the reason it's backfiring, raw fuel is being injected into the exhaust system. does it backfire regularly? or intermittantly? under load? or during a free rev of the engine?
a mis in the ignition system could load the exhaust up with fuel, but it less likely than a burnt valve or valve seat. or your injector could be still spraying when the exhaust is pumped out of the cylinder (which ever it is) very likely to be the one with low compression. Does the car see top end RPM very often?

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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