Disco V8 (3.9) & LPG sparkplug gap size info......

[nealgs]

Hi all,

First post here, and it's a question i'm afraid

Have a 300 series Disco (3.9 V8i ES) running on LPG. Have recently purchased new plugs and want to confirm what the plug gap size should be for best results.

many thanks
Gary

[webber1982]

if its a single point gas system they should be set to 0.7mm if its multi point just whatever the manufacturers say it shpuld be.

[nealgs]

thanks for the info webber1982. Need to go and get a feeler gauge tomorrow and get them put in.

We've been having problems with it backfiring, already trashed one MAF unit due to it and need to sort it out ASAP.

Any idea what could be causing it? plug leads are landrover genuine parts.

cheers again

Gary

[seriesdriver]

Welcome to the forum mate.
The major cause of backfire is a week gas mixture.
Is the system you have multi point or single venturi?
I have PDF on my old PC with installation and set up details , will see if I can dig it out.
Do you know how to richen the gas mix?

Standard LR leads are not the best for LPG you need a good strong spark.
I have Luminition leads, NGK plugs and tuned my system 2 years ago when I took off the Cats., not had a backfire since. ( just watch It’ll blow the air box off to-morrow )
Oh and I took off the air filter clips and replaced them with bungee cords.

[nealgs]

Hi seriesdriver - thanks for the welcome

It's a single venturi system (unfortunately) was on the vehicle when we got it.

Nope to how to riche gas mix - but I can learn

Ah!, plugs are Champions IIRC, bought from MM4x4 as part of their service kits (inform them running LPG for correct plugs etc).

Vehicle is CATless too, and may look into the bungie cord solution also, have 2 of the 4 af clips left

I may need to invest in two sets of leads/plugs - can you tell me from who/where you got your Lumenition/NGK stuff from (have 2 V8 ES discos both running LPG, although one runs really well)

cheers
Gary

[seriesdriver]

Hey just noticed your on the doorstep!
OK I will dig out the instructions, what make system is it?
I have the setting device for the Marini system.
Rimmer Bros for the plugs and leads.

[seriesdriver]

Gary just sent you a PM

[nealgs]

Well, got my feeler gauge yesterday, and regapped the plugs to 0.7mm - They were way out, also noticed that the pipe from the plenum chamber to the MAF unit was split - so replaced that with a new one i'd purchased a while back.

Also fixed the Cruise Control thanks to an article on these here forums

Took it for a test and it seems OK, starts and ticks over much quieter than it did (wife also noticed it was less noisy/tappity too)

No backfire yet, but early days - will se how it goes over next few days - fingers crossed.

Gary

[The Original Tom]

Bit late I know, but I've only just got this info myself, but I'm yet to modify mine so I don't know 100% that it'll work yet!

On a vehicle with a standard ignition system, (i.e points type, or with the standard ignition amp on the dizzy or behind the coil) then 0.7mm is the best you can do.
On my old 2.5 4-pot engine, I bought some 'NGK Iridium' plugs. These have a slightly longer, and much narrower electrode. It looks more like a pin than the small cylinder (like a lighter flint) on normal champion plugs. Normally this would lead to the electrode being burned away much quicker, but it's made of a very hard alloy (iridium) so has a similar or longer life, and produces a much more concentrated spark, and these can be opened to 0.8mm - my vehicle ran a bit better with these plugs and magnecor leads, and AFAIK they're still going strong on my mates engine now.
BUT: if you run an A&R power amp (from RPI) it dramatically increases your output voltage to 25-30kV, as well as automatically retarding the timing for petrol, and advancing for gas.
I've been told that this should allow me to open my plugs to 40thou (1mm), giving a comparatively huge spark!
I've yet to try this though I do plan to as soon as possible. I'll report back with my findings...

[nealgs]

It's never to late for information The Original Tom.

Thanks for the input

Car has been fairly well behaved upto know - Wife hasn't mentioned any backfires at all, and she (the car) seems to be running OK.

tick-over is a bit low at idle - think this could be due to a damaged MAF, but not 100% certain - how can I test the MAF, or is there another something else that could affect tick-over idle rate??

cheers all

Gary

[The Original Tom]

If Idle is running low, the first thing I'd check out is the air filter. If this is clogged it'll cause over-fuelling, and a rough idle.

Tom.

[nealgs]

It's a new airfilter - nope wait a min, 26/07/06 it was replaced, blimey that's flown a bit .

Cars has done around 6k miles since (appx) so maybe time for new filter anyway - will check that out

cheers
Gary

[seriesdriver]

Gary,
Drivers side rear of plenum black plastic socket, disconnect electrical connection and remove idle valve, " Motor-speed limiter F.I.P. Control" look out for thin rubber seal, this device rarely fails but can get really caked in carbon, burnt oil etc.
May help, there again you could just open the tick-over screw a wee bit

[Rossko]

Sometimes it helps to understand why to do this or that.

Lean mixture cannot cause backfires, after all how can an air-gas mixture set fire to itself? But it does make it far more likely as a lean mixture is more prone to flash-bang than gentle burn. Setting the right mixture really is important.

Twiddling mixtures 'blind' isn't very accurate and could be masking other faults. I'd recommend a session on an exhaust analyser if your car has no built-in help by the way of exhaust oxygen sensors.

So what can ignite the backfire then? A flame or spark gets into the manifold somehow. Crossfire amongst V8 ignition leads, an inlet valve that is a bit leaky or slow, worn valvegear, timing gear, a 'glowing coal' on the lip of a valve. Nothing predictable that we can say 'that'll fix it', just marginal things we can do our best to avoid.

In most cases then, a backfire is caused by a very slight problem outside of the LPG system. Getting the LPG mixture right will dramatically reduce the chances of the marginal problem actually igniting the mix - but sometimes the root problem is too bad and it still backfires once in a while

---------

Plug gaps can help - we want the biggest spark we can get, but want it reliably. It's harder for the spark to jump across LPG mix than petrol, hence the advice to close the gap. On modern systems (no rotor arm gap etc) the voltage is concentrated at the plug and usually there is no need to twiddle. On our older V8s the ignition system is a bit marginal at the best of times! Closing up to 0.7mm is good advice. This also reduces the stress on the rest of the system and makes it less likely that the spark will 'pop out' in the wrong place, like a crossfire to another lead.

Even on distributor less systems, LPG can show up a weakness like a bad lead, that's acceptable on petrol. Even when a misfire or roughness only shows on LPG, the sparks is the first place to look.

It doesn't make a jot of difference to the ignition if you have single-point or injection LPG system, the advice holds true.

I'm not a real believer in those iridium plugs, but the needle-nose may well help in concentrating the spark I guess. They may have a generally better build quality (as the life is much longer) than regular plugs too, so there could be more than one reason they work better.

Let us know how you get on !

cheers, Ross K

[nealgs]

well, it's been a long time since i've updated this, but here we go:

Bloomin' marvelous car has been brilliant since the gap size was changed. Did 1200 miles during our three week hols to cornwall - running was faultless.

So thanks for all the info, and help everyone.

regards
Gary