backfiring-sounds like machine gun

[MXMOVRDV]

Background: engine-stock(new stock injectors,cold air intake, 4-2-1 header's, test pipe for cat[no cat] 3" exhaust to small resonator to dual pipe's with muffler's out back) auto trans., 104000 miles. year old dist, new cap,rotor,wire's,plug's,coil,fuel filter,pcv vlave. THE PROBLEM: under wide *** open throttle around 4300 to 4700rpm's I get a "machine gun" backfire or misfiring. If i let up on the gas a little it stop's doing and run's fine. I spent 3 hour's looking for a vacuum leak.none found. timing is dead on where it should be. Any suggestion's would be helpful and save me some money. Okay I forgot this: EGR Valve still attached and working. everything else is allllll stock and working. Thanks. GO VERT!
Modified by MXMOVRDV at 6:19 PM 12/9/2008

[mrk442]

how are your spark plug gaps?.... had similar problem when i was lazy and didn't gap my plugs in a rush to see if new motor would fire.. it did but i was blowing out the spark at WOT..

just a thought?

and yes they had a ridiculously huge gap from that factory..

[mrk442]

ohh and you might get more help over in the 240sx tech forum or the nissan online mechanic forum.. zeroforum/17

zeroforum/23

then again i post here to most of the time since if you own a vert your automagicly smarter then normal folks..

[Eikon]

MX..

Let's reason this out and see if we can figure out what the problem is.

Backfiring is caused by unburned gas that exits the combustion chamber and then ignites later in the cat or headers or somewhere in the exhaust piping. You can get unburned gas by either having too much fuel compared to air (running rich) or not enough fuel to air (running lean and not having enough fuel to spark) or from having insufficient spark.

So now we have to figure out which of these is happening, and what is the cause of it.

The first solution I would try is exactly what MRK442 advised. Pull your plugs out and check the gapping. Stock is supposed to be .44. But with your go fast mods, if .44 doesn't solve the issue, you might try to tighten that up to .40.

If spark plug gapping doesn't help, I might try a new 02 sensor.. or even just cleaning your existing 02.

If those don't work.. report back and we'll keep working on it.

[MXMOVRDV]

Cool info guys. The o2 sensor was put on when the header/exhaust was put on. I will check the plug's(splitfire triple platinum-original style splitfire) gap and try going to .040. I have thought about the too much or not enough fuel. I do not have any way of checking for this unless I buy an air/fuel gauge to use on the single wire o2 sensor. I will check the plug's and report back here.

[Patarach]

i had the same problem after driving my turbo 240 vert after about about 10 miles, after a quick inspection one of my plug wires had popped out a little bit and i was only sparking on 3 cylinders, it was like once i hit boost my car would just take a **** (power wise) and started backfiring.

[GoinTopless240]

Have you replaced the fuel sock on the fuel pump. First thought is not enough back pressure. NA needs some back pressure to keep new fuel from being sucked out (unburnt fuel). Turbos hate back pressure for the most part. Did you set the timing correctly (with tps unplugged)? Just a thought. Good luck.

[bugity]

i had the same problem all the same symptoms as you i thought everything was wrong but i thought my timing was screwy but the cause of my machine gun backfire at around 5500- 6000 ish rpm was a small exhaust leak by my down pipe allowing air to get sucked in and ignite as machine gun backfiring......p.s. sorry to bring back up an old topic but when i was having trouble this topic helped thanks

[froggman23]

Yeah, I'm haveing the same issues, is there a chance that useing the super high grade gas could cause this as well?

[EPD]

Yeah, I'm haveing the same issues, is there a chance that useing the super high grade gas could cause this as well?

No using high quality gas slows the burn rate so you dont ping.

Sounds like it may be a ecu or possibly a slight timing issue.

[bugity]

it will cause some poping i am currently runnning an sr20det on e85 ethanol which is about 107 octane at around 24 psi of boost. i had to advance my timing slightly via the CAS (rotate clockwise) to stop the backfiring. this also created extremely noticable hp increase.