Well a true exhaust backfire would have no effect on your intake. This would be an intake back fire and they are normally much smaller. What happens is the injector leaks and makes a small puddle in the intake. Valves on older cars do not seal perfectly so if the cylinder next to the puddle leaks a little fire BEFORE the vacuum dries it up the gas you get more of a "Fast burn" in the intake. It sound very muffeled pop if no hoses pop off.

There are so many factors that need to be right to get this to happen, I have had customes that have had it happen every day at the same time and I have had customers only had it in the winter or even once every 6 months. I think in most cases the injector is leaking all the time but the conditions are not right. If you wait long enough the puddle with dry on it own. If the motor is hot enough the puddle drys up very fast ect...

I am not a big fan of tank fuel cleaners because of what you mechanic said, The problem is the cleaner can push stuff thru the filter but there is NO way to be sure it will not get stuck in the injectors. Most shops have a stand alone injector cleaner for on the car injector cleaning. That is how I would have done it. You disconnect the fuel lines at the rail and run the car off this machine.

Injectors can just be plain bad and not a dirt issue.

 

We had it done that way on the camaro that was wrecked. EGR replaced and injectors cleaned. This place doesnt believe in that. I asked about it when I first met them. The other place that does that is only 5 businesses down (Pro Tune) and coincidentally when I first was looking for someone to take on my original electrical issues, Pro Tune recommended these guys which is just a dealership. They moved the shop portion out when the owner of the dealership decided to go out of business. Anyway, this guy (Walt) said that he believed it was took quick of a process and he's heard of some harm with it. I guess nothing is fool proof but since Walt did my motor work and the warranty, I'll go with him for now. Are injectors an expensive job to replace? I know I haven't driven this car no whole lot but this is the first time this has happened and if I had only cut back on the key and tried cranking it again, it may not of done that.

 

I did mean to say the tank cleaner would not work or would not clean your injector. I was just explaining why I would do the new filter.

If it was me and the first round of cleaner did not work I would look for a place that does sonic cleaner them and test for flow and leaking. I think the going rate is around $100 for 8 injectors. I would go to a U-pullit yard and grad and couple extra and just ask for the best 6 back. But that is if the first cleaning does not work.

 

Back out the intake = backfire.
Out the exhaust = afterfire.
Many people mistakenly call an exhaust pop a backfire though!