I have a 2015 LT that has almost 49,000 miles on it. I replaced the spark plugs recently that were never replaced prior. I purchased the car from the original owner. Ok so when I come to a standstill while driving I can feel a slight sputter in the engine. Its not terrible, but its there. So I know that this could be the coil packs, the maf sensor or possibly a dirty throttle body. Before I jump into that I wanted to know if anybody had another idea or if anyone had a good path to proceed. I hate cleaning throttle bodies and I don't want to buy new coil packs unless I have to. I have no codes. No lights on the dash

 

So a shutter but only on light throttle? I assume that the issue was there before you changed the plugs and that changing the plugs had not effect on the issue?

In addition to what you said it could be an issue of the primary ignition (AKA Control module or wiring). If the EGR valve is a little dirty it can allow exhaust into the intake. Any small vacuum leak. Dirty injectors.

If you had a scan tool you should look up to see if you have a miss fire count. If a single coil is bad that cylinder should have a higher reading in miss fire counts. The computer pretty much ignores idle miss fires. Seeing miss counts in perfectly running cars is normal. Next, a scan tool with an ignition oscilloscope could be helpful but only if you know what you are looking for.

When I was a GM warranty tech these types of drivability issues where always the hardest to find with an absolute test. Missed under load was always easy to find compared to this. When I did it, each engine had it own quirks. The 305 and 350 TPI used to get micro cracking of the intake gaskets at the tune port runners. The V8 throttle bodies where more likely to have a dirty EGR. The 3.1's had issues with the throttle bodies. They would get dirty and even leak vacuum in some cases. You could never be 100% sure so you had to do a scan to look at all the sensors to look for input that where off "normal" but not bad enough to set a code. Vacuum test with spray and take a look at the Oscilloscope. I hate to say it but that is when the guessing started.

Are you a mellow driver? I have had cars that would have a build up of carbon in the intake, valves and pistons. A little fuel additive and run the crap out of it and they would be good for another year. I know I have to do it with my daily drivers. I tend to get stuck in traffic or I am in bumper to bumper not allowing for any fun. A little sea foam and some WOT play time and she is happy. Back in the day we used to pour water into a running engine to clean out the carbon. Don't do that on any modern engine but it is basically they same issue.

 

I am not always a mellow driver and I just bought the car a few weeks ago. It happens and red lights usually. Its a slight misfire almost like the system is burping. They might be a few little burps after a more noticeable burp. It does not happen all the time. The plugs are new and I just did a Royal Purple dose which seemed to help slightly. I don't want to start buying new coilpacks and things that turn out not to be the problem. I have seen where it can be a dirty throttle body. I do have a foxwell scanner but have not used livescan before for things like this. But there definitely is a misfire. Its slight and its not all the time. I do not feel it when accelerating or while at speed. Its always when the car comes to a stop while in drive.

 

These cars do not have ICM so cleaning the throttle body is pretty easy. Also I would clean the EGR valve seat (check youtube for how to) Next I would do either the spray vacuum test or a smoke test to look for very minor leaks before I bought any parts.

Are there any aftermarket parts on this engine. I have seen this reaction to a set of a headers and to some point even cold air intakes. GM spends an insane amount of time tuning these cars and any change air flow will effect the way the engine runs. If the factory intake cause turbulence and a new after market one get rid of the turbulence you could feel it at idle. As you load the engine the effect would become less and less.

Example. in 1989 we have a new Corvette that idled rough, I though it sounded cool the customer did not. At the time we had 5 other Vetts on the lot and none of them did this. The Car was suppose to idle smooth at 780 RPMs. If we turned the idle to 920 RPMs the issue went away. We tried to fix it for about 3 months. It went to the tech center for a month. They did not fix it and the car became the only car I ever worked on that was a buy back (AKA a Lemon). I had basically replace everything from the short block up. The whole top half of the engine. This made me think it was computer relater, but I replace the ECU twice and even the entire wiring harness. Everything I did after the third day was at a GM support direction. They could even log into the ECU through the GM computer system and watch the car run in real time. God know what the GM tech center did. GM releases bulletins every week. As a tech I have to review those as they come out. About 4 months after the buy back I got a bulletin for Corvette bad Idle. The cam was defective and off one degree. This did not require repair unless the customer wants it fixed. It was so minor it did not even effect emissions. I say all this just to show that even technical engineers are guessing with these low RPM shake/miss.

 

Mine improved a bit when I replaved one quart of motor oil with ATF non synthetic. I did not run it long as I was afraid of engine damage even though everything I read said it was ok. But the sputter was first like burp or hiccup at red lights or stops. After the ATF for a week or so it smoothed out completely. I still feel it a little but it go more smooth. Maybe I should have left it in longer but I don't trust it enough to be ok for the engine.

 

Did you see any info on what this is suppose to do? As a general rule oil cannot get to any of the systems that could cause a miss. Its job is to work as a fluid bearing, reduce friction and transfer heat. I have heard of people using ATF to clean the sludge out of an engine but that was older cars that had a lot more forgiving bearing clearances and with older crappy oil. This was used to clean out the top of the motor to keep the oil from pooling and causing a car to smoke. We would only see this on cars that did not get the oil changed.

A modern car is much less forgiving and would most likely have an engine failure before it reached the point it could effect the way it runs.

If it works, it works I am just curious as to how.

 

It was supposed to clean a lifter that is stuck. I guess my car does not have lifters. I got it out of the car. The sputter has lessened. It used to feel like a bubble pop. Not sure it fixed it or helped it but it changed it. It made it less noticeable. I was worried however that it was damaging the motor and that possibly it was destroying something. So while lessened maybe it meant that it was simply ruining it. Engine runs good. Not sure what it did in my motor. I only have 50,000 on the car. Like you I'm not sure what inside would make it sputter. Something that is in contact with the oil.
The car did sit alot. 50,000 in a 2015 it must have sat a lot.

 

Have you tried a fuel conditioner like Seafoam?