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LPG VX backfiring

jughead1

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My VX Series II, converted to dual fuel, has started backfiring when starting from traffic lights - occasionally.

Also, the idle is occasionally above 1,200 RPM when in gear (Auto) and the car is stopped.

But after the last backfire this morning, the idle dropped back to 600 RPM.

We recently had the heads reconditioned, and the oxygen sensors, plugs and leads were changed about 18 months ago.

The mechanic said we need new spark plugs.

Any ideas besides changing the oxygen sensors and plugs?
Could the mechanic have set the timing wrong when putting the heads back on?

Any help appreciated.
 

Cashman_309

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This would be a job for someone who's qualified with gas systems, it's something that shouldn't be mucked with by an end-user or unqualified mechanic.
 

oska

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change the plugs set the gap to 1.3 mm and replace the air filter

that should fix your problem
 

jughead1

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This would be a job for someone who's qualified with gas systems, it's something that shouldn't be mucked with by an end-user or unqualified mechanic.
I intend to get a mechanic to look at it. However I live on the Central Coast, and there are no gas mechanics in the area who know about the IMPCO system I have fitted to my car!

I am simply trying to find more info about the possible cause of the problem before taking it to a mechanic.

The air filter was changed witht he heads.

I will change the plugs and oxygen sensors (that does not need a specialised gas mechanic) and see how it goes.

Thanks.
 

phatvtcom

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My VT is on lpg too ive had this problem countless amounts of times. Check that your leads are intact and you've gapped the plugs accordingly. Sometimes it may vary depending on the amount of k's you've done. For instance I was told to gap mine at around 1.2mm but it kept backfiring at the same time so I went down to 1mm on all plugs. Never had a problem since.
If it hasn't been the plugs that you've replaced first, guaranteed it's them
 

jughead1

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How would plugs cause back firing? I would have thought it would be something to do with the timing.

Also, this problem only started after the heads were removed (after the car over-heated and damaged the heads).

Cheers,

JG
 

Cheap6

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Ignition timing no. That's not adjustable.

A lean or rich air fuel mixture can do it. As can spark plugs that aren't igniting the mixture in the cylinder (^). It ends up burning in the exhaust and, if you're unlucky, back through the open intake valve on intake and exhaust valve overlap. The leads may be at fault as well or instead.

Standard plugs - both gap and heat range - are best but they should be replaced at 20K km intervals (on petrol it's 40K km).

There will be electronic control (correction) of the LPG air fuel mixtures but the basic tune of the LPG system has to be right too. You can get a situation where it's rich or lean in part of the load range and vice versa in another. The electronic part tries to make up the difference but doesn't do it very well. Most noticeable during changes in engine load, like accelerating or decelerating.

It's possible that there has been a vacuum hose left off or reconnected incorrectly. There's only a few of them though.
 

Nut Kracker

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When I had my EF falcon a few years back, I went to get a set of plugs for it and was advised by Tom at my local car shop to get "Gas Plugs".
I don't know the difference between them and the normal plugs (hotter or colder or whatever), but the gap was pre set for gas and the car ran a lot better with them.
 

pc355

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Hi Mate, I had a similar problem with my vx, brand new top gun leads and would backfire under load, all the time! swapped the leads to Eagle's and didn't touch anything else, never had a problem since, no matter how hard I excelerate!
good luck with it.
 
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