New spark plugs, wires, and fuel filter = backfire???

[Doug Yoder]

Patient is a 1993 100s - FWD, 12V V6, ~163k mi (my sisters car).

Complaint of low power.  I asked when plugs & wires were last done, the
reply was "not since I've owned it".  Replaced them, the insulation on the
plug ends of the wires was almost gone on 5 of the 6 wires.  The old plug
electrodes were pretty worn, too.  Also replaced fuel and air filters, since
no history on them (air filter was pretty dirty too).  I also suspect a bad
head gasket on one side, as there was evidence of coolant splattering the
engine bay, and the coolant reservoir hissed when I loosened spark plug #6.

Put it all back together, started it up, and heard this horrible popping
sound coming from the exhaust.  Sounds kind of like misfiring into the
exhaust, but the engine itself is smooth.  STRONG exhaust puffs out the
tailpipe coinciding with the popping.  Seems almost like extra fuel in the
exhaust system is burning off, but if so, where did it come from?

I'm afraid to drive the car right now, it sounds like the exhaust is going
to come flying off the car or something.  Anyone have any ideas?

-- 
Doug Yoder

"Don't tell me the sky's the limit - there's footprints on the moon."