My car is 940 and non-turbo.

I've been having this backfiring problem for a long time. I've posted this problem before and checked several things suggested by fellow brickboarders but the problem remains.

I've inspected the following items:

Spark plugs - all wires are okay with ohm readings of 6000 to 7000. One of them has reading of 30,000. Replaced this wire. Spark plugs are okay too.

Air Mass Meter - Replaced

Vacuum hoses - Checked and found to be okay, no leakage.

Distributor cap - Checked and sand papered those metallic points inside cap. No cracks and no carbon built up in the cap. Rotor was sand papered too.

Bosch ignition coil - Checked and no damages. Ohm readings taken - primary 3,000, and secondary 15,000. Are these readings okay? Cable also okay.

Throttle body - Cleaned and adjusted.

There is no jerk while car is in motion. If ever there are any popping sounds while driving, these cannot be heard in the car cabin. The sound is loud only when the car is stationery (gear at N or P) and when I bring the tach up to 3000rpm (or more) and then letting go of the gas pedal promptly. It goes boom, boom, boom, boom (intermittent boom for several times) Sometimes there are crackle sounds.

Also the fuel consumption has increased noticeably. The exhaust pipe from the exhaust manifold was changed, the mid and rear muffler was changed and most recently the tail pipe was changed to one with a bigger diameter (meant for the turbo version).

What else can cause backfiring? O2 sensor? (diagnostic shows no problem). Someone suggested the ignition coil. Is this possible? I have logged 106,000km and it's the same coil. What about valve clearance? Can this cause backfiring? If any of the above items can be the culprit, please tell me how to test whether they are faulty or not.

Any and all help is/are appreciated. Thanks in advance.

Laozong.