Backfiring V8

[stocky]

What causes an engine to backfire? Is it too much fuel or bad spark? Was driving last night, drove hard in first, changed to second, as it went into second and i put the foot down, there was a bit of a bang, ive been told a flame out the exhaust, and then a couple of warning lights and engine went into limp mode. Over fueling possibly?

[azkwazere]

What causes an engine to backfire? Is it too much fuel or bad spark? Was driving last night, drove hard in first, changed to second, as it went into second and i put the foot down, there was a bit of a bang, ive been told a flame out the exhaust, and then a couple of warning lights and engine went into limp mode. Over fueling possibly?

its one of two things... either your running rich as or your timing is up the duff, in the advanced area.

If too much fuel is going into your exhaust manifold becuase of mixture or becuase your timing is advanced to a point where its pushing unburnt fuel out of the cylinder, itll cause it to fire like that.

thats my guess anyway.

aZk.

[stocky]

do you mean cam or dizzy timing? dizzy timing was 10 degrees last i checked, but i will have another look once i get a chance to. its weird thats its only ever done it once though. havent been able to get it to do it since

[azkwazere]

cam timing... you need to get fuel into your exhaust somehow for it to backfire.. soi would suggest that the exhaust valve is opening to early.

if its not doing it constantly i wouldnt worrry to much, may have just been a fluke.. keep an eye on your fuel usage though...

aZk.

[Cursed CX8]

stocky, get that cam checked, How well did you dial it in?

[ari666]

hmm, im not that great with efi in these things, but what azkwasere said is getting into the right field. back firing is usually unburnt fuel (hot as hell) getting into your exhaust and catching fire once it gets enough oxygen. so it may pop up near your heads or may pop once the explosive hot gas gets to the back of the exhaust tip. it depends on how much unused air is in your exhaust.

(i know poeople hate skylines but...) skylines 'pop' all the time when some muppet fits a blow off valve to it, cos' the blow off valve vents inlet plenum pressure into the atmosphere. the engine takes a second to realise that its not gonna get the air that it needs so it keeps feeding the engine a nice rich mixture, unburnt fuel gets into the exhaust, finds the air it needs in the exhaust and explodes.

with your app i really couldnt say cos' i dont know how the injector duty cycles work on. it does sound like the ecu didnt recognise that you were decelatating and kept up the rich mixture when it should have leaned.

have you checked TPS? does it have throttle pisition sensor? TPS, AFM and O2 work together to determin the mixture via duty cycles, maybe they are not talking to each other. i dunno if it would be a spark issue but you cant really rule it out. would be looking more towards the mixture system personally. could also be a dud injector over squirting, but that would be quite rare.

to be honest, from the other probs you have been mentioning, i would be getting a good vac guage (you can borrow mine for a while), running a vac line into the car and see what is going on with it cos i wouldnt rule out a manifold leak.

anyways, i hope that helps.

[vyjess]

hey i like that word "muppet"

[Commydoor]

back in the old days rule of thumb was backfire through the carby was electrical backfire out the exhaust was fuel so like the guys say check your fuel...what mods are done?...is it possible the injectors at that rev are maxing out to lean and when you back off are still fully open allowing that bit extra through and when you plant it again its igniting that unburnt fuel??

[faroogoo]

I broke a rocker arm before that caused backfiring through the carb,dont know if it has any relevance here though.

[Commydoor]

im trying to picture it in my minds eye lol...you break an exhaust rocker arm the valve closes the piston pumps the gases back out the inlet valve and fills up inside the manifold and exits out the easiest path...the carby?...

[ari666]

when the hell are you gonna drop by and let me play with this engine?!?!?!

[stocky]

when my uni exams are over couple more weeks and ill have 3 months off to fiddle with the thing, possibly new manifold time

[ari666]

it may be something as simple as a perished vac hose....

[VNracer34G]

How good are your plug leads and are they separated properly? If the insulation on the leads has degraded you can get crossfire- electricity will allways take the path of least resistance. Had a customers 253HQ many moons ago that was backfiring and running like crap, found the muppet (yeah great word) had taped his leads together to make it look tidy. I put some nice lead seperators on it and it flew, came back a couple of days later missing again-taped up again. Fixed it and explained the cause and how he could cause serious damage with the wrong plug firing, two days later was towed in with a broken crankshaft and taped up leads- the things ya see when ya haven't got a gun!