P38 V8 LPG backfire (again)

When I first bought it (last year), the P38 used to pop back a lot so based on advice on here, I bought new leads and platinum plugs and all has been well for the past 5K miles. Not one backfire at all in a variety of driving conditions. I even bought a new genuine air box & filter at the same time as the plugs/leads.

Today, it back fired and has blown the airbox apart!

So, first question is: How long do plugs/leads last?
Second question is: Has anyone got LPG and a K&N/pipercross etc cone filter?

Thanks!

i have one of those filters, in bits in the cellar now, get a stainless type, doesnt blow up when it backfires.

the bacfiing causes the filter to blow out which changes the airflow/misture which causes more bacfire. only time i get backfire is if i change whilst booting it!

the plugs prolly need changin, but when you are sorting it, get the lead and software for the system and set it up correctly. i reckon it could be running a little too weak so it will backfire when cone filter is on.

what system is it? what ecu does it have?

gavbriggs said:

i have one of those filters, in bits in the cellar now, get a stainless type, doesnt blow up when it backfires.

the bacfiing causes the filter to blow out which changes the airflow/misture which causes more bacfire. only time i get backfire is if i change whilst booting it!

the plugs prolly need changin, but when you are sorting it, get the lead and software for the system and set it up correctly. i reckon it could be running a little too weak so it will backfire when cone filter is on.

what system is it? what ecu does it have?

Click to expand...

Thanks for the reply!
Stainless type air filter?

The plugs are only 5K old, I thought they would last longer than that but I'll pull em out tomorrow to have a look.

It's a single point system, not sure of the make but I'll check tomorrow. The ECU is just the standard LR GEMS thing.

I am going to suggest dumping the mixer ring and get a new multipoint front end kit. It realy will make all the difference and those blowbacks will be a thing of the past. There is no need to get any fancy plugs, standard NGks are fine and cheap enough to change every month if you wanted to. There are a couple of supliers that are offering fully adaptive LPG ECU systems now that are a piece of cake to set up, and once done you can forget them.. worth thinking about.

filter= similar to this: Apiro DYNAMIQUE Single Cone, Stainless Steel Air Filter on eBay (end time 22-Jan-10 13:23:41 GMT)

if the cars running rich it could have killed the plugs, lpg needs a good spark to get it burning!

the ecu for the lpg kit. probably mounted near the cars ecu box, follow the wires from the electronic valve on the pipework that goes into the mixer ring on the air intake pipe.
will prolly look like the item in the top right pic thats on the right hand side
http://img.alibaba.com/photo/11362888/Autogas_Propane_Kit_Products_LPG_CNG_Gas_Kit.jpg

rewmer said:

I am going to suggest dumping the mixer ring and get a new multipoint front end kit. It realy will make all the difference and those blowbacks will be a thing of the past. There is no need to get any fancy plugs, standard NGks are fine and cheap enough to change every month if you wanted to. There are a couple of supliers that are offering fully adaptive LPG ECU systems now that are a piece of cake to set up, and once done you can forget them.. worth thinking about.

Click to expand...

single point is very much old hat these days, ive read a bit about the new (gen 5) type systems. they are getting more and more sophisticated and reliable!

i do however think that the single point is worth keeing going if you don't want to shell out for a new front end kit. they are quite expensive, and when you consider some, as in my case, cars arent worth the layout!

if i had a newer 4.4 or something i would be looking at the later kits.

my single point runs excellently (except for operator errors) and doesnt backfire.

if its just a case of new plugs, filter and the lead off ebay then for circa 50 quid its worth spending the effort.

maybe invest in a newer kit when its time to renew the car???????

I had considered multipoint but as is the case with P38 ownership, there have been plenty of other issues with took my hard earned. I paid £3.5K for it and have receipts for another £3.5K since I bought it in April last year!! The only thing thats not standard are the tyres so it's not like I've been blowing the cash on mods. It did come with old man emu nitro front shocks though, and shagged OEM rears (another job that needs doing).

Sadly the cost of a multipoint front end is out of the question at the minute. I have a look at plugs/leads etc tomorrow and at the LPG ECU and get some pics up, see if you chaps can spot on obvious issues.

Once funds allow, the long term plan is a Disco 3 diesel with a remap...

just thort, when i first got mine, the mixer ring wasnt on the plenum correctly, it was dragging in air. i liquid metalled it to the plenum and its not dragged air in since, worth a look!

my old one used to backfire for a past time, one evening when dark looked under the bonnet and found arcing across the coil pack,,, re-fitted leads and all was well... gotta be worth a look

Right chaps, had a poke around this morning and found a lead was loose were it sits on the coil pack at the back of the engine, not off, just loose and I clicked it into place. After starting it again, it idling a bit smoother
I'm guessing under load, the sparker may not have been strong enough and therefore bang - one backfire.

Checked all the connections are good and tight but I've put up some pics, see if you can spot anything that I've missed.

Gav - couldn't find the ECU as the wires go through bulkhead but I'll check for it when I refit the handsfree kit. There is a BRC stamp on the blue plastic thing near the vaporiser so I'm guessing thats the make.

Waited for it to go dark then fired her up again to check for arc-ing, could hear a clicking then bingo...saw sparks on the left hand side of the engine (looking from the front) towards the expansion tank.

It's a right pain to get at the lead but managed to pull it out and discovered:

Damaged housing where the lead plugs into.

Just waiting for the engine to cool to see if I can get my paws in to make a good connection!

Worst case scenario - new coil block and I bet it's an arse of a job due to its location.

what year is it? if pre 99 then the coil pack bolts on the top of the engine at the back, piece of **** to get to, if post 99 then its a bit trickier!

that system looks like its the older system or someone has replaced the electonic valve. the crew thing on the pipes is where the leccy one would be.

it also l oke someone has silicond the mixer ring on. it should be ok but mine was siliconed and it didnt perform very well.

i will bet tho your issues are down to the plug leads or coil pack or both!

Think they are fella, it's out of gas now as well but after checking all leads are good n tight, I'm gonna give it a blast in the morning.

try to keep it topped up as when it nears empty it will backfire due to less gas coming through, i usually use mine til half full then top up that way theres always plenty reserve if i need it!!!

Got to agree with Gav, if mine gets under half i will fill it back up.

Looking at the pics of the mixer ring, it does look very old, as stated, no electonic stepper valve.

The coil pack will probably be the main problem, mine used to back fire i gave up buying air boxes and bought a sports filter as it was chaeper if it blew to pieces, i have made an enclosure where the air box was, the new filter and the air temp sensor sits inside the enclosure. I then sourced an induction pipe and ran it from behind the front grill and into the enclosure.

the main reason i did it, so the engine breathes in cold air rather than the hot air from the engine bay, as the original air box gets its supply from the wheel arch.

I have since bought some iridium plugs and a set of Magnicor blue leads, since fitting it has not back fired once even under heavy load.

did not notice any performance gain, but i think that is partly due to the maf sensor is now duff after the last back fire.

If any one wants to see some pics of the enclosure i will take some pics and post them on here.

overlandrover said:

Got to agree with Gav, if mine gets under half i will fill it back up.

Looking at the pics of the mixer ring, it does look very old, as stated, no electonic stepper valve.

If any one wants to see some pics of the enclosure i will take some pics and post them on here.

Click to expand...

I used to have a 100L stako tank which took up most of the boot when I first got the car. I had a 80L doughnut fitted instead and I'm sure the chap who fitted it mentioned it had to change the valve for some reason. I also lost the fuel level indicator as the electrics weren't the same, again not sure why I the switch still works as does the indicator if it's running on petrol or gas.

I'd be interested in the enclosure pics, I haven't seen many cold air boxes for these and they're available for just about every other type of motor! (BMW E30 M3 Carbon ones being my fave)

BigJC said:

I used to have a 100L stako tank which took up most of the boot when I first got the car. I had a 80L doughnut fitted instead and I'm sure the chap who fitted it mentioned it had to change the valve for some reason. I also lost the fuel level indicator as the electrics weren't the same, again not sure why I the switch still works as does the indicator if it's running on petrol or gas.

I'd be interested in the enclosure pics, I haven't seen many cold air boxes for these and they're available for just about every other type of motor! (BMW E30 M3 Carbon ones being my fave)

Click to expand...

Not finished it yet, i will do some better pics as these are taken in the dark with my mobile, it is made from aluminium sheet, does the job.

Nice looking install.

Filled up with gas at my local LPG fitter and got him to give it the once over. He suspects that lack of gas and poor connection on the leads was enough to cause the back fire. After checking everything, he said it's a good as a single point will be!

He also had a bright yellow supercharged Hummer in for some work, it gets 7mpg on gas apparently!!!

Thanks for the help chaps, its running good today.