VX Backfiring (LPG)

[Fonix]

Hello Guys

Driving home tonight , i fueled up , drove the last 15 ks fine, car was its normal punchy self running on gas, then when i hit the last 1k on a dirt road something strange happened, i think i heard a bang (backfire?) not too sure, because i had the music up and it seemed the car didn't have the power it normally does, if put it in first on a dirt road it can normally spin the wheels, but it seemed to struggle to do this , even on petrol but i am not sure if i am imagining it or not.

I just took it for a quick 5k run then and it seemed alright on gas, until i tried it in first, put the first down , and i heard a bang again, so i am now assuming that this is a backfire. also know the car doesn't idle as it should hovers around 600-1000 all the time.

I did a service on the weekend , oil+filter, air filter, plugs, and it ran the next day fine on gas 150km plus trip then i was running on petrol from Tuesday morning until tonight.

Does anyone know what can cause this?

Also why i would loose power after a backfire? (or is that my mind playing tricks on me) and is there anything i should be checking after this backfire to make sure its all ok.

Thanks for your time

[petesmith]

Same thing happened to me on a few occasions, couple of things to check:

- o2 sensor
- any vacuum leaks (quite common)
- leads/coil pack

I would check for vacuum leaks first, happened to me on my VT couple of weeks ago and a vacuum hose had come off, the little one underneath the throttle body (prick of a thing to get to)

[Fonix]

Ok , ill check the vacuum hoses as soon as i can, if any are off is it still alright to drive the car? and would it explain any power loss? or anything else i can listen to on my drive to work tomorrow morning that would explain its them?

Also Ive Read about air boxes being destroyed , i'm guessing this would look pretty obvious if it happened?

[Skyworka]

Same thing happened to me on a few occasions, couple of things to check:

- o2 sensor
- any vacuum leaks (quite common)
- leads/coil pack

I would check for vacuum leaks first, happened to me on my VT couple of weeks ago and a vacuum hose had come off, the little one underneath the throttle body (prick of a thing to get to)

Your spot on... I am a mechanic at a gas place... backfires are most commonly caused by an ignition fault... a faulty o2 sensor could cause it to lean off tho this is an unlikely cause...

the VT/VX commodores are HORRIBLE for blowing off the vacuum hose that goes under the throttle body... a way u can tell this is a problem is under light throttle u can actually hear the leak... also make sure u check the other end at the drivers rear of the engine... cable tie or clamp it on if u can in anyway to stop this being a repeat problem

As to why the initial backfire happened... check the plugs brand/gap... there are a few things to do... generally that is step 1 that i would do...

[petesmith]

its ok to drive the car but it wont run well at all!! As skyworka said, you will be able to hear the vacuum leak as soon as the car is on, sounds like a hissing sound...
Air box, very obvious! I ended up super gluing my vacuum hose under the throttle body back on, i did cable tie it but it came off again...

[Fonix]

Well i had a quick look when i arrived at work this morning, couldn't see anything around the back of the throttle body, are the any pics as to what hoses i should be looking for ?, but i am presuming that i need to take the cover of and shine some light down the back.

I did notice that it seems to have blown a corner of my air box, yay this is starting to look like fun and expensive at the same time. That wouldn't be effecting power would it?

When i did the service and changed the plugs, i put NGK's in and pulled out what i think were original gm ones , not sure who put the originals in the gas installers, or the local mechanic, seeing as its the only think i have changed ignition wise, would it be a good test to put the original plugs back in and see if i can replicate the backfire?

[Tree cutter]

Did you buy plugs to suit running on gas.

[Fonix]

There Just standard NGK plugs from repco.

[Fonix]

Well i took it to the mechanic across the road from work today, he said he checked from vacuum leaks and didn't find any , but said the cooling system was low, and topped it up and re-bleed it, he belies there's a leak somewhere but ill deal with that latter, also said the air box will need to be replaced.

car seems to be driving ok when i drove it just before, guess i am going to just take it easy on gas until i can get it serviced/tuned can anyone recommend a place? (Dandenong area - i have been told about Gas Research on Frankston-Dandenong road.

[Tree cutter]

The low coolant level would have been the problem that caused the backfiring. Your gas converter needs to have coolant running through it to stop it from icing up and shutting off the gas supply. Your engine would have been running lean which will make it backfire.

[Skyworka]

The low coolant level would have been the problem that caused the backfiring. Your gas converter needs to have coolant running through it to stop it from icing up and shutting off the gas supply. Your engine would have been running lean which will make it backfire.

You'll actually find that the converter icing up will cause it to run rich... the gas ends up not converting from a liquid to a gas like it should but almost staying as a liquid as the converter starts to freeze up... it is an issue tho... check the plugs u put in... what gap are they... we fit the standard plug but gaped down to 1.3mm... it is also a posibility that when they did ur plugs they may have split a plug boot... or possibly cracked the ceramic... or it may just be bad luck...

a good way to check the ignition system is to get a water bottle... and lightly spray the leads and the plug area with water... this provides a easy path for the spark to jump... if you hear a ticking (arcing) and the car starts to miss fire... there is a problem there and get that seen to...

Just don't do what i saw one guy do and get ur garden hose and do it with that... hydro-locked his engine :P