1994 Explorer pinging with smoke

I have a 1994 Explorer that until recently has run great. On three separate occasions over the course of the past week, the engine has started "pinging"
for a few seconds followed by a large puff of smoke from the exhaust.
I have changed the plugs and wires, cleaned the MAF and have added fuel injector cleaner to the tank.
What else could be the culprit? Like I said it's intermittent but when it happens, it leaves a large cloud of smoke behind. By the way, the vehicle has 143K miles.
Thanks in advance,
Chris
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Timing chain tensioner going bad??? Do a search on rec.autos.ma...ford to check out this possible problem.
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