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Hello and thank you for your question,Are you able to scan the computer system and see if there are any fault codes setting?Have you verified your firing order is correct?
If you sure the firing order is correct and there are no fault codes then It could be possible the intake is leaking but it would have to be a pretty healthy leak. Dose the engine seem to be firing on all 6 cylinders?
Okay when it backfires is it backfiring out of the intake?
The one thing that comes to mind if its backfiring out the intake providing you are sure the firing order is correct could be a valve seating concern. Before you consider replacing the intake gasket I would consider performing a running compression test on all 6 cylinder to see if there are with in the same reading. This would tell you if there is a valve seating concern and should be checked first.
It is very possible and the running compression test would verify if that is the case. The running compression reading is usually about half of the cranking compression reading and should be pretty close to the same on each cylinder. Any cylinder with a valve concern will show lower readings.
It would have to be a very large leak and you would normally get the computer to set some lean codes due to the leak. I would start with running the compression test. In most cases a backfire can only be caused by either ignition timing (spark) or by a valve train concern.