Hi I'm new to the forum and was wondering if any one ever had a backfire issue while accelerating? It happens once in a while when I'm opening it up. Happens between 1st and 2nd gear...and its an 05 TL automatic transmission.

 

Any mods? Have you removed cats? Is the last cat broken? Chances are your TL is running too rich or too lean. That's usually a "problem".

Find the hole in your intake/exhaust that's messing up your ratio.

 

Can someone please explain what the heck is "backfire"?

 

Good explanation:
There are typically two kinds of engine backfires, one is generated from the intake manifold of the engine, and the second is expelled from the exhaust pipe at the rear of the car. A backfire is an explosion of sorts in the intake manifold or the exhaust system. A backfire occurs when there is an imbalance in the air to fuel ratio required for your vehicle to operate properly. If the fuel mixture is too lean (not enough fuel) you may have a backfire in the intake, or too rich (too much fuel) you may get a backfire out of the exhaust system. Most backfires are easily repaired by correcting the imbalance and providing a greater or lesser percentage of fuel to the mixture.

Full article:
What is an engine backfire?

 

Only thing I have done is upgrade my air intake to the K&N.