Why Do Skylines Backfire?

[harryg]

My car occassionally backfires when I shift between gears. Can anybody tell me why this happens?

I have been always from Day One been putting High Octane 98 petrol in it.

What causes a backfire?

[PHATR32]

1 of the things that causes backfiring is running a rich a/f mixture. basicly unburnt fuel goes in the exhaust manfold where it explodes.

another cause is no cat, which muffles small pops and little backfires which you normally dont hear.

another is running a venting bov on a car with a sesnative afm. dunno about the lines but on my turbo laser, a vented bov made it back fire like nutz cause the computer would measure the air coming, and add the fuel needed, but this air would leave the bov so the a/f mixture would go rich, therfore cuasing a backfire.

i thinks thats all right

steve

Edited September 20, 2005 by PHATR32

[harryg]

Does this mean the ECU is not working correctly? Or is it the oxygen sensor?

My car has catlyst converter, and no bov so I guess it's because the fuel is exploding in the exhaust.

How can this be fixed? It's really annoying especially when you're giving it a bit to kick some ass.

Edited September 20, 2005 by harryg

[PHATR32]

i guess its just running rich, so it may need a bit of a tune, although a stuffed oxy sensor can cause it to run rich.

check the ecu for error codes, it should tell you if something is wrong.

otherwise get a pro to check it out.

hows the fuel economy?

steve

[Bluprint]

I was once told by a fairly reputable mechanic that factory Nissan ECUs are tuned to run rich as all buggery in the top end, I guess that might explain it?

[harryg]

Everyweek, I put about 40-42 litres of petrol and lasts me for 350-400Kms, so it's close to 10-11litres per 100Kms. I have never run the car down to the red light to get a more accurate fuel economy from a FULL tank.

However, the backfire is only occassional like once a week and normally happens when I shift from 2nd to 3rd around 4500 rpms.

Also, about 2months ago, I had full injector service done. But is used to backfire before that as well, so i don't think I can blame the mechanic.

[Mr. Oizo]

Running rich.

Unburnt fuel goes through the exhaust.

Heat of the exhaust lights the unburnt fuel.

Boom.

[someonestolecc]

I reckon it's a great sound on occasion.

Might have something to do with the way you take your foot off the accel too. If you take it off abruptly at a certain RPM (where it's richest) it's more likely to backfire.

[PHATR32]

someonestolecc has a good point too.

e.g the engines up at 6000rpm, injectors are pumping the fuel, then you suddenly let go of the tb and it snaps shut,

it takes a split second for the injector to stop the fuel flow, which results in a rich mixture, which causes the pop (or bang) (or flame)

steve

[rukus]

Everyweek, I put about 40-42 litres of petrol and lasts me for 350-400Kms, so it's close to 10-11litres per 100Kms. I have never run the car down to the red light to get a more accurate fuel economy from a FULL tank.

However, the backfire is only occassional like once a week and normally happens when I shift from 2nd to 3rd around 4500 rpms.

Also, about 2months ago, I had full injector service done. But is used to backfire before that as well, so i don't think I can blame the mechanic.

<{POST_SNAPBACK}>

there's nothing wrong with a misfire here and there

as long as it doesnt backfire while u're accelerating, it's all good..

[WazR32GTSt]

what about timing too retarded?

[harryg]

there's nothing wrong with a misfire here and there

as long as it doesnt backfire while u're accelerating, it's all good..

<{POST_SNAPBACK}>

Thanks rukus, for clearing that up. And no the car never backfires while accelerating only on occassions while shifting.

Maybe I am letting the accelerator go a bit too quickly at high rpm as mentioned above.

Thanks to everyone for their contribution

[dezz]

backfiring sounds mad when you're gearing down!

lol

[sl33py]

What about while ur idling at the lights? Every 10-15 seconds... and bad enough that it feels like someone's kicking u in the ass of ur pants lol. The whole car seems to jolt sideways.. That's what i have and it annoys me. Regardless of whether A/C is on or not (thought it might be related since it cuts in and out and makes the engine do stuff while idling).

[Big Rizza]

What about while ur idling at the lights? Every 10-15 seconds... and bad enough that it feels like someone's kicking u in the ass of ur pants lol. The whole car seems to jolt sideways.. That's what i have and it annoys me. Regardless of whether A/C is on or not (thought it might be related since it cuts in and out and makes the engine do stuff while idling).

<{POST_SNAPBACK}>

faulty coilpacks or sparkplugs?

[Iron Chef]

harryg, another tip - be quicker on your gearchanges...

Mine does it a fair bit too, expecially when the exhaust is good and hot. My car was complianced under SEVS, and therefore has a new cat - given how well it flows with the cat-back above 5000rpm, I'd be curious to know how "high-flow" that cat really is!

[GTsauRus]

My gtr had an atmo bov, had to plumb it back for compliance since then it backfires on up changes only when the bov opens ie high boost

I think when the air decompresses it back flows through the afms causing rich mixture because the afms cant tell the reverse flow from forward.

[sl33py]

Well just bought the car and during the test drive(s) the engine cut in and out from 4k to 5k rpm. They changed the plugs which they said were incorrect model and also had wrong gap and after testing again, it pulls hard and fast through that range now. So.. the plugs should be fine. @ coilpacks: they tested them and said they're fine too.

A mate reckons it's just the engine running rich to "help cool the turbo down" after a "spirited" drive. Not sure of the logic there.. and noone else seems to get this 'kick' during idle on a stock engine.. seems mostly to do with post BOV installation and other engine tinkering!

[TTT]

no bov

<{POST_SNAPBACK}>

having no bov on a car that came with a plumback bov like all R32 and R33's, is like having an atmo venting bov.

your car still runs rich for a split second during gear changes or when you back off the throttle.

to fix it, fit a stock bov and plumb it back like it should be and the mixtures will be fine and no more backfire.

[datzlr]

change the coolant temp sensor