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
On or around Sun, 19 Dec 2004 00:43:25 GMT, "Highbeam" enlightened us thusly:
it would do...:-)
what petrol fuel system? If it's hotwire, check the wire grille thing in the upstream side of the AFM.
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
Is the LPG system fitted with a "flapper" blow back valve ?
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
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.
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
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
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