[T3] Backfire Cause and Effect

[Dave Hall]

The most common cause of a backfire on over-run (ie foot off gas and car
slowing) is an air-leak into the exhaust system, but I've also had it happen
when I replaced an injector with a NOS one due to a fuel leak around the
plastic of the old injector.  It happened a few times, but then stopped
happening, presumably once the seals swelled a bit.

If the oil smells of gas, maybe it was caused by a leaking injector, but if
the extra exhaust noise started with the backfire, it sounds more like some
damage was caused to the system with the backfire.  Take a good look and
listen when you next start it - I supect you will find a hole.  
If you're running a stock system, it may be the insides of the damper pipe
got blown out - an empty pipe with no internal damping sounds much louder -
the damper pipe makes the exhaust much quieter.  I doubt it could happen
easily on a Type 3 with the tailpipe bend, but that was common on the
straight-through Beetle tailpipes. 

Keep an eye on the oil level for a while (a good idea anyway!), and keep
smelling the dipstick for gas.  An oil/gas mixture will feel colder on the
finger as the gas evaporates, whereas a layer of oil tends to feel warmer as
there's no water loss by evaporation from your finger.

Dave
UK VW Type 3&4 Club
===================

-----Original Message-----
From: type3-vwtype3.org [mailto:type3-vwtype3.org-bounces at lists.vwtype3.org]
On Behalf Of J. Jonik
Sent: 06 June 2015 23:38
To: type3 at vwtype3.org
Subject: [T3] Backfire Cause and Effect

71 Std FI...Cruising on highway at around 70 mph, had to do a sort-of
quicker than usual slow-down, just taking foot off the gas...no clutching. 
Ka boom boom..   Two backfires. Mental note to check timing, points, dist
cap, spark plugs etc...BUT, the loud Harley I thought was coming along
behind me was...me.  It sounded like there'd be a nice visible hole in the
exhaust but looking at everything visible, under and over and at pipe
connection at the next stop found nothing obvious.   It was still too hot to
feel around.
 It's a deep-sounding loud "exhaust leak" (or....).    The car made it back
the 20 some miles  home driving at 60 mph)...but with a couple instances of
a kind of hesitation...like more effort, instead of smooth flow, that came
and went a few times.
 Will check muffler by hand for leaks after things cool down.  But, what
else, maybe more serious, could it be? That is...what harm can a backfire do
to an engine?

PS:    Not sure if car was quite level when checking oil before this
trip...but level was just a tad ABOVE the "full" mark.  Will double check,
but what if some fuel is getting into the oil?  Would that cause backfiring?
   Then.... to look up info about Why Fuel Gets Into An Engine. Happened
before...but forget solution(s).
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