Shutter/Misfire under low RPM and load

ic3man5 · February 3, 2020

2015 Silverado 5.3L LT 4WD 63,000 miles.

Recently started developing a shutter / misfire under load. Happens the most around 1250-1500 RPM. Doesn't seem to have lost any power and no codes being thrown.

Here's a list of things I've done:

-No misfires actively being logged over 20 minute drive.

-Replaced Spark Plugs

-Replaced Spark Plug Wires

-Replaced MAF Sensor

-Inspected transmission pan - no shavings - transmission shop doesn't think its transmission - shutter happens in 2nd-6th gear when converter is forced locked.

-Tripled torque converter lock up pressure and removed TCC slip

-Highly concentrated amount of seafoam through gas tank (entire bottle for less than 5 gallons of fuel)

-Individually turned off each fuel injector @ 2000 RPM to see if any are failing. Nothing seemed off.

-Not burning any oil.

-Catch can since almost new.

Only thing I've noticed is running 87 Octane instead of 93 Octane makes it worse. I'm completely out of ideas here and hoping maybe someone else has run into something similar.

CamGTP · February 3, 2020

When you say it happens with the torque converter locked, that says to me it's a possible torque converter issue itself.

A nicer scan tool that could do transmission controls could be used as you drove the truck. When the shutter happens with the TC locked, just unlock it and see if it stops.

ic3man5 · February 4, 2020

21 hours ago, CamGTP said:

When you say it happens with the torque converter locked, that says to me it's a possible torque converter issue itself.

A nicer scan tool that could do transmission controls could be used as you drove the truck. When the shutter happens with the TC locked, just unlock it and see if it stops.

When I unlock it, it does stop but its also moving it outside that RPM range. I originally thought it was a torque converter issue but my trans shop said it feels/sounds like the engine. I'm using HP Tuners for the controls.

Doublebase · February 4, 2020

Well, sounds like you’ve checked/considered everything...except fuel. Check the fuel trims with a scan tool, if it’s something like -25%...fuel is being taken away (because too much fuel is being use). Leaking injectors could be the cause.

ic3man5 · February 4, 2020

46 minutes ago, Doublebase said:

Well, sounds like you’ve checked/considered everything...except fuel. Check the fuel trims with a scan tool, if it’s something like -25%...fuel is being taken away (because too much fuel is being use). Leaking injectors could be the cause.

I did "look" at it when I had the scanner going but didn't check close or for long since I was looking at other stuff. I'll report back what I find. Thanks for the advice.

ic3man5 · February 4, 2020

Well it looks like STFT and LTFT are actually almost perfect so I think I can rule out Fuel. Looks like MAP needs to be 90kPa or higher to reproduce the shutter/misfire. I'm starting to lean more and more towards torque converter.

ic3man5 · February 7, 2020

Well small update, looks like something might be going on with fuel after all. My LTFT values are peaking towards negative 17-18% over time but only at idle.

The little orange "squares" spiking upward are me hitting the gas. All trims start out a 0% when engine starts. LTFT takes about 15-25 minutes to hit -17%.

I noticed with my tune I'm adding fuel around the RPMs I'm feeling the issue and it gets a lot better. Not sure what to think about this yet.

ic3man5 · February 7, 2020

Doing a little bit of research it looks like its potentially PCV Valve or a vacuum issue somewhere. I ordered a valve and going to replace tomorrow. For $8 its worth it to swap out just to see.

Doublebase · February 8, 2020

15 hours ago, ic3man5 said:

Doing a little bit of research it looks like its potentially PCV Valve or a vacuum issue somewhere. I ordered a valve and going to replace tomorrow. For $8 its worth it to swap out just to see.

But does that make sense to you? A vacuum issue would introduce air into the intake, the computer would compensate and add MORE fuel, not take away fuel, right? So you'd have + fuel trim, not negative. Vacuum leaks skew the fuel trim positive.

And there is no PCV valve on these trucks, it's a fixed orifice, it won't get any larger and cause excess flow. You can replace it, but it's not a PCV valve. And if you did have a vacuum leak in that part of the system, I think it would cause a misfire at cylinder 5 (because the PCV line dumps right above cylinder five at the intake). If you have a catch can, potentially you may have a leak at one of the lines causing a vacuum situation, but I still think your fuel trim would trend +, not -.

I could be totally wrong, let me know what you find out/how it goes.

carnau · February 8, 2020

Whats your fuel injector balance?

Adam Ferguson · May 19, 2020

Did you ever find out what the issue was?

Ozer · May 19, 2020

you are out of warranty based on build date? 5yr/100k

ic3man5 · May 20, 2020

I never found out what the issue was so I modified the VE tables in HP Tuners to reduce the fuel a bit and made it upshift easier under load for right now. I figured once it gets worse it will actually trip a code or make it so I can diagnose it.

I did figure out messing with the VE tables that I was already in a rich condition because I bumped the idle up to 700 which put it in the next "section", so if you are going to increase the idle keep it under 700, I think they run 700-1200 RPM rich for cold starts in the winter.

ic3man5 · May 20, 2020

14 hours ago, Ozer said:

you are out of warranty based on build date? 5yr/100k

That is only powertrain which I think doesn't apply to the engine unless its like a total failure. 3 yr/36,000 mi basic, 5 yr/100,000 mi powertrain. No extended warranty, I won't buy a product I feel needs one ever. I beat on it within the first warranty to make sure whatever is going to break, breaks fast and not slow outside the warranty. I've heard too many stories of "This broke about 100 miles outside my warranty and I drove it nice the whole time."

Edited May 20, 2020 by ic3man5

ic3man5 · May 20, 2020

On 2/8/2020 at 12:17 PM, carnau said:

Whats your fuel injector balance?

Everything I could test with the fuel injectors showed they were okay from the computer side, I can't test more without actually taking it to a shop and seeing if they are balanced properly. I did individually turn off each injector idling the truck @ ~2500 RPM and there was no difference in any of them.