Odd "Misfire"? on 2013 Yukon Denali

[Hunter79764]

I'm trying to diagnose an issue on my wife's Yukon. It's a 2013 Denali with the 6.2l, I have deleted the VVT and DOD quite a while ago, and did an engine refresh with a mild cam about a year ago, transmission was rebuilt about 3 years ago by a good local shop with upgraded parts including a semi-billet converter and was running fine until a month or two ago, and it has been a crazy busy time with lots of out-of-town trips and family issues, so time to diagnose has been scant and I need some help.

On to the issue -

There is a very noticeable hesitation/misfire/stumble/cut-out? at very certain conditions, essentially 1800-2100 RPM and ~60% throttle or more. It shows up easiest when cruising 65 or so, and rolling into the throttle to speed up to 75 but without downshifting. It feels like the engine completely loses power maybe twice a second, fairly rhythmically but not like a cylinder misfire, but not like a carburetor bogging down either. Kicking into 5th, going to 2200 rpm or so, backing off, or anything else to get out of that zone makes it go away. It is not limited to 6th gear, I can force it to do it in other gears, and even power braking in 1st gets a similar action, so I think that rules out a torque converter lockup cycling (which was a thought). The entire vehicle moves with it, I would not say "violent", but it isn't something that you have to really pay attention to notice.

I have logged with HPTuners all the regular stuff and the only thing I saw was that the fuel trims that are -3 to -7 for most of the map flip to 2 to 5 in the couple of cells where this issue happens, but I'm not seeing any lean codes (or codes of any kind), misfire indications, O2's seem to be switching normally, fuel pressure dips a little bit, but not crazy (it has the variable fuel pressure, at that load it has a 43 base psi and dips to 41 or so at tip-in, but recovers quickly and I believe does the same at other load points at tip-in without causing the surging). Otherwise, I can't actually see the surge on the datalogs at all, other than maybe seeing the RPM drop or flutter.

It reminds me of an issue I had with a loose ECM ground on a swapped vehicle I had, where the computer was basically dropping out, I have checked the major grounds and saw no issues, but I do have corrosion on the negative battery cable. I need to check the minor grounds, and I'm going to replace that cable anyway, but bypassing it temporarily didn't change anything so I don't think that's my source issue.

What other things should I check? Other items to log in HPTuners? I'm going on a ground and power hunt this weekend, what else should I be looking for? New-ish alternator, good battery, dash gauge shows steady voltage although I have had a bit of headlight flicker the last year or so, if that seems like it could be related. I'm just at a loss right now.

 

[bucket]

Does it have a fuel pressure test port? Is your reading from a factory pressure sensor?

 

[Hunter79764]

Does it have a fuel pressure test port? Is your reading from a factory pressure sensor?

Factory sensor on the datalog. I'm not sure if there is a test port, I do have a small gauge that I can check it against for
Static pressure, but not WOT pressure. I'll add that to the list tomorrow.

 

[TotalyHucked]

Fuel pressure is also my knee jerk suspect. 41 seems a bit low even in a variable pressure setup. I almost wonder if the regulator/bypass is getting weak so even though you see XX pressure supplied, that may not be what's actually getting to the injectors? Just a spit ball there

 

[Hunter79764]

Fuel pressure is also my knee jerk suspect. 41 seems a bit low even in a variable pressure setup. I almost wonder if the regulator/bypass is getting weak so even though you see XX pressure supplied, that may not be what's actually getting to the injectors? Just a spit ball there

It felt like low fuel to me too, but when I unplugged the MAF to do a Seafoam treatment (which didn't help, both in the intake and heavy dose in the fuel tank), it went to 58 psi and held there to problem, the fuel trims went crazy as expected, then after relearning, the issue was still there. I can't say for sure if it went away at all, but the tables were pretty set by the time I was able to run it on the highway where I *Know* the issue occurs. I'm going to try testing pressure at the rail with a mechanical gauge, especially now that I know I can get some stuttering from powerbraking it in the driveway. I didn't know any of them ran 43 psi, but apparently the 6.2 Flex Fuel engines do. I think 43 is "Standard", then it increases for higher ethanol content, rather than adjusting injector duty cycle? Either way, 58 is the fault mode pressure, apparently, and it didn't help.

 

[TotalyHucked]

It felt like low fuel to me too, but when I unplugged the MAF to do a Seafoam treatment (which didn't help, both in the intake and heavy dose in the fuel tank), it went to 58 psi and held there to problem, the fuel trims went crazy as expected, then after relearning, the issue was still there. I can't say for sure if it went away at all, but the tables were pretty set by the time I was able to run it on the highway where I *Know* the issue occurs. I'm going to try testing pressure at the rail with a mechanical gauge, especially now that I know I can get some stuttering from powerbraking it in the driveway. I didn't know any of them ran 43 psi, but apparently the 6.2 Flex Fuel engines do. I think 43 is "Standard", then it increases for higher ethanol content, rather than adjusting injector duty cycle? Either way, 58 is the fault mode pressure, apparently, and it didn't help.

Hmm, interesting. Didn't realize the newer trucks had reduced commanded fuel pressure that far

 

[Hunter79764]

Hmm, interesting. Didn't realize the newer trucks had reduced commanded fuel pressure that far

I didn't either, from what I saw it was fairly limited to certain combinations of 6.2 trucks, mainly the Denali and Escalades.

I ran a temporary extra ground, no change, watched the ECM voltage through the condition, didn't really budge. I'm going to look and see if maybe there is an injector ground that I need to check, but otherwise I'm trying to get it to the local tuner again and see if they can find anything. I'm at the end of my rope on this one. And strongly considering selling the truck, I hate working on it, and it feels like I have spent more time and money on this one than any other vehicle I've owned. If my wife is ok with either the previous generation (2000-2006) SUV or an Express conversion van, I want to get back to Gen 3 engines and deal with weak 4L60's instead of Gen 4 engines and stuff AFTER the 2008/09 crash... Ugh.

 

[bucket]

If it comes to that... focus on a 3/4 ton truck or van. Lots of 8-lug conversion vans too. Better brakes and no 4L60.

 

[Hunter79764]

If it comes to that... focus on a 3/4 ton truck or van. Lots of 8-lug conversion vans too. Better brakes and no 4L60.

Yep, 8 lug conversion, preferably extended length, would be ideal. We don't tow much, but it does need to realistically pull 5-6k without going through heroics to fit in the fleet. The GMT800 1/2T SUV's are good for me, 4L60 aside. I've had a couple of them and know the few quirks pretty well. If it has more than 150k on a 4L60 that hasn't been rebuilt, at this point that will just be part of the purchase price to take it to a good local trans shop and have it done. I'm also done with the AWD/4WD stuff. We don't need it here in this part of Texas, and it just gets in the way.

The wife requires remote start and a power liftgate though. Remote start is easy to add aftermarket, I think the power liftgate is just because she's short and doesn't like to reach for the pull strap to close it. Getting barn doors on a van takes care of that one.

Once I get this fixed, I'll see how pissed off I still am and go from there...

 

[winfred]

the flex fuel sensor changes the ratio of fuel injected based on alcohol content, it basically adds a multiplier to the fuel trims, if you can see the alcohol content data reading, and trigger a reset to see if it changes, that can tell if it's stuck/wrong, your fuel trims will change, sometimes drastically if the alcohol reading changes

on occasion the system will brain fart and the reading will drift and it will report wrong fuel trims, doesn't really mess with the running unless it's in like the 20-30% plus range, i don't think that is what's going on by the description

i'll try and check back in when i have more time, but i may forget, dealer tech and the new garbage is way less fun to work on and needy, i miss the LS in my bays, easier to make money on

 

[Hunter79764]

the flex fuel sensor changes the ratio of fuel injected based on alcohol content, it basically adds a multiplier to the fuel trims, if you can see the alcohol content data reading, and trigger a reset to see if it changes, that can tell if it's stuck/wrong, your fuel trims will change, sometimes drastically if the alcohol reading changes

on occasion the system will brain fart and the reading will drift and it will report wrong fuel trims, doesn't really mess with the running unless it's in like the 20-30% plus range, i don't think that is what's going on by the description

i'll try and check back in when i have more time, but i may forget, dealer tech and the new garbage is way less fun to work on and needy, i miss the LS in my bays, easier to make money on

My understanding was on a small set of vehicles, the flex fuel adjusted fuel pressure instead of fuel trims. Then GM figured out it was better to just leave it all in the fuel trims, and a few years of 6.2 SUV's have an odd configuration to show for it. I could be wrong, but piecing together a few different things led me to that conclusion.

I'm going to do more on the grounding side of things, some other folks have pointed me in that direction as a pretty good chance that's the cause, and at minimum, it isn't helping a few other minor issues I've had. We'll see how that goes.

 

[Bextreme04]

Fuel pressure is also my knee jerk suspect. 41 seems a bit low even in a variable pressure setup. I almost wonder if the regulator/bypass is getting weak so even though you see XX pressure supplied, that may not be what's actually getting to the injectors? Just a spit ball there

These Gen IV trucks that were intended for flexfuel use the fuel control module from the CTSV and don't have a return or regulator/bypass. They are direct PWM control of the pump and the pump module can support up to 600+hp on E85. 41-43 is the correct pressure when not running E85.

My understanding was on a small set of vehicles, the flex fuel adjusted fuel pressure instead of fuel trims. Then GM figured out it was better to just leave it all in the fuel trims, and a few years of 6.2 SUV's have an odd configuration to show for it. I could be wrong, but piecing together a few different things led me to that conclusion.

I'm going to do more on the grounding side of things, some other folks have pointed me in that direction as a pretty good chance that's the cause, and at minimum, it isn't helping a few other minor issues I've had. We'll see how that goes.

It sounds to me like you might be getting some false knock(or real knock) that could be driving the timing to be cycling. Have you logged "knock retard" and ethanol percentage? Unless you have added a flex fuel sensor, your truck would not have had one from the factory. It uses a "virtual flex fuel" algorithm that is a known problem. If you are running regular gas and the percentage is showing over ~15-20% ethanol, your PCM is having a minor meltdown due to something and is trying to run like it is on flexfuel. This will change the timing and the fuel trims based on ethanol percentage. If it is reading more ethanol than it should, it will go too rich, run the timing to far advanced, and then start knocking... which will start the knock retard cycling and a "surging" feeling.

 

[TotalyHucked]

These Gen IV trucks that were intended for flexfuel use the fuel control module from the CTSV and don't have a return or regulator/bypass. They are direct PWM control of the pump and the pump module can support up to 600+hp on E85. 41-43 is the correct pressure when not running E85.

Ahhh ok, good info. I'm not as well versed on the gen4+ stuff

 

[Hunter79764]

It sounds to me like you might be getting some false knock(or real knock) that could be driving the timing to be cycling. Have you logged "knock retard" and ethanol percentage? Unless you have added a flex fuel sensor, your truck would not have had one from the factory. It uses a "virtual flex fuel" algorithm that is a known problem. If you are running regular gas and the percentage is showing over ~15-20% ethanol, your PCM is having a minor meltdown due to something and is trying to run like it is on flexfuel. This will change the timing and the fuel trims based on ethanol percentage. If it is reading more ethanol than it should, it will go too rich, run the timing to far advanced, and then start knocking... which will start the knock retard cycling and a "surging" feeling.

I need to talk to my tuner again, he discussed that with me, and my recollection was that he "turned it off". I never run E85, too difficult to get and the 21 gallons with 11.5 mpg on premium sucks bad enough already for frequent fill-ups, especially only a few stations that carry E85 in the area. I changed the gas cap for a black one and haven't given ethanol a second thought in a good while now. The log did not show any knock, and timing didn't seem to be out of line from what I could tell, not being any kind of expert. But I'll go back and check the ethanol percent shown, and I'm going to re-log the issue to send to my tuner and see what he can see from it.

 

[Grit dog]

Yep, 8 lug conversion, preferably extended length, would be ideal. We don't tow much, but it does need to realistically pull 5-6k without going through heroics to fit in the fleet. The GMT800 1/2T SUV's are good for me, 4L60 aside. I've had a couple of them and know the few quirks pretty well. If it has more than 150k on a 4L60 that hasn't been rebuilt, at this point that will just be part of the purchase price to take it to a good local trans shop and have it done. I'm also done with the AWD/4WD stuff. We don't need it here in this part of Texas, and it just gets in the way.

The wife requires remote start and a power liftgate though. Remote start is easy to add aftermarket, I think the power liftgate is just because she's short and doesn't like to reach for the pull strap to close it. Getting barn doors on a van takes care of that one.

Once I get this fixed, I'll see how pissed off I still am and go from there...

Getting your wife a short bus to daily may induce other issues that are easier to diagnose but emotionally harder to solve…..I’d stick with the snack wagons if you value the relationship with your wife! Lol