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LPG dual fuel gas backfire / frontfire fix


 
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Pajero Stue
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Age: 49
Zodiac: Virgo
Joined: 25 Nov 2021
Posts: 3
Location: Brisbane

PostPosted: Thu Nov 25, 2021 4:34    Post subject: LPG dual fuel gas backfire / frontfire fix Reply with quote

HI all,

So I have been reading everything I can about an NM Pajero LPG vapor mixer backfire. I have had the car for a while and ever since the timing belt was done by the previous owner, it would blow up the air box many times a day. It is a 6g74 Exceed.
This was mainly when accelerating under load. Normally taking off from a roundabout of after a right angle corner etc.
I thought I had better get to the bottom of the problems. I am doing this so someone in my shoes someday can stop pulling their hair out and rectify a very frustrating issue.
Having back fires / front fires like this shows there is an issue somewhere. A mixture of gas and air in to an engine is a simple process that really cant have too much go wrong with it. Rich or lean is about the only thing that changes. My car has a OMVL R90/3 mixer, and a Gastec FP32 controller.
Anyway, this is what I did.
I checked the compression. I cant remember the values but they were all similar.
I replaced the spark plugs - numerous times with different varieties. - Still backfired
I looked at the leads at night and could definitely see cross firing between the plug leads. I changed them with new NGK leads and thought this is obviously the issue. - Still backfiring (although less I think)
I wrapped the leaps in EPDM rubber of about 1.5mm  thickness - Still backfiring
I took the head off as I thought it may be the valves leaking. I re-seat the valves and redid the timing belt to go back to square 1. I also did the valve stems seals while I was there. - Still backfiring!
I changed the crank angle sensor - still backfiring
I bought the 5mm lead holders and spaced the leads so they were not touching each other or any metal by at least 1.5mm. My plugs were re-gapped to .9mm as most people recommend as I thought the path of least resistance would be through the plugs. - Still backfiring (although less I think)
I rebuilt the OMVL mixer with new seals and diaphragms as I was thinking that a lean mixture in a hot engine may be the cause. - Still backfiring
I re-programmed the Gas controller - Still backfiring

Now, I was close to giving up but thought that the gas/air mix shouldn't spontaneously combust. Even when hot. It would need an ignition source. After re-seating the valves and fixing the cross fire, I was struggling to see what it could be. I started thinking it may be the throttle controller or something like that giving off a spark. This is like looking for you missing phone in the fridge as you MAY have put it in there. We have all done it, and I bet it has NEVER been in there...

My last resort was then to go back in to timing (thinking it may be too advanced). Also replace the O2 sensor.
My airbox at this stage was half plastic, half silicone and an OKY strap. The aluminum honeycomb in the MAF sensor resembled a ball of Alfoil, which made life very hard in seeing how the engine was running. I was constantly trying to straighten it out as best I could. They always run rough without the comb to change the air from a turbulent flow, to a laminar flow across the diode.

I only had a few booms left until the airbox was no more.

Before I looked at the timing again, , I went and bought the cheapest set of plugs I could find and I re-gapped them to .7mm. I was clutching at straws at this stage. (Autolite 3924)
Anyhow I put them in and the backfiring stopped. Well it stopped for about a week and I had 2 booms in this time. I had found something that had a very obvious change.
I then re-gapped the platinum plugs I had in previously (Autolite APP3924) to .6mm and have been backfire free for a couple of weeks now. It starts on gas and runs fine.

Now I dont know why the plugs need to be gapped from 1.1mm all the way down to .6mm but it seems to have solved my issues. The only thing I want to do now is replace the O2 sensor which is coming via ebay.
The car has over 350 000 kms on it so the engine is tired.

I hope this helps someone one day. As everyone says, start with the leads and plugs. 90% of the time, this is the issue. Don't be scared to re-gap the plugs down to .6. I dont know if there is anything detrimental in this, (or even lower), and someone please chime in if you can see anything.

Good luck

Admin, please move this to the relevant section if you think it is posted incorrectly. This is my 1st post and may be wrong here.
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PostPosted: Thu Nov 25, 2021 4:34    Post subject: Google Ads keep the POCUK free to join!


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