Lpg Backfire???

A couple of days ago my Disco blew back through the inlet side of the engine
whilst I was running on lpg at 90mph down hill wind at my tail ( very fast
but quite a low load).

It exploded the air box and made the engine run very rough at tick over.

after putting the airbox back together the car runs fine but I am most
concerned that this will happen again. What causes this backfiring and how
can I prevent it happening again.

Thanks in advance
Pete



--
"Reason has always existed,
but not always in a reasonable form"
Groucho

"Those are my principles, and if you don't like them...
well, I have others"
Karl

On or around Sun, 19 Dec 2004 00:43:25 GMT, "Highbeam"
<[email protected]> enlightened us thusly:

>
>A couple of days ago my Disco blew back through the inlet side of the engine
>whilst I was running on lpg at 90mph down hill wind at my tail ( very fast
>but quite a low load).
>
>It exploded the air box and made the engine run very rough at tick over.

it would do...

what petrol fuel system? If it's hotwire, check the wire grille thing in
the upstream side of the AFM.

>after putting the airbox back together the car runs fine but I am most
>concerned that this will happen again. What causes this backfiring and how
>can I prevent it happening again.

that's a bit more tricky. The conditions are not what I'd normally expect
for a backfire.

check the system tuning. You don't say what system it is, so no advice from
me there...

check the ignition system thoroughly and make sure that the leads are in
good nick - you can sometimes get sparks going astray between leads, leading
to one appearing int he wrong place at the wrong time. magnecor leads are
as good as everyone says they are, even though they're not cheap.

"Austin Shackles" <[email protected]> wrote in message
news:[email protected]...
> On or around Sun, 19 Dec 2004 00:43:25 GMT, "Highbeam"
> <[email protected]> enlightened us thusly:
>
>>
>>A couple of days ago my Disco blew back through the inlet side of the
>>engine
>>whilst I was running on lpg at 90mph down hill wind at my tail ( very fast
>>but quite a low load).
>>
>>It exploded the air box and made the engine run very rough at tick over.
>
> it would do...
>
> what petrol fuel system? If it's hotwire, check the wire grille thing in
> the upstream side of the AFM.
>
>>after putting the airbox back together the car runs fine but I am most
>>concerned that this will happen again. What causes this backfiring and how
>>can I prevent it happening again.
>
> that's a bit more tricky. The conditions are not what I'd normally expect
> for a backfire.
>
> check the system tuning. You don't say what system it is, so no advice
> from
> me there...
>
> check the ignition system thoroughly and make sure that the leads are in
> good nick - you can sometimes get sparks going astray between leads,
> leading
> to one appearing int he wrong place at the wrong time. magnecor leads are
> as good as everyone says they are, even though they're not cheap.
>


The major problem with buying a car aready converted is you really have no
Idea what system is fitted.
I will have to get under the bonnet and do some research

--
"Reason has always existed,
but not always in a reasonable form"
Groucho

"Those are my principles, and if you don't like them...
well, I have others"
Karl

Is the LPG system fitted with a "flapper" blow back valve ?


"Highbeam" <[email protected]> wrote in message
news:[email protected]...
>
> A couple of days ago my Disco blew back through the inlet side of the
> engine whilst I was running on lpg at 90mph down hill wind at my tail (
> very fast but quite a low load).
>
> It exploded the air box and made the engine run very rough at tick over.
>
> after putting the airbox back together the car runs fine but I am most
> concerned that this will happen again. What causes this backfiring and how
> can I prevent it happening again.
>
> Thanks in advance
> Pete
>
>
>
> --
> "Reason has always existed,
> but not always in a reasonable form"
> Groucho
>
> "Those are my principles, and if you don't like them...
> well, I have others"
> Karl
>
>

On Sun, 19 Dec 2004 10:02:10 +0000, Austin Shackles
<[email protected]> wrote:

>On or around Sun, 19 Dec 2004 00:43:25 GMT, "Highbeam"
><[email protected]> enlightened us thusly:
>
>>
>>A couple of days ago my Disco blew back through the inlet side of the engine
>>whilst I was running on lpg at 90mph down hill wind at my tail ( very fast
>>but quite a low load).
>>
>>It exploded the air box and made the engine run very rough at tick over.
>
>it would do...

>>after putting the airbox back together the car runs fine but I am most
>>concerned that this will happen again. What causes this backfiring and how
>>can I prevent it happening again.
>
>that's a bit more tricky. The conditions are not what I'd normally expect
>for a backfire.

Nor I, I thought that it was low revs and a wide open throttle that
provoked it.

I'd like to know the various theories, I realise the greater hazard
with gas is that the whole of the manifold from the gas "diffuser" to
the valve is full of gas:air mix, so if a spark gets in there and the
gas velocity is less than the flame speed it will backfire.

So the various sources of ignition I can see are:

1) Weak mixture cuts flame speed to the extent that there is a flame
path back into the manifold at overlap around tdc of the exhaust
stroke.

2) stray spark on any cylinder whilst the its inlet valve is open.

Badger points out that anything interrupting the current to the coil
will induce a spark, but I thought electronic systems suppressed
this??

If not it should be possible to provoke a backfire by twiddling the
ignition key, but I don't seem to be able to do it.

AJH

>Is the LPG system fitted with a "flapper" blow back valve ?

Don't forget these valves, and the poppet type, don't prevent
backfires, they're only an attempt to alleviate pressure and limit
damage. In practice their own inertia means they don't work so well,
it's all over before they have opened.

The flapper type is like a cat-flap to shut off the backfire path,
surrounded by an elastic band allowing the pressure out of the
induction system. After a big bang as described, it's worth checking
the flap isn't bent and sticking if one is fitted.

If you havent any fitted, a flapper at the throttle housing and maybe
also a poppet on the air box side could prevent damage in the future.

>> A couple of days ago my Disco blew back through the inlet side of the
>> engine whilst I was running on lpg at 90mph down hill wind at my tail (
>> very fast but quite a low load).

humm, high vacuum = lots of ignition advance, high revs = possible
valve bounce, both encouraging backfire. Another factor might be ram
air pressure from simple airspeed, tending to weaken the gas/air mix
and also encourage backfire.

The problem with simple LPG systems is that sooner or later
*something* will trigger a backfire, it' s inherent in filling up
plenum chambers designed for fresh air with a fuel/air mixture.

cheers
Rossko

> The problem with simple LPG systems is that sooner or later
> *something* will trigger a backfire, it' s inherent in filling up
> plenum chambers designed for fresh air with a fuel/air mixture.

Its quite common at the garage where I work part time (postgrad student).
Had a V6 Omega in not long ago which had back fired on LPG, made a hell of a
mess! One guess is an lean mixture resulting in a backfire. Older engines
tend to complain if there run a bit lean but moden, multi valve combustion
chambers are more tolerent.. up to a point! Ether way, it seems to stem from
an overly tight budget - ether as many MPG as posible (ill informed), a
cheap kit or poor calibration. Toby