Twice in the last several months, I thought I heard a onetime backfire at start up. No codes , stock C7,stick.

 

A backfire is when you get combustion in the intake manifold rather than in the cylinder. Given that it almost never happens with port injection I seriously doubt we will ever see that with direct injection.

If you are just talking about a pop sound in the exhaust when it initially hits it is probably just moving air. It's especially loud with the NPP exhaust, which maybe you have.

 

Right , I do have NPP. Thanks

 

And even more prevalent with NPP combined with long tube headers.

 

I get it once in awhile on cold starts and it sounds like a gunshot. Loud backfire, not air moving. Has me a little worried.

 

Again, a BACK fire is combustion in the intake manifold.

You are trying to say you are having combustion in your exhaust system on a cold start but this is impossible. For combustion to occur in the exhaust the pipes need to be very hot, like you just ran around a road course at full throttle hot and have unburned fuel (poor combustion quality) getting into the exhaust.

Your cold exhaust system has plenty of moisture in it and dead air. When the engine fires to life it blows it the moisture and dead air out in one shot and then airflow goes to normal and sounds like regular exhaust does.

When you let it sit overnight, go start it and then jump out and go stare at your exhaust tips while it idles and look at all the condensation it spits out until the exhaust gets hot and evaporates the rest, it's the same reason your car exhausts steam in cooler weather the first 10 min of driving.

When you start your car the stagnant air and moisture get blasted with sound wave/exhaust flow and is loud for the first RPM or two and then it's normal sounding (aside from the higher idle RPM used for quicker warm up).

 

My C7 has backfired big time on start up - more like a cannon. It set a check engine code when it does this. Problem is GM cannot seem to pull the code when this occurs. Car has been in the shop more than in my garage and the tech has driven it more than I have. No solution yet. Lemon Law clock is ticking away...

 

Might be a BCM code related to NPP exhaust, do you have it?

 

You and I seem to have the same thing. I am 68 years old and have played with cars all my life. It's not normal. On my car it only happen on the first cold start of the day & only about once every three weeks. If you find a code please pass it on. The first couple of times it happened my wife ran into the garage because she thought I had a negligent discharge from the .45 I carry. I am sure the dealers are aware of this complaint and will either find the cause. My only concern is doing damage to my engine that will not show up for awhile or rob me of a few HP. I think I will set up a recorder in my garage that will capture the sound. Also keep a log. Good luck.

 

have you ever had a direct injection engine or an NPP style exhaust?

 

Yes and Yes.

 

I am also from the generation that brought you lake pipes, dump tubes, cutouts, headers, mechanical fuel injection, four speed transmissions etc. I have had turbo chargers, superchargers, duel fours, tri power etc. I think folks like me are using the term backfire as a generic word for a loud bang not combustion in the intake. We are familiar with automobiles and know when something is so wrong there is a problem.

 

just wondering, because my car fires up with a loud pop as well, it has since new and now it is not stock and still does it. i have personally recalibrated almost every table in the ECM and it still does it.

i just don't think it's an issue, never did. it IS different from previous gen cars/engines but it pops into life and is otherwise nothing. i start my car maybe once every couple of weeks and it does it every time. when i drive it somewhere and let it sit while my wife and i eat or whatever it doesn't do it (referencing warm startup table values). i think worst case scenario, the tune commands more air and fuel than is necessary at cold coolant temps and really tapers off when warmer. i will post the tables later this evening unless i forget.

i am interested to find out what code the other guy is throwing to see how it could be related. interesting that there is no code though...

 

Dont forget about wot shifts that also bang the exhaust like a shotgun or a blow out it is rather regular compared to start ups.

 

Is there a way of getting the car to do it consistently? Mine seems to occur when it's colder outside and it's random. I have a manual transmission. I shift near redline and sometimes it pops and sometimes it doesn't. My NPP is set to off.