Exhaust/engine backfiring (petrol)- ideas/experiences?

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  • Exhaust/engine backfiring (petrol)- ideas/experiences?
  • hora
    Member

    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?

    Premier Icon BadlyWiredDog
    Subscriber

    Do you not have a simple diagnostics gizmo you can hook it up to?

    craigxxl
    Member

    Exhaust leak manifold or close to it on the exhaust itself?

    Premier Icon somafunk
    Subscriber

    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 car

    globalti
    Member

    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.

    hora
    Member

    I don’t have a gizmo and theres no emmissions/EML etc showing.

    Checked the exhaust- can’t see a leak. If there is one I ‘think’ its high up right back the manifold/semi-hidden.

    Premier Icon wwaswas
    Subscriber

    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?

    [edit] 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.

    hora
    Member

    back fire is unburnt fuel in the exhaust burning on contact with hot manifold etc.

    Yes- correction rich not lean.

    Premier Icon BadlyWiredDog
    Subscriber

    You might also, if it’s been running very rich, have killed the Lambda sensor or similar. I did that with the ‘rado once when a coil pack was dying. What fire-breathing, hot-rodded monster of a car are you driving?

    Premier Icon wwaswas
    Subscriber

    “Are you sure it’s a bit rich?”

    hora
    Member

    Citroen Picasso BI-turbo V12 AWD

    (some of the above may not be true)

    In addition- on ‘stabbing/releasing the accelerator theres a crackling noise from the engine side. Thats the coils? The weird thing is the car is not down on power/sluggish but it is lowish on mpg.

    Premier Icon Rusty Mac
    Subscriber

    You could try a diagnostic tool, they are not too expensive these days.

    You can get Bluetooth ones that work with android phones likeThis

    and the free carista app from the play store.

    or if you are an apple fanboi you’ll need a wifi ones like THIS

    Rockhopper
    Member

    Backfiring like this is almost always an air leak into the exhaust somewhere (probably close to the manifold).

    Premier Icon molgrips
    Subscriber

    Don’t think it’s anything to do with the ignition circuit or coils. It’s either sparking or it’s not. The timing could be messed up I guess..?

    Bloody petrol nonsense, diesel is so much simpler 😉

    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).

    hora
    Member

    Cheers (using the below link/scanned through) I’m not getting any of these really apart from a slightly down mpg

    http://www.ehow.co.uk/list_6005864_symptoms-faulty-lambda-sensor.html

    I’m going to aim for leak first. If I block the exhaust (temporarily) it wont cause any damage?!

    Premier Icon molgrips
    Subscriber

    No. But don’t listen to me 🙂

    Premier Icon wwaswas
    Subscriber

    well, don’t bung a potato up your exhaust (ahem) and then rev the engine you’ll take out someone’s upstairs window.

    Just a hand placed over it and see how quickly the engine dies and if you can hear a fluttering noise from the front of the car.

    hora
    Member

    Quick-test, it doesn’t stall.

    Premier Icon wwaswas
    Subscriber

    It’s not a twin exhaust is it? 😉

    does suggest that exhaust gases are escaping where they shouldn’t?

    You’d normally expect a bit of lumpiness after you block the exhaust even with a bit of a hole somewhere.

    hora
    Member

    I could also hear something further up- I’ll need a second person of course.

    It’s not a twin exhaust is it?

    Its a V12 AWD bi-turbo Picasso so of course its got 4 exhaust pipes 😉

    Premier Icon wwaswas
    Subscriber

    pop it into kwik-fit – they’ll get it on a ramp while you wait and have a good look underneath for you.

    Premier Icon wwaswas
    Subscriber

    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.

    [edit]

    weird – I did an edit and got a second post…

    Premier Icon binners
    Subscriber

    Its French. Its just its way of letting you know its disdain for you

    Premier Icon wwaswas
    Subscriber

    Is it a Pfffft! sort of sound?

    hora
    Member

    Hey Monsieur! I hear you want a road bike huh? Well you are too stocky and fat to grace such a beautiful french race beauty so I am going to take that money away from you Adieu!

    Is it a Pfffft! sort of sound?

    Yep and easily audible even stood right at the back.

    Premier Icon pictonroad
    Subscriber

    It’s cold, the engine will be adding more fuel to richen the mixture, if you release the throttle quickly I’d expect a bit of fuel to make it’s way out the exhaust.

    maxtorque
    Member

    hora
    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!)

    hora
    Member

    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.

    Premier Icon molgrips
    Subscriber

    It could be mechanical – engine shifts when it’s decelerating and accelerating.

    Premier Icon somafunk
    Subscriber

    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.

    hora
    Member

    Here it is/listen through:

    http://www.flickr.com/photos/30625376@N06/11996540046/

    Yes I have odd taste in shoes.

    maxtorque
    Member

    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.

    Premier Icon somafunk
    Subscriber

    That’s a rattly exhaust manifold shield or similar 😆

    hora
    Member

    The garage said they had checked for loose heatshields. I wonder if they really did check thoroughly enough.

    Premier Icon somafunk
    Subscriber

    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.

    chrisdiesel
    Member

    Hora I’ve not read through the whole feed but if it’s a 1.6 16v I have a new coil pack you can try and it is a fairly common issue

    Milkie
    Member

    That is an excellent description MaxTorque. I wondered why the Puma backfires after fitting a free flowing exhaust and know I know!

    As long as its not mis-firing, leave it and just rev it at the lights and enjoy the backfiring! 😉

    Premier Icon wwaswas
    Subscriber

    Yes I have odd taste in shoes.

    Benny Hill up top, Alan Partidge from the knee down?

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