LPG backfire when starting...?

[smaddock]

I'm starting to ask this question across several forums as its a problem I've had for a while but am trying to sort it out now...

Not after a lpg debate, just need some feedback if this is a common issue or not, and any remedies...

I've got a 2.7ltr hilux engine (3rz) converted to dual fuel using a Gas Research carby and running a Haltech interceptor ecu mapped separately for gas and petrol, and drives perfect on both. In fact, it puts out almost identical power on each fuel, around 90rwkw.

The problem is however, if I attempt to start the car on gas, it backfires probably 80% of the time. No damage occurs due to a vacuum hose blowing off each time, but its a pain to put back on. If parked on gas, when I restart it I've tried flicking it to petrol to attempt to soak the cylinders with petrol and fire it, but it still backfires every now and again.

The only way I can stop this happening is to obviously flick it to petrol before parking, and start on petrol. My only prob is if I'm offroad and it stalls when on gas on a steep incline, its pretty much going to backfire on start up...

The gas place that did the installation are quite knowledgeable, and said this is a common issue on 3rz hiluxes and 1fz landcruisers with carby gas systems, as they run a wasted spark arangement and the gas vapours get ignited on the wrong stroke hence backfiring, but he doesn't have a solution.

Does anyone have this problem, and if so, have you fixed it and how???

All my leads and plugs are new and of top quality by the way.

Cheers,
Simon

[PGS 4WD]

Do you have a primer? Gasresearch needs a primer or alternativly petrol start.
Other contributors, lack of timing at crank speed, slow crank, excessive throttle on crank.

Joel

[coxy321]

Could also be a vacuum leak, although leaks only tend to affect the car when it is running (not starting). We had a S11 AU that was backfiring. It went to a Ford dealership, then a gas workshop, then another workshop and they couldn't fix the problem.

I left it with one of my mates who is a qualified mechanic and gas fitter and he found that it was a tiny vacuum leak on the inlet manifold gasket. Ran like a taxi after that.

[smaddock]

Do you have a primer? Gasresearch needs a primer or alternativly petrol start.
Other contributors, lack of timing at crank speed, slow crank, excessive throttle on crank.

Joel

I do have a primer but don't use it simply coz if it backfires it just increases the 'bang'. It only seems to help if I hold the button for like 10 seconds. Every now and again I've tried starting on petrol after parking on gas, and it backfires much the same, I'm assuming this is due to the gas vapour still in the inlet track.

Throttle is always closed 100% when cranking, and cranking speed is good. Haven't checked cranking speed timing though, possibly an issue, will have to get my tuner to have a look

And as far as vacuum leaks go, as you mentioned coxy I would think this would only affect the low speed running performance, not the starting, and it drives bloody grouse on gas at all throttle positions, so not sure if it is an issue or not...

Thanks for the replies.

[bazzle]

I would check to ensure leads are not touching. May be cross firing.

Bazzle

[smaddock]

I would check to ensure leads are not touching. May be cross firing.

Bazzle

Yeah was thinking that, but have checked and have installed lead spacers so they're definately not touching

[bazzle]

Does it have a distributor cap?
Check the gap between the rotor and the contacts. If too big it can jump back to prev contact esp if timing advanced.

Bazzle

[smaddock]

Nah no dizzy, it got 2 coil packs firing as a wasted spark arrangement, that being that 1 and 4 fire at the same time, one will need it one won't, 2 and 3 then fire next and so on... I doubt it would be coil pack issues coz again its only when cranking and first firing that it backfires, never and I mean never when driving.

Cheers

[coxy321]

Nah no dizzy, it got 2 coil packs firing as a wasted spark arrangement, that being that 1 and 4 fire at the same time, one will need it one won't, 2 and 3 then fire next and so on... I doubt it would be coil pack issues coz again its only when cranking and first firing that it backfires, never and I mean never when driving.

Cheers

What about the wasted spark igniting excess LPG vapour?

[roblrc]

Keep your foot of the accelerator when starting. Also check your ignition timing on initial crank it maybe over advanced at base setting.

[smaddock]

Nah no dizzy, it got 2 coil packs firing as a wasted spark arrangement, that being that 1 and 4 fire at the same time, one will need it one won't, 2 and 3 then fire next and so on... I doubt it would be coil pack issues coz again its only when cranking and first firing that it backfires, never and I mean never when driving.

Cheers

What about the wasted spark igniting excess LPG vapour?

And thats exactly what I think the problem is, in which case I'm assuming there's no cure for, unless going aftermarket ecu with individual igniters or liquid gas injection, both of which are not on the cards yet...

I did have an idea of setting up a blow-off valve type arrangement where if there is positive pressure in the inlet manifold, it vents it to atmosphere, so if there's a backfire, the valve will open and let it out, then close again so no more hoses blowing off... although I'm not sure of any products like this

[BEEPJEEP]

Hi there Smaddock, the "wasted spark arrangement" is your problem!! how many kms has the engine done?? DO NOT use a blow off valve cause when it backfires there'll be a huge flame in open air and if you do have a slight gas/lpg leak under bonnet!!!!!!!!!!!!!!! You any many others have this problem because like you said 1 and 4 fire then 2 and 3 fire, that means when 1 fires thats the down power stroke at the same time 4 fires going down to burn any un burnt fuel during its "scavenging" stage just when the inlet vavle opens to let fuel in, this is the stage it backfires. Other makes fixed this problem by having the ECU remapped but also putting on seperate coils X 4 one for each cylinder and re-wire it. IT can be done as an after market computer, i"ll find out for you as i used to be in the game B4. Also some reckon by having there vavle seats re-done has fixed it too!! means taking off the head MMMMMMMM not to sure in your case, talk soon.

[smaddock]

Hey beepjeep, hmmm aftermarket ecu it may be then... Was hoping for a cheaper option, but if it fixes it then I don't mind. Wiring it up is no biggie, just the tuning fees as it'll be a whole tune, at least I'll get some cash for the interceptor ecu so it might not be that bad.

Yeah you've now turned me against the pressure release valve, didn't even think of the flamage coming out Lucky I didn't do it, coulda had a slightly more expensive problem of burnt out engine bay

Oh by the way, the engine's done around 180k kms, and is around a 2000 model

Thanks for your help

[PGS 4WD]

Generally the issue can be fixed with a vapour lock, it goes on the vapour line to the carby, the tendency for the gasresearch converters to leak a bit can make the vehicles backfire on initial crank due to residual lpg in the manifold and cylinder.

Cheers

Joel

[smaddock]

Generally the issue can be fixed with a vapour lock, it goes on the vapour line to the carby, the tendency for the gasresearch converters to leak a bit can make the vehicles backfire on initial crank due to residual lpg in the manifold and cylinder.

Cheers

Joel

Thanks mate, but should have mentioned that was one of the first things I did when I had the old impco system on it. Still using the vapour lock, but not making any difference unfortunately.

[PGS 4WD]

The next step is to use a petrol start timer to delay the ignition from firing on crank, you can wire it through a 5 pin relay and set the delay so the engine gets up to crank speed before the ignition starts.

Joel

[smaddock]

The next step is to use a petrol start timer to delay the ignition from firing on crank, you can wire it through a 5 pin relay and set the delay so the engine gets up to crank speed before the ignition starts.

Joel

Hmmm, you just got me thinking... I did buy a relay thing from the gas place that basically if you crank the engine on gas, it will inject petrol into the engine for a preset amount of time, like several seconds to try to get the engine to fire rather than using the lean gas mixture thats still there, but I haven't fitted it yet. Is this what you're talking about? Or, do you mean a relay that has a timer on it, so it will crank the engine for the preset amount of time before allowing the ignition to fire? If this is the case, would you inject petrol at the same time, or just leave the gas to be drawn in?

Cheers mate

[PGS 4WD]

Thats the device, if it backfires on hot start I'd run it as a delay on the LPG(as described earlier), if it backfires on cold start I'd run it as a petrol start. It can work in both circumstances by causing a slight flood on hot start as well, this makes the mixture to rich to combust and can help with backfires although a little more cranking will be required.

Joel