What causes backfire?

[smagnotta]

I set the timing as per the TSRM and it smoothed out my idle but still have the backfire problem between gears, especially between 1st and 2nd.

What causes the backfire upon deceleration?

(I have the Lexus/550, S-AFC, HKS SSQV BOV, new plugs, new coil packs, new coil pack ground wire. The AFM is a new Suprasport Lexus AFM replica.
I also have the Zeitronix Wideband and a TANABE CAT-back.)

I figure too rich? Or is this just a function of the turbo spooling and having a 3 inch exhaust versus the stock exhaust (in other words is this normal) ?

 

[LamaRossa]

your after market BOV cuasing rich condition when you shift venting metered air to the atmosphere making the computer conpensate the lose air with extra fuel

let off the gas totally and wait for the bov to blow off all the air in the system when shifting in to 2nd will stop back firing


Lamarossa

 

[smagnotta]

your after market BOV cuasing rich condition when you shift venting metered air to the atmosphere making the computer conpensate the lose air with extra fuel

Thanks but my BOV does not vent to the atmosphere - it recirculates. I think it is too rich and needs tuned some more...

After 6 years, the car is finally where I want it to be -- soon I can go do this :drink1:

 

[bluemax]

Mine almost back fires when I let off the throttle to shift gears or just letting off at high RPM.
Its almost stock GTE. I just have a modified CT26, K&N, HKS exhaust.

 

[Bigdough666]

U have any cats? Mine backfires almost everytime I shift because its running SUPER rich (untuned) and I run no cats. WIthout a cat, excess fuel (from running rich) builds up in the exhaust, then when you let off the gas, it combusts, POP - backfire. Shoots a cool lil fireball out of the exhaust pipe, but its annoying. When I get the rest of the fuel system Im heading to the dyno to do some tuning with the S-AFC.

 

[foofers]

Does backfiring due to bov venting to atmosphere cause any damage to the engine?

 

[Ckanderson]

also make sure your TPS is set correctly.

 

[MisterTurbineTwister]

"Does backfiring due to bov venting to atmosphere cause any damage to the engine?"

Yes it can.

Any time the engine allows unburned fuel to pass the combustion cycle without being burnt, the fuel can wash down the cylinder walls and dilude the oil and create more wear on the rings and cylinder walls.

Having a turbo, it can also damage bearing seals within the turbo, damage O2 sensors, catalytec converters, valve seals and even valves and seats if left for a long enough time.

The computer (ECU) has little to nothing to do with the bad air to fuel ratio given when the BOV vents into the atmosphere.

The reason that it runs rich on BOV operation is because the ECU has allready metered the air before the BOV opens and the Air flow meter can not detect air escaping the system after the meter (or entering the system for that matter) and that means the ECU has no way to know that it needs to correct for the lack of air entering the engine when the BOV is functioning.

I suppose you could make some sort of adjustment for it in a stand alone ECU that adjusts the Air to fuel ratio when RPM is suddenly dropped or perhaps put some kind of sensor at the BOV, but it's just not worth it.

Simply plumb the air back into the air intake system behind the air flow meter in front of the turbo's intake impeller and you will not have this problem.

The turbocharger needs a pressure release when the engine RPM drops off because the exhaust turbine of the turbocharger slows down due to lack of exhaust to propel it. This means that the positive pressure inbetween the intake impeller wheel of the turbo and the throttle body will fight it's way back into the turbocharger causing surging and overheating of the turbocharger. This is where venting the pressure takes place.

A BOV is one way to do it, but a diverter valve (like your stock setup) routes the pressurized air from behind the intake impeller wheel back in front of it to equalize the pressure surrounding the intake impeller wheel, relieving the positive pressure built up behind the wheel at RPM drop as needed.

 

[bluemax]

I run a NA-T motor with the NA cat and a 85mm HKS exhaust.
I'm thinking the 85mm exhaust is my problem. I guess I could put on the 65mm exhaust I have before the NA-T conversion to test my theory. But that's too much trouble.

 

[MisterTurbineTwister]

If you are hearing exhaust scavenging between shifts on a N/A motor, that is normal. If you are experiencing actual backfiring due to unburnt fuel, your air to fuel ratio needs to go leaner.

Don't ever go to a smaller exhaust system if you plan on a forced induction system. Backpressure is good to a certain degree on N/A cars, but if you have the bigger exhaust system in it now for your planned turbocharger, just keep it.

 

[Fastway135]

Do autos with ssqv backfire under any circumstances?

 

[NDBoost]

im auto on open 3" DP @ 12lbs w/ Greddy Type S BOV, & Apexi Intake. Under WOT or repeated on/off of the throttle i backfire from time to time.

SOmetimes its horrid and i can feel the backfire in my stomach and chest lol..

 

[jetjock]

By "backfire" I'm assuming you mean muffler explosion. The term backfire refers to combustion in the intact track. ME is always caused by fuel being combusted in the exhaust system. You have a rich mixture, a misadjusted dashpot, an overheated cat due to the rich mixture coupled with exhaust leaks, or perhaps a TPS problem. Since the ECU shuts off the injectors during decell it could be any of several things.

 

[whopwood]

The fix is to go with a maft in blow-through mode or a maft-pro in blow-through or speed-density mode. Then your ECU will stop seeing air that isn't there.

Or just shift differently so it doesn't pop

 

[IJ.]

I always thought the ECU had a fuel cut on trailing throttle?

 

[jetjock]

I always thought the ECU had a fuel cut on trailing throttle?

It does. What part of "the ECU shuts off the injectors during decell" didn't you understand?

 

[IJ.]

LMAO the bit that I didn't actually read !
(hey it was first thing in the morning and I'm NOT at my best)

 

[jetjock]

Lol, just yankin yer chain mate. At least you know the difference between a quart and a liter

 

[IJ.]

LOL yep "not very much "

 

[tlo86]

Do autos with ssqv backfire under any circumstances?

just recirculate it back into the accordian my car died at really high rpm's, and ran rich (check your spark plug, will be covered in carbon, smell it maybe... don't taste)