Backfires.....

Message

felix · #1 Post by **felix** » Tue Mar 06, 2007 12:31 pm

I have a R reg Seat Alhambra running on LPG. It has been running fine with the only problem being it’s a bit juddery when it travels with out accelerating. I put this down to possible ram air into the air box. I therefore took the inlet pipe off the air box which made the running a lot smoother. Only problem now is I’m experiencing backfires. Any thoughts? Would this be rectified by a blow back valve or placing an open pipe in the breather system to allow the pressure to escape – or am I curing the symptoms rather than the cause.

MarkJones · #2 Post by **MarkJones** » Tue Mar 06, 2007 1:27 pm

backfires are usually caued by the ignition system not being up to scratch.. lpg is an insulater ...and its hard for spark to jump gap at the plug ...spark will find the easiest route to earth if your plug leads aint up to scratch when running on lpg. i dont know weather your seat has plug leads or coil over plug ...but if it has plug leads get them replaced with magnacor ones ...around £45 a set for a four cylinder car . also change and regap your plugs 0.2 mm less. all the best....mark

felix · #3 Post by **felix** » Tue Mar 06, 2007 3:56 pm

Plugs and leads are new And its only been backfiring when i took the inlet pipe off i'll try and reduce the plug gap and see what that does. I've been quoted 0.7mm in the past

MarkJones · #4 Post by **MarkJones** » Tue Mar 06, 2007 4:30 pm

plug leads can be new ..but cheap and nasty particilary the bargain basement dayglo ones. these still cause problems on lpg .. thats why i sugested magnacors . its up to you fit magnacors and forget ...or buy cheap every 12 months. not saying magnacor will solve your problems though ...but will help to eliminate one cause....and make your system more reliable for the future. also it is advisable not to buy cheap as regards distributor caps ,rotor arms etc ... bosch or lucas .......not intermotor all the best..mark

morticiaskeeper · #5 Post by **morticiaskeeper** » Tue Mar 06, 2007 7:16 pm

What engine is it? Where is the LPG going into the system? I ask because I've got a VW Sharan that also has the intake pipe upstream of the filter removed. My "mixer" was a nozzle fitted into the intake pipe bend a few inches upstream of the throttle. I had a few backfires, blowing the airbox apart even though it had a valve. It also got a bit juddery on constant throttle. Last weekend I fitted a proper mixer onto the throttle and was amazed by the difference. The juddering has gone, throttle response is vastly improved and I've had to lean the vaporizer off to corect the mixture. I've got NGK B6 plugs at 0.7mm, remember to retorque them after a few days. 8mm HT leads and a recent coilpack after it started tracking down the body. I had the Orange Lumenition leads sold by Halfrauds and they lasted less than a year. The 8mm leads I have now are unbranded, but I do trust that particular store to supply good leads. If it's only started backfiring since you took the pipe off, could it be a mixture problem?

classicswede · #6 Post by **classicswede** » Tue Mar 06, 2007 8:15 pm

What you will probably find is that the mixture was over rich causing kangaroo effect. By removing the air intake pipe the mixrure will have leaned off. My asumption would be that the lean running is causing the backfire.

felix · #7 Post by **felix** » Wed Mar 07, 2007 12:58 pm

Cheers guys Paul - Its a VW engine, probably the same as yours. I have a vpm mixer just before the throttle body. The car runs perfect and is no different to running on petrol for accelerating. The only problem is when cruising, when it becomes a little juddery - nothing much but enough to be annoying. Its more noticable on a windy day and taking the air intake pipe off has done the trick, but now i'm experiencing backfires. To be honest i haven't had chance to re-adjust the plug gap yet and is on my 'to do'list. I have a set up computer software disk also which i need to do to calibrate the system which may have an effect. Its a close loop system but at present the stepper motor is closing itself down too much causing the car to stall. The stepper is off the car at the mo and is replaced by a manual valve. Another question that springs to mind is - why does the air box go bang on back fires. Surly the increase in pressure can escape out of the inlet pipe as the air filter is not one way..?

morticiaskeeper · #8 Post by **morticiaskeeper** » Wed Mar 07, 2007 7:09 pm

John I was thinking that the backfire pressure wave was just too fast for the filter to cope with, but then I remembered that my airbox suffered all it's damage in the lower section, after the filter. The biggest problem i've had with a backfire, other than the airflow meter being blown off the hose, is the damage sustained by the Vaporizer. I've got a Zavoli 'vap and it's just about impossible to get any info, let alone a rebuild kit. I think the diaphragm is taking a hit and I have to richen the mixture up. Give it a month after a backfire and I can start to lean things off again. Gradually turning the screw in half a turn at a time. My closed loop controller has mixture lights on it, so I can set things up without a pc, I just wish the lights were inside the car so I could look at the mixture under operating loads. If I was you, i would firstly do the plugs and HT system. If it still plays up, I would look at the mixture. Go a bit richer first and not the results. Does your closed loop system allow you to set a fueling map or just a single value? Where are you based? I'm not very experienced with LPG, although I worked on racing engines in a previous life, I just want to see your system, if it's the same as mine, at least we can swap notes

trinity · #9 Post by **trinity** » Thu Mar 08, 2007 9:31 am

Hi, Just a thought on your mixture controller with mixture lights , I cut the wires on mine & threaded tham through the bulk head & refitted inside the car. This way I could watch the mix lights & set my system. Another temporary way is in fine weather bring the controller out from the bonnet & tape to the windscreen temporary in a visable place. As for the jerking when you lift your foot off , is the mix correct at idle?

lazydocker · #10 Post by **lazydocker** » Sat Mar 10, 2007 10:23 pm

It's worth having a search on fleabay for "Formula Power" leads. They're 10mm, custom made 3 core leads and are superb. They work out similar in price to 8mm Magnacore's (for a V8) but have given fantastic results!! Improved burn, thus better economy, and no problems whatsoever with tracking... My Rangie gets VERY wet under the bonnet and I've had no problems at all (touch wood!!!)<img src="http://www.my90.co.uk/lpgforum/clipart/biggrin.gif" border=0> Just a suggestion!!