Misfire...but only on LPG

[JumboBeef]

My Disco (1996, V8, LPG) has started backfiring sometimes, and running a bit rough, but only on gas    It runs fine on petrol.  The plugs were changed about 1,000 miles ago, and the leads last year.

Any ideas please!

[dxmedia]

Electrics, change the plugs, leads, dizzy cap and rotor arm (if it's got em - landies aint my thang) again.

If it runs fine on petrol then 99.99999999999999% of the time with LPG it's electrics.

Try hotter plugs and pull the gap in about 0.05 and see if that helps too.

[morson4x4]

i used to have this problem in my 1990 disco i took to a garage and they told me the lpg was on its way out i dont trust it no more if what he suggested not working i would take it to a garage

[dxmedia]

i used to have this problem in my 1990 disco i took to a garage and they told me the lpg was on its way out i dont trust it no more if what he suggested not working i would take it to a garage

Just as a matter of interest, do you have single or multipoint?  What did the garage say was on it's way out?   All the parts for building / repairing systems are very commonly avaliable and LPG systems are supprisingly easy to work on, especially if it's single point.

[JumboBeef]

I love my LPG, it has saved us so much money over the nearly three years we've had the motor.  Ordered a new cap and arm and will fit those first to see if that will sort the issue.

Thanks for the replies.

[martyn]

I've had two disco with LPG ninety nine times out of hundred its the leads put the best leads you can buy on also good quality plugs champion  etc i never bothered with making the gap bigger or smaller put what the manufacture states

 if there is any weakness in the system along these lines gas WILL find it out petrol is more forgiving and thats why you don't get the back fire

[Devon-Rover]

And you did buy a Genuine cap and Rotor arm? Not cheap but will work. Having a gassed V8 means having to have top notch Ignition system.

[JumboBeef]

Well, replaced cap and arm.  The old cap and arm looked OK, but put the new ones on.

Still backfiring.

Checked the leads, they *seem* fine.  They are 12 months old.  Should these be next on my list?

Plugs are about 6 months/2,000 miles old.

Thanks,

[lcruiser100s]

Ive Had the same issue on a single point system. It would be backfire after initial burst of acceleration, Ok on motorway stretch.  Just turned the mixture up a bit, seemed to have cured it. fingers crossed.

[Range Rover Blues]

Close the plugs to 27 thou, if you are buying new then fit NGK BP6ES, not BPR6ES.

Check compresion on the engine too, backfiring can be a symptom of a burned out valve.

If you can get one, a Gunson's flashtester will check the strength of your sparks.

[JumboBeef]

OK, thanks.  It runs perfect on petrol, the problem is only on LPG.  I have ordered new leads.

[martyn]

It will be the leads 99 % certain just make sure they are Good quality

[JumboBeef]

Changed the leads and............all is well in Disco Land!  :dance:

Thanks for the replies.

[Devon-Rover]

Good to hear it was a simple fix. That's why i run Magnecor.  :dance:

[martyn]

i couldnt remember the name of those leads ill have to write it down tried and failed to get them for disco 2

[JumboBeef]

Update:

mmmmmmmmmmmmmmmmmmmmmm

The backfire has *almost* totally gone, but not quite.  I still get a rare misfire

The leads certainly helped.  The only thing left to change now is the plugs, so I will do them this week.

[Range Rover Blues]

Just to put it into perspective, regular unleaded is about 95 OctANE (RON), LPG is approximatley 115 octane.

Octane is a measure of how hard it is for fuel to self-ignite under compression and as spark ignition works by causing auto-ignition due to a shock wave, the octane rating of the fuel relates directly to how hard it is to ignite.

That's more than 20% harder to burn.

Also the burn temp of LPG is different and it takes longer to burn.


The backfire could be due to a weak spark, one tha's fine for petrol but not LPG.  Alternatively it could be a weak mixture which can in turn lead to overheating plugs or other problems associated with missfiring.

I've just traced my missfiring to a dodgy coil, it was fine when cold but if the car sat in traffic for example it would missfire on LPG and oddly it begain to effect it's petrol performance too, not missfiring but it wouldn't pick up from idle smoothly, it was all or nothing.
Worth mentioning that if the spark doesn't happen in the cylinder then it has to go somewhere, in time that can cause damage.

[JumboBeef]

If you are buying new then fit NGK BP6ES, not BPR6ES.

Fitted new BP6ES today, then drove 60 miles and no backfire!  :dance:  The plugs which came out were BPR6ES (fitted by me last year :oops: )

What's the difference between the two types?

[Range Rover Blues]

R is for resisted that's all, it's a radio surpression resistor but if you have quality silicone leads then they will be surpressed already.  The resistor saps a little of the spark power so BP6ES are better for LPG.

[JumboBeef]

Thanks!

[JumboBeef]

My misfire has come back (only now and then, but it is still there).

Which are the best leads I can fit, and where can I get them from please?

Thanks!

[Range Rover Blues]

Magnecor 8 or 10mm, but TBH you don't need them, I bought some decent silicone rubber leads form E-aby for half that price.  If you still have a missfire then you need to check everything else.

How good is your coil?  might be worth trying to get hold of a Gundon's flashtester to check spark strength.

[JumboBeef]

Thanks for the reply.  I have pushed all the leads back on REALLY tightly, and it seems OK now.  We shall see.................