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Discussion Starter #1
My 87 S3 has started to backfire after downshifting, particularly from 4th to 3rd gear. I also noticed greyish smoke coming out of the exhaust whenever it backfires. On my daily drive home from work, I shift to 3rd gear when descending a 1 1/2 mile hill and the backfiring has gotten worst over the past few months. Recently, I have been getting 6-8 pops during that 3 minute stretch.

Over the past 2 years, I have replaced or serviced the following parts related to the fuel system:


  • All vacuum hoses (replaced with silicon)
  • Intake plenum hoses (replaced with silicon)
  • Fuel line
  • Fuel injectors (serviced by OK injectors)
  • Spark plug wires
  • Distributor rotor
  • Head gasket
Any ideas what might be going on?
 

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Pull your spark plugs and see if one is fouling, like oil residue on it. Clean them and see if the backfire goes away for a little while. If so, you probably have a cylinder going bad. You can also perform a compression test to see what's going on.

Sorry man, but I do not think there's any good news.
 

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When you go down this hill are you descending with the throttle closed? Under those conditions the computer should cut off fuel to the injectors. No fuel should mean no backfire. If the Throttle Position Switch is misadjusted then the computer would still be telling the injectors to squirt. So, check the TPS.
 

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Discussion Starter #5
Eric,

I tested the TPS according to the Spider diagnosis link and it checked out. I heard the click in the TPS as I activated the throttle and the ohm test was as expected.

I next went out for a test drive on the same route I take to work. I got about 10 backfires on a 15 minute drive. On each occurrence, I was in third gear transitioning from throttle off to throttle on; it never occurred when the throttle was off. It seems more frequent when I was driving downhill, but it also occurred a few times driving level and uphill. I could not get it to occur in any gear other than third.

My daughter said it smelled like fireworks after a backfire.
 

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Late night, random guesswork here...Sounds like intermittent ignition problems...loose connection somewhere or even a bad ignition switch. Does the tach needle fluctuate while you're getting the backfires. Fireworks smell could be from unburned fuel hitting the cat.
 

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I'd be checking primary electrical system, battery connections and hold down. starter main power and engine grounds, coil and plug wire connections, etc. Another track would be to find the exhaust leak that is letting air into the exhaust system, manifold nuts, down pipe gasket, etc
 
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