3.9 EFi V8 backfire on LPG help

[geoffbeaumont]

Hi Geoff, I set the plugs to the standard setting. I did not reduce them for LPG. The current trouble I am getting on petrol too.

I have ordered an A+R amp from RPI to get the bigger spark. Hopefully will come early next week.

Dunc

Once you get it running you'll want to pull all the plugs then and regap them.

No idea whether the amp will help with LPG running (people seem to get mixed results with gizmos like this - some swear by them, others find they make no difference), but you definitely shouldn't need it to get it running nicely on petrol (at least providing it's not a damp day, you're within 100 yards of a puddle, etc... ).

[duncmc]

I got it running nicely at tickover on petrol this evening. Turns out that I was putting the HT leads on according to the RAVE manual, and that doesn't work with mine. I needed to be 45 degress further round. So I just put no 1 where no 8 was etc, and it ran. So next I will dynamically set the timing then clean/service the vaporizer and see how I go from there.

[webbersdad]

Thanks for the suggestion, it has the magnetic pickup type of distributor rather than the older points though. I still have to check the gap though. I need non ferrous feeler gauges for this apparently.

You may have misunderstood.

Gap the spark plugs to 0.7mm

webbersdad

[p76rangie]

LPG does not really need different gaps to standard with any standard Rangie electronic ignition system. You can go one stage colder plug if you like, but would not muck around with the gaps. For a dual fuel car you should set your timing up around 10 to 12 degrees BTDC. It is advisable to also get your distributor regraphed for LPG as well.

[ThreeSheds]

OK, OK - I give up... I'll be the first to mention it...

MEGAJOLT

Ahh - I feel better now

For a simple LPG system on a V8, there is no better ignition system than MegaJolt - there is plenty of experience and guidance available on here, and you get computer controlled dual advance maps (one for petrol and one for gas) and sparks like this:

You won't believe how good it is...

oh, and it's waterproof.

Matron! I am ready for my nap now...

Roger

[duncmc]

OK, OK - I give up... I'll be the first to mention it...

MEGAJOLT

Ahh - I feel better now

For a simple LPG system on a V8, there is no better ignition system than MegaJolt - there is plenty of experience and guidance available on here, and you get computer controlled dual advance maps (one for petrol and one for gas) and sparks like this:

You won't believe how good it is...

oh, and it's waterproof.

Matron! I am ready for my nap now...

Roger

ok, I am going to have to read up on Megajolt! I like the look of that. Maybe a project for later once I have sorted the rest of the car.

[p76rangie]

I have found that too much power can cause different issues. I have a strong ignition system and I find that when I loose a plug, and the spark can't go there, it starts to jump to the next lead within the distributor cap and causes a worse situation with two cylinders misfiring rather than one.

[andy2986]

I fitted Megajolt to my disco, probably one of the best mods I've done to it. It's not to hard and there are loads of people on here ready to help.

I also went the Rpi ignition amp route first, more for the clever ignition timing thing it does, it worked for me at the time, but now with Megajolt my V8 is waterproof.....

Andy

[p76rangie]

From my understanding, megajolt does not use a distributor but uses coil packs like on the later GEMS motors. This makes them unsuitable for single point LPG systems. As the sparkplug in each cylinder is fired when not required (with 2 coils each cylinder is fired 4 times per cycle instead of once with a distributor system) it fires often when the intake valve is open. This can ignite the LPG in the intake manifold and cause a bad backfire. This is why most reputable shops will only fit injected LPG systems to these types of motors. I am not saying that it will happen all the time and some people get away with it not happening at all. However, the risk of a backfire is many times higher if you run a single point LPG systems on such a motor. The damage that a backfire can cause is usually not worth the risk.

[andy2986]

Have to say, since fitting megajolt to my Disco it has never backfired, its waterproof and it ticks over really smoothly pulls well up to the red line on both fuels (mine is single point lpg).

the dual map system it offers has the advantage of tailoring an ignition map suitable to whichever fuel you are running.

I believe the waisted spark system it uses fires only two plugs at a time not four (it uses two x 4 way coilpacks)

the pairing as far as I can remember is:

1&6

2&3

3&2

4&7

5&8

6&1

7&4

8&5

so if 1 is firing then the waisted spark is on 6 so no danger of firing when an intake valve is open.

Andy

[ThreeSheds]

From my understanding, megajolt does not use a distributor but uses coil packs like on the later GEMS motors. This makes them unsuitable for single point LPG systems. As the sparkplug in each cylinder is fired when not required (with 2 coils each cylinder is fired 4 times per cycle instead of once with a distributor system) it fires often when the intake valve is open. This can ignite the LPG in the intake manifold and cause a bad backfire. This is why most reputable shops will only fit injected LPG systems to these types of motors. I am not saying that it will happen all the time and some people get away with it not happening at all. However, the risk of a backfire is many times higher if you run a single point LPG systems on such a motor. The damage that a backfire can cause is usually not worth the risk.

You logic is a little off (see below) since there are four coils on a 'jolted' V8, and the cylinders are sparked twice per cycle (once per revolution).

But from personal experience, MegaJolt it brilliant with single point LPG...

Just to expand on what I said above:

Firing order is 1-8-4-3-6-5-7-2 and cylinders are fired in pairs as follows:

As 1 is firing on compression before TDC, 6 is approaching TDC on the exhaust stroke and also fires

similarly:

8, 5

4, 7

3, 2

6, 1

5, 8

7, 4

2, 3

Although the 'spare' spark is actually always going off during the exhaust stroke - the inlet valve may (depending on ignition advance) be starting to open when it does (the inlet valve starts to lift at 22deg BTDC on a standard 3.9 ), but the exhaust valve is still wide open and the speed of the gasses down the header may well even be creating a partial vacuum in the cylinder as the piston is slowing down.

Also the energy expended in this 'spare' spark in that relatively low pressure environment will be well down on that of the 'main' spark, and these two taken together should mean that (if all other things are correctly adjusted) the chances of a backfire are significantly reduced.

Certainly in my case, I have 'got away with it', in that I have have never knowingly had a backfire while using MegaJolt with single point LPG over the last four years.

I would be very interested to hear of other peoples experiences with single point LPG and MegaJolt - does it cause more backfires than a dizzy setup? It's a valid point that needs addressing I think, since backfires can cause a lot of damage ...

Roger

[duncmc]

Tonight I found that the engine will still not run very well on tickover, and is very low on power.

So I took out the Idle Air Bypas valve and cleaned it. I have read that there should be a washer to recover, but there was none, and I cannot find one in Microcat? Does anyone know if there should be one or not? It looked ok by the way.

I also found a loose vacuum hose that was probably blown off during one of the backfires. I cannot find where is has come from though. It comes from the front right corner of the pleunum chamber and ran underneath the throttle assembly towards the bulkhead.

Does anyone know where it should go? Thanks.

[p76rangie]

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. Therefore the backfire will usually occur when de-accelerating. A weak mixture also occurs when the unburnt fuel left in the cylinder is ignited by the second spark. Rover V8's are notorious for backfiring on LPG when running coil packs. Unlike some other parts of the world, here LPG systems can only be fitted by licensed operators. They of course have to warrant their installations and repair any damage that may occur from a questionable installation. The risk of backfire on a coilpack Rover V8 is high enough that you would find it extremely difficult to find someone prepared to fit a single point system to the later Rover V8s. They will only fit injected systems.

[duncmc]

Tonight I found that the engine will still not run very well on tickover, and is very low on power.

So I took out the Idle Air Bypas valve and cleaned it. I have read that there should be a washer to recover, but there was none, and I cannot find one in Microcat? Does anyone know if there should be one or not? It looked ok by the way.

I also found a loose vacuum hose that was probably blown off during one of the backfires. I cannot find where is has come from though. It comes from the front right corner of the pleunum chamber and ran underneath the throttle assembly towards the bulkhead.

Does anyone know where it should go? Thanks.

I think that the only place this can go is to the automatic transmission as the hose is so long it must have gone down the back of the engine towards the auto box. Now I just need to figure out how to get it back on.

[andy2986]

Hi,

I cannot get to my Disco to have a look at the moment, so this is from memory Im afraid.

It might be either the pipe for the fuel pressure regulator or if you have a Charcoal canister the vacuum for that.

Don't think there is a vacuum for the transmission...

Andy

[Bowie69]

On an RRC there's two pipes like that on the RHS of the plenum, one to go to the heater control sphere, and one to the cruise control vacuum control thingy next to the throttle. The heater sphere is on the LHS inner wing.

[andy2986]

Ah yes sorry just realised yours is a RR not a Disco...

[duncmc]

On an RRC there's two pipes like that on the RHS of the plenum, one to go to the heater control sphere, and one to the cruise control vacuum control thingy next to the throttle. The heater sphere is on the LHS inner wing.

Brilliant!

Thanks, connected back up and now together with the stepper valve it idles properly now.

Timing and ignition next for the LPG, but I won't take the distributor out again until the bits arrive to rebuild it.

Any tips on how to easily get the distributor back in and the timing correct?

[ThreeSheds]

Any tips on how to easily get the distributor back in and the timing correct?

1. Turn the engine forward until it is on compression on No 1 cylinder.

2. Keep moving it forward until your timing mark of choice lines up.

3. Place the dizzy back in the hole with the rotor pointing roughly where No 1 lead is on that diagram you have, and the body conveniently placed for pipes etc.

4. Note where the rotor arm is pointing.

5. Replace the cap and see if the rotor lines up with a terminal:

5.1. If it does then this is where No 1 lead goes and for all the rest follow the sequence in an earlier post.

5.2. If it doesn't then move the dizzy body until a terminal lines up and then do 5.1.

5. Use a strobe to set it properly.

Roger

[duncmc]

1. Turn the engine forward until it is on compression on No 1 cylinder.

2. Keep moving it forward until your timing mark of choice lines up.

3. Place the dizzy back in the hole with the rotor pointing roughly where No 1 lead is on that diagram you have, and the body conveniently placed for pipes etc.

4. Note where the rotor arm is pointing.

5. Replace the cap and see if the rotor lines up with a terminal:

5.1. If it does then this is where No 1 lead goes and for all the rest follow the sequence in an earlier post.

5.2. If it doesn't then move the dizzy body until a terminal lines up and then do 5.1.

5. Use a strobe to set it properly.

Roger

Hmm, that is how I have been putting the distributor in, but something is still not quite right I dont think. The engine ticks over nicely but has no power and the strobe shows timing of around 6 degrees AFTER TDC. I think I must be a tooth out on the distributor drive gear.

Before I started messing it ran at 10 degrees After TDC. I will see if I get chance to have a look tonight. Thanks.

[Escape]

I would be very interested to hear of other peoples experiences with single point LPG and MegaJolt - does it cause more backfires than a dizzy setup? It's a valid point that needs addressing I think, since backfires can cause a lot of damage ...

Roger

I've been running single point LPG on 3 different GEMS V8s. The only times I had backfires, it was due to another, mechanical problem (bad HT lead, vacuum leak...). I certainly can't see a reason to retrofit with a sequential system, though I might opt for one if starting from scratch.

[mickeyw]

But from personal experience, MegaJolt it brilliant with single point LPG...

........

I would be very interested to hear of other peoples experiences with single point LPG and MegaJolt - does it cause more backfires than a dizzy setup? It's a valid point that needs addressing I think, since backfires can cause a lot of damage ...

Roger

I've been running my MJ and LPG on a 3.9 in my Ninety for a couple of years now. I can safely say I have had only a couple of backfires in all that time, and those were during a setting up period. The only casualty was the intake hose.

The dizzy used to cause backfires on a regular basis, normally on overrun. I used to keep a roll of gaffa tape handy for intake hose repairs

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. Therefore the backfire will usually occur when de-accelerating. A weak mixture also occurs when the unburnt fuel left in the cylinder is ignited by the second spark. Rover V8's are notorious for backfiring on LPG when running coil packs. Unlike some other parts of the world, here LPG systems can only be fitted by licensed operators. They of course have to warrant their installations and repair any damage that may occur from a questionable installation. The risk of backfire on a coilpack Rover V8 is high enough that you would find it extremely difficult to find someone prepared to fit a single point system to the later Rover V8s. They will only fit injected systems.

I have never heard or read of such concerns before. Is this from personal experience? And what country are you in?

[Bowie69]

I have fitted an MegaJolt n EDIS setup on an RPI 3.9 Rover V8, and it has done well over a thousand miles with the only issue with backfiring being when a coil pack failed -during the first couple of hundred miles.

I fitted it the same times as a lambda feedback control unit to the open loop system and his MPG went from 8 to 12(!)

He was... impressed

[p76rangie]

I've been running my MJ and LPG on a 3.9 in my Ninety for a couple of years now. I can safely say I have had only a couple of backfires in all that time, and those were during a setting up period. The only casualty was the intake hose.

The dizzy used to cause backfires on a regular basis, normally on overrun. I used to keep a roll of gaffa tape handy for intake hose repairs

I have never heard or read of such concerns before. Is this from personal experience? And what country are you in?

Look at what kit they sell for a D1 compared to a D2 here: http://www.v8engines.com/faq-lpg.htm http://www.range-rov.com.au/21.html

You can read bottom paragraph of this http://www.amrautos.co.uk/Range-Rover.htm

[Bowie69]

Well you can ignore ANYTHING that RPi write on their site as complete tosh if you ask me.