Thread: backfire through air box

Originally Posted by krazy kiwi

im new to lpg i have a 3.9 d1 thats started to backfire through the airbox every now and then 3 times on a 100km trip how can i fix it or does it just need a tune up

Almost certainly your spark plugs are not optimum for gas. They need to be 1 heat range colder than standard for petrol, and the gap no more than 0.8mm/0.032". 0.7mm is best. I recommend Bosch part no. WR7DC+ as being very good for standard engines on gas.

Timing sould be set at 6-10 degrees BTDC at idle. ( I set it at 10 for the 3.9) Plug leads should be inductive core (stainless steel spiral core) types and NOT tied together in a bundle. Rather they should be run with about 20mm air gap between them where possible.

Once you have done all that it will normally cope with lean mixtures without backfiring.

thanks i will give that a try

Thanks for the advice.
I'm having the same trouble and will give it a go. I'm running a 179 holden in a 109 2A.

Originally Posted by Sideroad

Thanks for the advice.
I'm having the same trouble and will give it a go. I'm running a 179 holden in a 109 2A.

Similar issues. If it's got the flat seat plugs use WR7DC+, if its the 202 head with taper seat plugs use HR8BC. Also the electronic dissy (blue motor type) upgrade is always worth while.

Originally Posted by krazy kiwi

im new to lpg i have a 3.9 d1 thats started to backfire through the airbox every now and then 3 times on a 100km trip how can i fix it or does it just need a tune up

If you have had 3 back fires through to the Airbox, then also check you MAF, that's often how they end up dead, the little thin platinum sensor wire gets blown to smitherines. So running on petrol might not work too good, try running it on petrol and see if all is still ok.

You should also look at getting a spot to relieve the pressure before it reaches the airbox. In my old playdough we had a hole about the size of a 50c coin in between the airbox and the gas car/mixer that was covered by a thick airtight/fireproof sleeve that went all the way around the pipe. It would expand like a rubber stocking to take most of the pressure from the backfire and not damage the air filterbox, in the prado if the airfilterbox was opened, or one of the sensors in it was loose, your engine would not start and you would get more backfiring as a result! That is why when we got the newer playdough it was put on gas injection so never backfired while we owned it

Thank Bee Utey,
It has the 179 head but bored to 186; and is running the blue Elec. dissy. I will have to check the leads, they may be getting a bit old and were from before the gas conversion. I was running a touch fast at idle and I asked the mech. to adjust it. Now it back fires randomly over 80kph+. A 30 min run can get 0-3 back fires resulting in gas ring and filter being blown loose and damaging the top of carb. I now have the filter loose-ish so it is blown off only.

Originally Posted by bee utey

Similar issues. If it's got the flat seat plugs use WR7DC+, if its the 202 head with taper seat plugs use HR8BC. Also the electronic dissy (blue motor type) upgrade is always worth while.

Originally Posted by Sideroad

Thank Bee Utey,
It has the 179 head but bored to 186; and is running the blue Elec. dissy. I will have to check the leads, they may be getting a bit old and were from before the gas conversion. I was running a touch fast at idle and I asked the mech. to adjust it. Now it back fires randomly over 80kph+. A 30 min run can get 0-3 back fires resulting in gas ring and filter being blown loose and damaging the top of carb. I now have the filter loose-ish so it is blown off only.

The probability of a burnt out lead with these dissy's is quite high if you run the standard plugs with a large gap. Definitely worth putting new leads on.