued. Last weekend, I cleaned the K&N air filter, MAF, throttle body, and even changed out the tranny filter and fluid. The truck did great until the way home today, when it sputtered while making a left turn. Thinking now it can be the fuel pressure regulator, fuel filter, and/or pump; all of which are in the gas tank. Thoughts?
 
Those minor exhaust leaks could be your issue. I also have a 2000 Mustang, and it ran rough and I replaced coils, plugs, and fuel filter, only to discover a slight leak at a clamped exhaust section. I had an exhaust shop weld it and the car has run great since. That small leak caused a misread by the O2 sensors with no codes. Note: the car was accelarating rough with some idle issues before the welding.
 
Last edited:
Recently replaced:
  • O-Ring on Oil Pickup Tube
  • Passenger Side Motor Mount
  • Starter (Seems like I got lucky when doing the Motor Mount as it was on its way out)
My rough idle is still there. 0 misfires being detected by any scanner.

I'm at a total loss on what to do at this point.
 
Live with it?

Truck is trying to set idle @ around 500 rpm but any little electric load increase or AC kicks on will make it "BURBLE" and the rpm will drop a bit but then ECM tries raise it back up with least fuel possible.

Seems all cars/trucks 2000 onward (OBD2) do this to save fuel and lower emissions...
Some are better at it then others and you do not notice unless you had high speed rpm scope.

Now if yours is like a yo yo and rpm up/down quickly then check air leaks in intake system maybe?
 
Do these trucks have an idle air control valve? That would be my first guess if they do.

Next I suspect a vacuum leak or exhaust leak if before the upstream O2 sensors. Spray a little carb cleaner around suspecting vacuum and intake areas and listen for a change in idle if you don't have a smoke machine.
 
Live with it?

Truck is trying to set idle @ around 500 rpm but any little electric load increase or AC kicks on will make it "BURBLE" and the rpm will drop a bit but then ECM tries raise it back up with least fuel possible.

Seems all cars/trucks 2000 onward (OBD2) do this to save fuel and lower emissions...
Some are better at it then others and you do not notice unless you had high speed rpm scope.

Now if yours is like a yo yo and rpm up/down quickly then check air leaks in intake system maybe?
It's gotten to the point where you can watch the water in a bottle splash around. It didn't do it the last 10,000 miles / has been getting progressively worse over the last 500.

I replaced the Intake manifold gasket / have checked every vacuum line that I could find. The second I get over 1500 RPM everything smooths out and you would never notice it on a longer cruise.

I think I'm going to end up at the dealership as everyone I've gone to shurgs and says "bring it back once it has a misfire or an actual code"
 
Any updates on these rough idle conditions? I left my 2007 with my mechanic last week. I even let him take it home overnight. Of course, the truck ran fine during this time and my mechanic couldn't find anything wrong. The truck ran perfect for me on Friday and Saturday as well. Then this morning it sputtered slightly at the same distance it always sputters after making a right turn on a main road. I'm thinking crud in the gas tank or a weak fuel pump. Any ideas?
 
Any updates on these rough idle conditions? I left my 2007 with my mechanic last week. I even let him take it home overnight. Of course, the truck ran fine during this time and my mechanic couldn't find anything wrong. The truck ran perfect for me on Friday and Saturday as well. Then this morning it sputtered slightly at the same distance it always sputters after making a right turn on a main road. I'm thinking crud in the gas tank or a weak fuel pump. Any ideas?
None.

My hunt continues but I’m going to check the brake booster for a vacuum leak and potentially replace all 8 coil packs.
 

Attachments

  • 3786B0A1-6737-465F-9B21-4E384AF2DFC5.jpeg
    3786B0A1-6737-465F-9B21-4E384AF2DFC5.jpeg
    73.3 KB · Views: 4
These type of problems are the worst. Did coil packs; O2 Sensors, and plugs on my Mustang, only to find a bad catalytic converter and after that was replaced; a slight leak from an exhaust clamp by the converter which threw off the O2 sensor. It's like a repeat of history with this truck.
 
Back
Top