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SM Backfiring like an AK47. Suggestions?

- - - - - backfire AFM Tune

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#21
ECOBRAP

ECOBRAP

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  • Location:Bay Area, CA
  • Region:Nor Cal
  • Car Year:1999
  • Car Number:59

200-800 rpm is a different story than the usual one we see with tuned AFMs.  What spark plug heat range? 

 

Is your dyno tuner an SM guy, or just some guy?  Low RPM is meaningless if you are talking about RPM lower than what you ever run on track at WOT. At part throttle the closed loop will keep you safe.  

 

NGK Iridium Plugs. Dyno tuner is an SM guy. Yes the RPM where I have issues is 99.99% of the time not an issue during a race, however during a restart last race it stuttered once before taking off, luckily didn't cost me a position but it might in the future. As I said, I am not too worried about it unless the backfiring under deceleration has any negative effects i.e. cracked exhaust manifold or unhealthy backpressure if there is such a thing (I am a noob mechanic if you haven't noticed!)

 

CA emissions car?  A/C car?  What is your coolant temp when the behavior starts?  That video (which is apparently a supercharged car) is looking at problems once the fan comes on.

 

Repeat:  "The other clue is to check the idle speed when fully warmed up, and then check it again with the TEN and GND pins shorted.  When everything is in spec, the idle will be the same.  If there is a difference, it means the IAC valve is having to work to keep the idle down - and once the tuned AFM is introduced into this situation, it can produce fast idle and cycling idle."

 

But check the timing and coolant temp when the behavior starts.

 

Yes it is an emissions car, but it won't pass when I have it in SM trim (adjustable FPR, advanced timing, straight pipe). No A/C. It seems the car only behaves like this when the car is warmed up, but can't say for sure because it is intermittent. For example it started hunting back and forth so I disconnected the TPS as per someones suggestion, and the car stabilized. I plugged the TPS back in, and it did not start hunting again, just ran normally as it should. Next time I will check timing.

 

Matt, did you swap the AFM on the car?   Is so the one on when you purchased it was adjusted by Mr SpeedWell.   Are you still running the 5X racing AFPR?  Perhaps something to consider when you get the gauges hooked up.  

 

You can also look at the butterfly value with it closed and test WOT position to ensure as suggested it hasn't been damaged by pressing down to hard when you need extra HP... :)

 

Yes the AFM was swapped when I was trying to diagnose the rough idle when the new motor went in. Forgot to change it back before hitting the dyno, so the one I borrowed was tuned... and it's fast so I don't want to put the old one back in unless I can afford more dyno time! I will check the butterfly valve as well for over/underextension.


-Ecobrap

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