Exhaust/engine backfiring (petrol)- ideas/experiences?
On accelerator lift off whilst stationary the exhaust backfires. You can hear this when sat in the car but mostly out back behind the exhaust. Its not a ‘bang’ but its definitely backfiring. The spark plugs have been changed (they were well passed their best). Before I change the coil pack I was thinking could there be a fuel rich/build up in the system still (i.e. in the cat)?
Bad spark plugs basically mean they aren’t sparking properly and as such the engines been running ‘lean?
If I run a full tank of high RON fuel on a long run and/or put two bottles of fuel additive in to ‘clean’ the system through will this help?
I really don’t want to spend circa £80 on a new coilpack if theres nothing wrong with it and in reality the cat’s fub’arred. i.e only spend money once rather than twice to fix one issue.
Anyone had experience of a petrol backfire and how did you cure?Posted 4 years ago
Give it an italian tune up, preferably on a quiet section of road and hold it in 2nd/3rd till it bounces off the rev limiter (once fully warmed up of course)
It could be carbon build up in the exhaust valve chamber that ignites the unburnt fuel – why is there unburnt fuel though? – faulty air flow meter?.
Impossible to diagnose with any certainty without make/model year etc of carPosted 4 years agoglobaltiMember
Backfiring can sometimes be caused by a breathing problem – either a leaking inlet manifold or exhaust manifold, I can’t remember. But you’d hear a leaking exhaust manifold; even a small leak sounds like a tic tic tic in the engine bay. I think it can also be caused by a leaking valve – how do the exhaust pulses feel when you put a hand over the exhaust pipe? Is there a smell of petrol in the exhaust even when warmed up? A compression test would confirm a damaged valve.Posted 4 years ago
the engines been running ‘lean?
If anything, it’s running rich as not all the fuel is being burned during the combustion process.
back fire is unburnt fuel in the exhaust burning on contact with hot manifold etc.
My wife’s Golf has 4 coil packs and when one went she had similar symptoms but they were able to diagnose it at the garage using their ‘puter and the engine management system.
Pointless spending money on clever flushing if the problem is electrical?
 put your hand (or have a helper do it) over the end of the exhaust pipe you can usually hear if a gasket is blowing then and if engine doesn’t die immediately there’s def a leak somewhere.Posted 4 years agothisisnotaspoonMember
In addition- on ‘stabbing/releasing the accelerator theres a crackling noise from the engine side. Thats the coils?
Sounds like an exhaust leak to me.
Low MPG would imply a fueling issue, MAF or Labda sensor. A leaky exhaust or poor combustion could both kill the labda sensor so I’d start there (and figure out the leak or it’ll do it again).Posted 4 years ago
Cheers (using the below link/scanned through) I’m not getting any of these really apart from a slightly down mpg
I’m going to aim for leak first. If I block the exhaust (temporarily) it wont cause any damage?!Posted 4 years ago
pop it into kwik-fit – they’ll get it on a ramp while you wait and have a good look underneath for you.
Its a V12 AWD bi-turbo Picasso so of course its got 4 exhaust pipes
shoes and socks off and feet over two holes, hands over two.
weird – I did an edit and got a second post…Posted 4 years agomaxtorqueMember
On accelerator lift off whilst stationary the exhaust backfires.
Eh? what does that^^ even mean? You are reving the engine in neutral, and if you back right off the accelerator, with the engine at high rpm, it “bacvkfires”?
If so, and there are no misfires when you are actually driving under load, who cares?
(i could go into a long complex explanation of transient fuelling characteristics and DSFO (Deccel Fuel Shut Off) but i’m not sure there is actually anything wrong!)Posted 4 years ago
Its hard to describe the noise from when you are sat in the drivers seat revving – as you come off the accelerator – its a sort of cascading (say 3 sounds)- its a plastic/catching (possibly crackling) sound. Stood at the rear its a soft-backfire sound out of the exhaust. I’ll try and capture/record the noise this weekend.Posted 4 years ago
My car makes that crackly/popping noise on the overrun when changing down as the rather basic K-Jet injection system catches up with the metering head in the air box but it has a ported and flowed cylinder head with rather lairy cams, a custom 4 branch stainless manifold and larger fuel injectors, i doubt your picasso has similar so i can only come to the conclusion that it is possibly over fuelling and the unburnt vapours are igniting on the exhaust side, i’d start looking for problems with the lambda probe/cat check/carbon build up on valve seats etc.Posted 4 years ago
Here it is/listen through:
Yes I have odd taste in shoes.Posted 4 years agomaxtorqueMember
So, assuming you have no misfires under load, when you back out suddenly on the throttle, the sudden fall in intake manifold pressure as the throttle closes, pulls the fuel puddle mass off the walls of the intake manifold, this transient “extra” fuel gets pulled through the engine, resulting in a rich mixture in the exhaust system. In conjunction, the spark mapping almost certainly retards the ignition timing, resulting in higher exhaust gas temperature. As this flow of rich, hot gas meets areas of higher oxygen content it can ignite, and burn. Sometimes (at low loads/temp) this is a gentle sort of “whumpf” kinda burn, at higher loads (like you get on a racing car for example) you get the full “bang” as the mixture explodes much more violently. As at idle, your exhaust system volume is large there will be plenty of trapped pockets of “fresh” air, to cause this, especially if the engine transitions into DFSO during the run down (and in effect is just pumping clean air into the exhaust). Of course, you may have an exhaust air leak as well.
If you have a scan tool, pull up the long and short term fuel adaption values and see where these sit.Posted 4 years ago
Sometimes they only rattle at a particular rpm frequency, i.e. between 4000rpm to 4500rpm or suchlike, definitely a heat shield issue or possibly loose baffles in one of your silencer boxes that are rattling but it’s hard to narrow it down to either on that vid.Posted 4 years ago
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