2015 GM 3500 6.0L - Need help with engine stumble

[burbguy82]

So for a while now I have had this issue that is kind on intermittent...let me explain.

2015 GMC 3500 6.0 L96, 6L90e 4x4 DRW

About a year ago, I stated to have this issue where upon startup on a warm\recently stopped engine. In short, I would start the truck up, and upon acceleration there would be a massive stumble, and if I held the pedal down, there would be what sounds like several thumps, that sounded like backfires. Let go for a second, and the engine returns to "normal". It happened a couple of times, and later in the week, my check engine light came on. Code upstream sensors, heater specific. So, I figured lets go ahead and change the sensors, all of them, and since it was close to 100k miles, change the plugs and wires. All GM Genuine parts used.

There is also a noticeable reduction in smoothness and power. The 'backfire" sounds to me like a starving for fuel condition, but the ECm is taking out -10-15% fuel on the LTFT

Check engine gone, codes gone=fixed right? Wrong

I have messed with this thing and messed with it, checked this and that on my TopDon bidirectional scanner. So far I have replaced the MAF with a new Gm Genuine part. As my first real try. Still nothing. Same issue

Recently Home Depot had a sale for Seafoam for $2.50 a bottle, so I got 10 bottles. Just to see what would happen, I put 4 bottles in the tanks (66gallon tanks combined) which is really close to the proper ratio per the bottle=

problem disappeared, ran smooth as glass, good power, no stumbling and backfire sounds, everything was great. Good power, smooth everything.

Since, I have got gas again, (which would have diluted the Seafoam mix, and the problem returns.

I have :
cleaned all grounds
did visual check on all wire harness that are visible
checked fuel pressure=65psi at all times
done a injector balance test, all start at 60+ and end at 28 or so
vacuum is at 18-20 ish
TPS and Pedal agree
probably forgetting something.

So in conclusion: weird "backfire" sounds after warm start, with hesitation, loss of power, sometimes a weird shift between gears, unsmooth acceleration, and a full dose of Seafoam solves the condition.

So far the Seafoam is the ONLY singular thing that has solved the issue, how? And how can I use this to find out what is actually wrong?

 

[spasm3]

Knock sensor?

 

[CKN]

I was going to say possible lifters....

 

[GMBoy]

I have seen this on a similar truck and in that case we found a bad fuel injector - ended up replacing them all while we where in there. The bad injector had bad windings and was leaking when sitting/engine off when under pressure - such as when you shut off an engine and pressure is still in the lines. Then it would stumble on start up due to the leaked fuel and then the windings acted up when hot and the fuel trims were all messed up.

Do an injector test/power balance test again but compare hot to cold starts... Hope that helps!

 

[burbguy82]

I have seen this on a similar truck and in that case we found a bad fuel injector - ended up replacing them all while we where in there. The bad injector had bad windings and was leaking when sitting/engine off when under pressure - such as when you shut off an engine and pressure is still in the lines. Then it would stumble on start up due to the leaked fuel and then the windings acted up when hot and the fuel trims were all messed up.

Do an injector test/power balance test again but compare hot to cold starts... Hope that helps!

And I assume that on a hot test, the injector test will end with a lower PSI? For verification.

 

[burbguy82]

I have seen this on a similar truck and in that case we found a bad fuel injector - ended up replacing them all while we where in there. The bad injector had bad windings and was leaking when sitting/engine off when under pressure - such as when you shut off an engine and pressure is still in the lines. Then it would stumble on start up due to the leaked fuel and then the windings acted up when hot and the fuel trims were all messed up.

Do an injector test/power balance test again but compare hot to cold starts... Hope that helps!

Also, any thoughts on how the Seafoam would tie into that?

 

[clinebarger]

Also, any thoughts on how the Seafoam would tie into that?

It could temporarily help a sticky injector, Seafoam isn't very "hot" & a poor injector cleaner in my opinion. I use Top Engine Cleaner cut 50/50 with gasoline in it's own pressure vessel....Do not add TOP to the fuel tank as it is to "hot" for the fuel pump & other materials in the fuel tank.

 

[burbguy82]

It could temporarily help a sticky injector, Seafoam isn't very "hot" & a poor injector cleaner in my opinion. I use Top Engine Cleaner cut 50/50 with gasoline in it's own pressure vessel....Do not add TOP to the fuel tank as it is to "hot" for the fuel pump & other materials in the fuel tank.

Makes sense. I suppose you have to remove the rails and introduce it out of the truck?

I searched TOP and came up with nothing. Thanks for the help BTW

 

[clinebarger]

Here

 

[BrendanC]

you mention tanks combined, is the second tank OE? could be a EVAP leak making a leak condition on tip in.

 

[clinebarger]

I unhook the fuel line at the rail, Remove either the fuel pump relay OR the Fuel Pump Module Fuse depending on the system on your truck.

Then I hook up my injector cleaning rig (Shop air pressurized regulated pressure vessel) Adjust the pressure to @ 70 psi. Run the engine 'till all the product is used.
Reconnect fuel line & install fuse or relay.

 

[D60]

When I was shopping the Lang cleaner (not the chemical, the tool) was cheapest through Zoro with a code. FWIW

Lang also has a vid on their site but they tell you to "cheat" the FPR by staying below it. Perhaps a touch risky and clinebarger seems to indicate higher pressures are better. That said, I used the cheat method on a '99 Vortec and it seems to have cured his problems.

SKU : TU-470B - Fuel Injection Cleaner – 2 Gauges

Manufacturing professional tools since 1932

www.langtools.com

 

[burbguy82]

you mention tanks combined, is the second tank OE? could be a EVAP leak making a leak condition on tip in.

Tanks are OE. I have thoughht of evap. charcoal tank was solid, and it has been replaced, purge valve and vent is verified good. or so i think

 

[burbguy82]

Im going to give the cleaning a shot, and report back.

 

[CharBaby]

Just shooting from the hip here...Don't overlook the CAT's

 

[ls1mike]

I have had injectors do similar things on the Trans Am, didn't know what it was until one failed completely open and caused the #7 cylinder to not fire at all. I replaced all of mine recently.

 

[burbguy82]

I have had injectors do similar things on the Trans Am, didn't know what it was until one failed completely open and caused the #7 cylinder to not fire at all. I replaced all of mine recently.

It makes sense, but hard to see on a scanner, as i am still a novice at looking at data. Not sure that one would see it on a scanner anyway. Weird thing is, on the fuel injector note, is that the fuel trims are negative on both sides, which would or could mean that there is the same issue with one injector at least on both sides, which would be weird.

One thing I forgot to add was I recently had a look at the throttle body, and when it goes to 30% the movement in the butterfly is not smooth, and kind of clunks from 20% to 30% command or maybe 30-40% cant remember, which is likely where me foot is on the pedal when this instance happens..... But no CEL, no fault codes, and scanner shows that the pedal tha throttle agree.

@clinebarger
@GMBoy
@ls1mike
and all others

forgot to mention that...above throttle body info.

I am running some more stuff throught the tank as we speak, and then I am going to do the "Professional" injection service when I can be without for a day or two. Plus I need to get one of those pressurized vessel kits.

 

[ls1mike]

It makes sense, but hard to see on a scanner, as i am still a novice at looking at data. Not sure that one would see it on a scanner anyway. Weird thing is, on the fuel injector note, is that the fuel trims are negative on both sides, which would or could mean that there is the same issue with one injector at least on both sides, which would be weird.

One thing I forgot to add was I recently had a look at the throttle body, and when it goes to 30% the movement in the butterfly is not smooth, and kind of clunks from 20% to 30% command or maybe 30-40% cant remember, which is likely where me foot is on the pedal when this instance happens..... But no CEL, no fault codes, and scanner shows that the pedal tha throttle agree.

@clinebarger
@GMBoy
@ls1mike
and all others

forgot to mention that...above throttle body info.

I am running some more stuff throught the tank as we speak, and then I am going to do the "Professional" injection service when I can be without for a day or two. Plus I need to get one of those pressurized vessel kits.

I had no faults for the injector either, but with the mods on the WS6 it may have never set a code. Been a long time since I have been into the tune.
Now Cline will know better but all my electronic throttles are smooth and the feedback shows it being a linear movement. You might be on to something with that.

 

[burbguy82]

You might be on to something with that.

I suppose you mean the throttle body situation. Maybe. But that does not explain or gel with the fact that the running of "fuel injector cleaner" smoothed out everything. This is the time where I wonder why we even have computers if they lie to us, maybe it is not.

This is one reason why I bought a CUCV recently, it is hard for those things to lie to you.

Anyway, if yall think of anything at all, please bounce it off my chest. I am probably not remembering every single detail, and a eureka moment would be great.

Thanks again fellas.

 

[clinebarger]

It makes sense, but hard to see on a scanner, as i am still a novice at looking at data. Not sure that one would see it on a scanner anyway. Weird thing is, on the fuel injector note, is that the fuel trims are negative on both sides, which would or could mean that there is the same issue with one injector at least on both sides, which would be weird.

One thing I forgot to add was I recently had a look at the throttle body, and when it goes to 30% the movement in the butterfly is not smooth, and kind of clunks from 20% to 30% command or maybe 30-40% cant remember, which is likely where me foot is on the pedal when this instance happens..... But no CEL, no fault codes, and scanner shows that the pedal the throttle agree.

Scan tools have latency issues, This is where data loggers & scopes come in. A couple here has HpTuners but I always forget who?? Their scanner is more of a Data Logger than it is a Scan Tool.