The backfire is an explosion that you hear coming out of the exhaust, not a backfire into the intake manifold?

Actually, whichever it is, I would suspect that you have a fuel injector that is leaking down the rail fuel pressure after you shut off the car. You have a big slug of unburned fuel going through when you restart the car.

Try some fuel injector cleaner (I like Techron) and see if it makes a difference.

Try unpluging the cold start injector and see if it makes a difference.



I've seen posts of recomended shops for refurbishing fuel injectors, and having them done is a good opportunity to replace those pesky injector O-rings that tend to cause vacuum leaks.

By the way, unless you live in the FAR north, you don't really need that 5th injector. When my brick was new, the time/temperature controller failed so I had to unplug the injector to keep it from running all the time and flooding the car. I started my car in Iowa at 10 below zero with just the main injectors.