Help diagnosing stumbling/back firing LQ4

The motor in my C20 Suburban was built by Thompson Motorsports back in 2014. Its a basic LQ4, stock crank and rods, Mahle forged pistons, small cam, 317 heads milled to 67cc chambers....nothing exotic.

I'm running the Holley Terminator TBI set up and finally got a proper tune from Andrew a couple months ago. The truck has never run better. Within the past few weeks, I've been getting intermittent back fires and the truck has really started to run poorly at light/part throttle. Everything with regard to the tune still looks correct....temps, AFR, sensor readings, etc. I've also noticed its developed a slight tick at idle. Its more than what you would associate with an exhaust leak (which I checked for with soapy water on the headers and did not find) but not as pronounced as what you would associate with rod knock. As you increase the RPM, the tick/knock goes away.

I've got a new set of coils, plugs, wires, and coil harness arriving today. All of that currently on the motor was pulled from the junk yard, so this will at least eliminate the possibility of an ignition issue.

My other thought was a bad/spongy lifter? As best I can tell, the noise is coming from the driver side bank. From what I've read, with a bad lifter, the sound you hear is the valve slamming closed, missing the ramp on the cam and smacking the seat.....does that sound accurate? If that was the case, would that valve being out of timing result in the periodic backfire I'm experiencing?

To make matters more interesting, I have movers arriving tomorrow to pack up the entire house as we move from Norfolk to Austin. This truck is my only mode of transportation for the next two weeks, before I have to drive it cross country to Texas. So I need to get this sorted....even if that involves pulling some cylinder heads in the parking lot of my buddy's condo over the next two weeks.

Thanks for the help guys!

How many miles has it been doing this? Did you maybe get a bad batch of gas? I've heard of that happening.

At this point, I think I've got two tanks through it, but it is close to empty, maybe Ill fill up and put a few more miles on it before I start tearing into it...

fresh gas didn't do anything. Tomorrow Ill pull the valve covers and check each rocker on the base circle of the cam for excessive play.

Are you able to datalog? Might have a code pending if you check them? The ltft's around 0?

Is it backfiring through the intake *lean* or the exhaust *rich*?

Unfortunately you may be on the right track, but if it was a bad lifter the sound would be more centrally located, would sound like it's coming from under the intake. Might have somehow bent a pushrod, valve spring may be bad?

Are you running stock rocker trunions?

Just pulled the valve covers and checked all the rockers individually, on the base circle of the cam.....nothing out of spec.

Backfiring through the exhaust....though the AFR isn't showing overly rich whenever it backfires. Its strange, because I can almost predict when its going to backfire 3-5 seconds in advance because you can hear the flutter in the exhaust pulse prior to the actual backfire.

Pushrods are aftermarket and have never seen over 5500 RPM so Id be shocked if I bent a rod and from what I can tell, the valve springs are all good. Stock rocker trunions at this point....need to upgrade those eventually. Right now the motor has about 30K miles on it.

Its a Dominator ECU, and I have the laptop and software set up....if you can give me a brief step by step of how to data log and then what to look at, I can tell you what its showing....I'm tuning illiterate and learning as I go here.

Thanks for the help!

Since Andrew did your tune, I would contact him. He can pull any stored or previous codes. Also you can get a obd Bluetooth adapter on Amazon and download a free car scanner app to read and clear codes yourself.

Watched a Holley youtube video....headed out to do some data logging now...

got two good logs.....if someone can point out what I should be looking for as far as codes or abnormalities, I'll take a look.

Are there any codes? If so what are they?

I'm very unfamiliar with the software...learning as I go...what menu do I go to in the V5 software to pull up any codes?

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Originally Posted by FLYNAVY53

I'm very unfamiliar with the software...learning as I go...what menu do I go to in the V5 software to pull up any codes?

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Holley EFI does not generate any trouble codes.

Andrew

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I've got a new set of coils, plugs, wires, and coil harness arriving today.

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Did you get these on yet?

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Originally Posted by andrewb70

Holley EFI does not generate any trouble codes.

Andrew

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thats what I thought, thanks!

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Originally Posted by dontlifttoshift

Did you get these on yet?

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not yet…..Im in the middle of a move from Norfolk to Texas. Today was pack up day, tomorrow they load the truck. As soon as thats done tomorrow afternoon, Ill swap out all the ignition components and see where we are.

Got all the new ignition bits installed....truck fired right up and runs like a top. No issues at all. Big thanks to Andrew for his assistance in tuning the truck initially, and following up with the trouble shooting help.

Is self-tuning disabled? Or at least reduced? If there is a deficiency somewhere, and it's still self-tuning it will mess itself up trying to compensate. To me, sounds like the fueling table went crazing, try reinstalling the original tune, or clearing the learned table. If you clear the learn table and it goes back to how it was, there's something going on (i.e., exhaust leak before O2, vacuum leak, etc.)