We call this problem a TB (Throttle Body) or Tuberclosis. When it happens, the Throttle Body becomes a combustion chamber as pre-dominated by a spark or valve problem.
As known to us, there is two areas that you need to be checking for:
The most common is the spark plug. Over here, we recommend that Bosch Super 4 or NGK BP6ES to be used. These spark plug can tolerate high temperature or commonly known as 'Cold Plugs'.
Using the right spark plug is important as mentioned by several experts in this forum. The reason is simple.
A poor spark plug (or likely Hot Plug) may be still very hot (glowing hot) and can cause a Pre-detonation to next incoming Air/CNG mixture. Since the Intake Valve is still open on 'Chamber 2', the Hot Plug detonate the gas unnecessarily in this chamber.
Case 1 - HOT Plug
The fire will travel out from the No 2 Chamber Intake Valve and lighting a fire fuse to all the way up to the Throttle Body, and exploding pre-mixed Air/CNG in the TB. Your vehicle should jerk or choke immediately after that. In worst case, your EFI car will have the Throttle Body Butterfly valve blown out or a burnt airfliter
Spark Plug Cable Cross-Fire
Alternatively, using non-original spark plug cable can cause a cross-fire. As the electric coil/distributor gives power (voltage) to Cable 1, the current jumps over to Cable 2 as well. Both spark plug 1 & 2 open fires, and you will experience a situation similar to Hot Plug. But only this time around, the cross-firing is lighting up cable 2 and fire fuse to the TB again.
[b] Intake Valve
Some vehicle has an adjustment Valve Tappet. These vehicle needs to be regularly adjusted if NOT the intake valve will not close tightly. Again, the firing of that chamber will light a fire-fuse (via Air/CNG) from the Intake Valve gap and into the TB. Hydraulic Valve is likely not to have this problem.
Once we experience a special case, the owner of a EFI vehicle complaint about backfire. He has been using his car for 12 months and not such problem before. It just started recently, after he has a radiator flushout. The problem turned out to be - he also replace a non-original electric coil on his EFI-based distributor. We have changed all components and nothing work. Finally we found the problem to be changing the low-amps/voltage electric coil.