This is not meant as a flame (no pun intended), only a little
clarification, without getting too technical -
I . . .disagree with the following comment -
[the car is supposed to backfire on the "overrun". "]
When the one on my Truck failed (1.8 liter inline four), I got a nasty,
VERY loud backfire whenever I let off the gas, especially during shifts
when I was accelerating hard, which was most of the time with that small a
motor :-). When I replaced the non operating "diverter valve", the
backfiring stopped. That's WHY it's there. On my truck it's actually
called an anti backfire valve, but performs the same function as the
diverter valve. (vacuum controlled air diversion or re-routing so to speak)
it's there to PREVENT backfiring, *NOT* to prevent a controlled burning in
the exhaust, which is what the air injection is supposed to accomplish.
Early air injected, emission controlled vehicles are purposely designed to
run a richer fuel/air mixture, so there is additional unburnt fuel in the
exhaust (just outside the combustion chamber, in the exhaust header or
manifold) to actually promote this burning, when the air is injected, and
just one of the reasons that early pollution controlled vehicles got a bad
rap for poor fuel economy. Backfire, SEVERE backfire, not the nice
popping/burbling sound from a cam at deceleration, is an indication that
something is not operation correctly. Were talking about a violent
explosion, at the tail end of the car, not a controlled burn. No auto
manufacturer would actually cause it to happen, besides the damage it could
cause, the noise would be unacceptable in any production vehicle, (we *are*
talking about production, not racing vehicles) and would not be considered
"normal operation". If it's backfiring LOUDLY, at ANY time, something is
wrong. The purpose of all those controls, is to promote controlled
burning, and reduce pollution of some form or another. A loud backfire is
an uncontrolled burn, something definitely not the norm-
Barry Schwartz (San Diego) bschwart@pacbell.net
72 FI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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