Misfiring/ Hesitation/ Loss of power under load and High RPM

R32R1 · February 11, 2019

Hey guys,

I've been a member of a few other car forums over the years, but recently decided to join here after hours of searching online with minimal results; hopefully others experiencing this issue can be armed with suggestive data for their dealers when going in for service. I'm no expert, by any means, but have tinkered on my VW R32 long enough to build it to 600hp, break, rebuild, repeat...

I recently purchased a 2015 Sierra 1500 Denali 6.2l and have been experiencing 'misfire' like symptoms right off the bat while at 40-50% load (full throttle was fine). I knew about the 8-speed transmission issues, and the dealer showed me about 15 TCM updates that they can perform. To my surprise, everything from the slamming into reverse to poor shifting cleared right up, and the truck drives perfectly fine when driving like a grandma.

We went on a test drive together and I duplicated the misfiring to him, while he read the ECM live. He saw 4 injectors not performing, so the dealer replaced those 4 injectors. While in there, he spotted coolant (common issue with the T-stat), and replaced the Radiator and T-stat under warranty.

Within a week after that, I bought and installed the following mods:

Range AFM Delete
Corsa Sport Exhaust
Corsa CAI

EDIT** Returning back to stock for proper diagnostics.

Now that I can actually hear the exhaust note, the 40-50% load misfire is still there, along with full throttle being painful. at 5-6k RPMs, I've had misfiring, backfiring, loss of power, struggling or 'confused' in wanted to shift to the next gear (truck held itself at 6k RPMs for 1.5-2 seconds) and honestly feels just like when I bent 3 valves in the VW.

For the record, the dealership is a friend of mine and the service department is outstanding; they will do anything to help me out, and I'm trying not to be "that guy" when going back for the same problem.

I believe the 4 injectors replaced were all on the same bank. Looking outside of injectors, has there been any known issues with collapsed lifters or fuel pump/line issues that they can possibly address?

Thank you,

Eric

Edited February 11, 2019 by R32R1

diyer2 · February 11, 2019

You won't like this......but.

Why buy a problem truck? Find another one.

Who's your friend at the dealer?

The owner, salesmen?

Unless it's the owner anyone else will be limited on repairs by management and GM for warranty.

That guy bought a used truck under warranty from a friend. So forget that guilty feeling.

Make them fix it.

R32R1 · February 11, 2019

Just now, diyer2 said:

You won't like this......but.

Why buy a problem truck? Find another one.

Who's your friend at the dealer?

The owner, salesmen?

Unless it's the owner anyone else will be limited on repairs by management and GM for warranty.

That guy bought a used truck under warranty from a friend. So forget that guilty feeling.

Make them fix it.

Ha believe me, it wasn't known when purchased that it was a problem truck. Coming from a 2012 Sierra 1500 5.3 SLE, seeing the Denali come in on trade was like a godsend.

I'm friends with the owner and mostly all staff there. They definitely go the extra mile to assist, but it's also that 'think outside of the box' thought process that might help a little more with troubleshooting. I've spent many days in the waiting room with my other vehicles while mine was in for routine maintenance, and I've never seen another dealer treat their customers better than this one.

Like anything, it's a business - I get it - when the book time is up and the warranty money for that ticket runs out, time to button her up and send it. I am just trying to provide them with a little more info, so they can attack this with a new ticket, new aspect to hopefully figure out the issue. Problem is my patience is wearing thin, and part of me just wants this engine to lean out and blow up so they have a concrete warranty claim instead of chasing a 'what if it's this'...

diyer2 · February 11, 2019

Got it.

Good Luck.

R32R1 · February 11, 2019

Thanks lol

swathdiver · February 11, 2019

You need to see what the computer is seeing and do a data log to look for problems. Check fuel pressure at idle and under load, check the gap of your spark plugs. Check the injectors again and so on. These engines have enough sensors to quickly figure out what's ailing them if the right people are trained up in the way they should go.

diyer2 · February 11, 2019

Agreed.

A good tech will figure this out.

R32R1 · February 11, 2019

1 minute ago, swathdiver said:

You need to see what the computer is seeing and do a data log to look for problems. Check fuel pressure at idle and under load, check the gap of your spark plugs. Check the injectors again and so on. These engines have enough sensors to quickly figure out what's ailing them if the right people are trained up in the way they should go.

Thank you, I am returning everything back to stock so they can properly diagnose it. Hopefully I'll have an answer this week and THANK GOD I didn't have to cut my exhaust off like we did on my old truck. Still in one piece and can easily be re-installed

R32R1 · February 11, 2019

They mentioned something about the back pressure being off due to the Corsa exhaust and how the 'flapper' in the stock exhaust is what maintains back pressure.

I get that if they bring the tech from GM in, there obviously can't be an aftermarket exhaust on, but is the computer that sensitive these days that it'll be thrown off by that?

swathdiver · February 11, 2019

I doubt that the Corsa exhaust will have anything to do with it unless there's a major defect in workmanship.

R32R1 · February 12, 2019

That's what I'm saying!!! This is my 4th GM, 4th Corsa (TBSS, Escalade, '12 Sierra 5.3, Denali) and all of them had zero issue with the corsa. Exhaust has no leaks or anything either, but I'm swapping the stock on so It's 100% by the book.

R32R1 · February 14, 2019

Update-

Truck is back to stock and still showing the same symptoms: Hesitation/ Misfiring/ etc

I have a feeling it's something as stupid as an 02 sensor or MAF, or as serious as a cracked valve spring.

It'll be going to the dealership tomorrow, so hopefully they can figure it out.

R32R1 · February 21, 2019

Final Update:

Dealership replaced the other (4) injectors and the truck runs perfect again. As for a cause, who knows...but at least the problem is solved. 8 bad injectors is a little concerning though...

Exhaust and intake are back on

Thanks for the help everyone!

Sierra Dan · February 21, 2019

On ‎2‎/‎11‎/‎2019 at 4:14 PM, swathdiver said:

I doubt that the Corsa exhaust will have anything to do with it unless there's a major defect in workmanship.

Thousands of K2 owners have modified exhaust systems without a flapper in place and have not experienced engine problems.

Sounds like your dealer is playing the guessing game R32R1

R32R1 · February 22, 2019

Spoke too soon...exhaust back on and it's doing it again. Not sure if it's coincidental, but something tells me I'll be swapping the exhaust back to stock (yet again) for them to look at this.

Im starting to really dislike this truck, and that's the last thing I need right now.

Symptoms are in 3rd gear and up, 40% throttle- I'll get the hesitation/ misfire feeling. I checked the maintenance records and this truck already had the torque converter replaced at 35k miles. All updates are done on it, so I'll try swapping the MAF to see if that's the cause.