Thread: LPG Issues 3.5L S1 Disco

PS Forgot to ask if I should be fitting a backfire valve - and what does one look like and where does it go - hopefully there is already one fitted!

Set of plugs & set of the best quality leads you can get should stopthe backfires

Dunno the best brand to use on a disco,a search on here may help.We only use Genuine ones on our work vans,they seemto work the best stopping missing & backfires,usually get around 140k out of a set

hope this helps

Dunno what a backfire valve is

Buy original cap and rotor otherwise expect trouble regulary. Buy a good set of leads and plugs suitable for gas. Tune your gas and power valve and all should be right.

Hi Steve,

I have '96 3.9Lt V8 Disco with LPG and before that I had an '88 3.5Lt V8 Rangie on LPG so I've seen quite a few of the problems and I hope I can help you.

It's most likely, for the age of your vehicle, that you have a fumigation system which takes the liquid LPG from your tank, feeds it to a water-heated converter in the engine bay, converts it to vapour, regulates the flow using manifold vacuum and then feeds the gas into the air intake (using a mixer ring) close to the plenum chamber i.e. just before the throttle butterfly.

Backfiring in my experience has tended to be the result of two possible problems; running lean or ignition problems (especially cross-firing between leads).

Running lean can result from a couple of problems:

1. Easiest to deal with is that you have developed an air leak somewhere around the plenum chamber. Check all the hoses on the plenum chamber (the big rectangular cast aluminium box on top of your engine) and make sure they're all connected and there are no splits (at your engines age that's quite likely). Also check the vacuum line down to the distributor, especially if it has little rubber elbows on the ends - they can split (this will not only allow extra air into the plenum but muck up the ignition timing). I think your motor will have an air bypass valve (to speed up the idle when the air-conditioner is running) bolted to the rear of the plenum chamber against the firewall and I've heard that these come loose and leak air into the plenum (they also stick and need cleaning out).
2. The converter has a large rubber diaphragm inside which responds to manifold vacuum and adjusts an orifice for the gas which lifts a rubber cap off a brass seat (if it's the typical type) to regulate gas flow according to engine demand. The diaphragms become stiff with age or develop holes and the rubber pads develop indentations where they sit on the seat and they become hard and don't seal properly or stick and seal when they shouldn’t. If the converter is 10 years old it probably needs to have an overhaul kit fitted which replaces all these parts. Also the converter tends to fill up with a waxy residue from the gas which makes it not work properly. At that age it's quite likely that the converter has reached it's use-by date and it might be more economic to replace it entirely (the converter isn't hugely expensive).

Ignition problems are a bit harder to pin down but here are some considerations:

1. LPG seems to be much less tolerant of timing problems than petrol, although usually the timing is left set for optimum timing on petrol which leaves it a few degrees retarded from the optimum position for LPG – and being over advanced is more likely to cause back-firing so this situation gives you a bit of protection if anything. The distributor needs to be in good condition and if the distributor cap and rotor are in poor condition (dirty, cracked or showing any signs of carbon inside) that can cause the coil energy to track to the wrong spark plug and fire the wrong cylinder at the wrong time potentially causing a backfire. Replace the dizzy cap and rotor with good quality parts (many people say genuine Rover parts only) as a start.
2. The V8 has a lot of high tension leads and they run all over the engine and they're hard to keep clean and keep apart. The thinner the insulation, the harder it is to keep the high voltage from exiting trough the side of the lead and the dirtier they are the more likely the spark is to track down the outside of the lead and across to another lead where the two touch. The easiest way to check if you have a cross-fire problem is to run the engine in the dark with the bonnet up and look for corona (blue coloured arcing). However this won't show you anything if the arcing is between 2 leads which are touching - you'll only see the spark running down the outside of leads. Unless they're quite new and clean, buy a new set of good quality leads with 8mm insulation and rubber caps which enclose most of the porcelain insulator on the top of the plug, and fit them carefully (replace one at a time so you don't forget which socket on the distributor goes to which plug. Some of the commercial sets of leads don't have leads the optimum length so it is sometimes better to measure what length you need and buy them individually. Clean the rocker cover under the leads and route the new leads so that the sit properly in the plastic cradles to keep them all spaced apart from each other. Also don't let them lie on top of rubber hoses (the hoses contain carbon to make them black and that makes them a little conductive). Clean the top of the coil well too and keep the HT lead from the coil to the distributor spaced away from all metalwork and hoses. You can make small spacers for the leads using cable ties - form one into a ring about 8mm in diameter and place it between the 2 leads, putting 2 cable ties through it, one around each lead to hold it in place. The cable ties are plastic and are good insulators. Now run the engine in the dark with the bonnet up and check for corona - if there is any you'll need to separate the lead a bit more from whatever it's arcing to. Main thing is to stop them arcing to each other.

Those are the most likely things to help you sort the problem. Obviously the engine used to run well on gas and something has changed so it's really a matter of finding out what that is - and if you can make some improvements along the way that will be worthwhile.

You asked about a backfire valve. When the engine backfires you risk damaging the airflow meter (AFM) which is probably a hotwire unit just behind the air-cleaner box. It also tends to blow the lid off the box and stretch the clips and rip the plastic lip moulded onto the box for the clips to lock onto. You can re-bend the clips to re-tension them, and check that the air cleaner element is seated properly before you clip the lid back down.

Sometimes you can put a flapper valve between the LPG mixer ring at the plenum chamber and the airflow meter. It's a sort of cat-flap arrangement inside which is held open by the air rushing into the motor. When there's a back-fire inside the engine / plenum chamber, the explosion pressure slams the cat-flap shut which protects the airflow meter. The valve usually has a big rubber band around it's neck and the neck is perforated with big holes which let the pressure from the backfire out by blowing the rubber band out - it recovers without you having to do anything.

On my Rangie I had room in the big air hose from the airflow meter to the LPG mixer ring to install one of these valves. On my Disco I don't have room and yours may be the same but it's worth investigating. The other thing I found on the Rangie was that the backfire would usually blow a hose off the plenum chamber and it was always the most difficult one to get at to put back on. I clamped all the hoses except one very well to make sure they never blew off, and the one I left not too tight was the big one on the driver’s side which was easy to check and put back on.

I hope this helps you sort out the problem. At least you know the engine runs well on petrol so there’s no fundamental (expensive) problem with it.

A final tip – usually with the LPG system the petrol pump is powered permanently to keep the fuel rail pressurised so that if you want to change from LPG back to petrol there’s no delay getting fuel to the injectors (actually the pump only runs occasionally and stops when the rail is pressurised). Never leave the petrol tank less than about quarter full for long periods or the pump is left high and dry. This makes it inclined to overheat and die – always just when yo need it. Keeping enough petrol in the tank to cover the pump keeps the pump lubricated and cooled.

Good luck and I’ll keep an eye on the thread to see how you get on.

Cheers,
Graham

GrahamH,well written post,maybe we should have an LPG section on this forum,as there are always heaps of threads about it.

We have run our workvans on LPG for the last 20yrs,but my typing is not up to speed,to write a post like yours would take me a week

Off topic,looks like the new ones we are getting shortly have to be gas injected,apparently Sprint gas have been having heaps of trouble with the new systms,apparently its the quality of the gas.....

We have a 3.5 disco, the only time we have had problems with backfire was after we picked it up from the local workshop.
It seems they slipped with a tool and cut the air intake hose that attches to the air flow meter.

So it may simply be an air leak,
We replaced the hose and all the clamps and every thing is now fine.
The last thing you need is to blow the Air flow meter, they dont like back fires.

Hi Graham,

Thanks a lot for the detailed reply - I'll print it off at work tomorrow and let you know how I get on!

Cheers

Steve

Scarry, thanks for your comments. And regarding your comment on the new vehicles having to go to sequential gas injection - I'm not sure of the exact legislative requirements but the story is something along the lines that LPG installations on vehicles manufactured after a certain date must comply with new exhaust emission standards on LPG which can only be achieved using Sequential Gas Injection. Refer to LPG Australia: LPG Autogas for Vehicles - New Requirements for more info.

Looking forward to hearing how you get on Steve.