'95 v8 misfire

[ian_s]

i've been getting a misfire on my discovery lately, sometimes accompanied by a intake blowback when running on LPG (3 times this morning within a mile of home).

so far:

i have changed the plugs - NGK bpr6es - the ones that came out were slightly sooted on one side of the plug only, all 8 looked the same.

changed the rotor arm as the tip of the one that was on there looked like it had been burnt/eroded slightly.

swapped the coil for a spare bosch one that i removed last year.

(the spare coil was swapped out trying to rectify my horrendous fuel consumption which turned out to be a fault with the LPG vapouriser)

while swapping the coil, i noticed that the older bosch coil and the newer intermotor coil had the terminals on them the other way round.

It looks like i got it wrong when i swapped the coils last year, I connected the terminals wrong.

i've done something like 22,000 miles with it connected wrong, but it ran OK like that until recently.

Is it possible that i've done damage to any other parts in the ignition system, like the amplifier?

also, i noticed the RF suppressor was attached to the -ve side of things but i think it should be attached to the +ve, can anyone confirm which is right?

Thanks

Ian

[Bowie69]

I'd be surprised if you had damaged anything running it backward, you end up with just a weaker spark IIRC.

There's two reasons for LPG backfires, one which you have started on is the electrics, the other is the LPG leaning out for some reason.

From the electrics side I would go and get a genuine dizzy cap and rotor arm, if the rotor arm you have is a pattern part, throw it away, they are a big cause of running issues... Also, how old are the leads...? could be worth swapping them as arcing between the leads can cause the wrong plug to fire at the wrong time and therefore a backfire.

From a fueling POV, I would change any LPG filters you may have in the system, and then get the mixture checked, any garage can do this, check it at idle and at 3000rpm to see if it is at least close.

Check for air leaks after the mixer ring as this will weaken the mixture ( I am assumig here it is a single point rather than sequential...)

Then go from there

[ian_s]

Thanks

it misfires on petrol as well, but doesnt blowback, which was why i was going down the ignition route. also the sooted up plugs.

the HT leads were replaced last year with genuine parts, they are all routed so they dont touch each other, or anything else apart from the plastic clips that hold them. i've watched it running in the dark, and couldnt see any flashes/arcing.

the new rotor arm is genuine.

dizzy cap i think was a pattern part last year so that's on the list to be replaced.

the LPG system is a single point one, but it's a closed loop system, vip100 i think, which has 2 LEDs to show the mixture, green is lean, red is rich.

on idle it sits mainly on red, flashing green occasionally. at roughly 3000rpm it flips red/green regularly.

of course, this is assuming that the lambda sensor is working as it should, and is giving the correct output to the lpg box

i really would have liked to be able to see the lights on this while the engine is under load, but it's under the bonnet and the cable on it is too short to move it anywhere else.

it's MOT was only a few weeks ago, and the mixture was spot on according to that, but then it wasnt misbehaving back then.

i've only got the one LPG filter on it, will take that apart later and see whats inside it.

Ian

[Bowie69]

Thanks

it misfires on petrol as well, but doesnt blowback, which was why i was going down the ignition route. also the sooted up plugs.

Right, once you have the cap changed, I'd try and known good ignition amp, it's the only other thing that can cause this sort of misfires.

the LPG system is a single point one, but it's a closed loop system, vip100 i think, which has 2 LEDs to show the mixture, green is lean, red is rich.

on idle it sits mainly on red, flashing green occasionally. at roughly 3000rpm it flips red/green regularly.

of course, this is assuming that the lambda sensor is working as it should, and is giving the correct output to the lpg box

i really would have liked to be able to see the lights on this while the engine is under load, but it's under the bonnet and the cable on it is too short to move it anywhere else.

Know that system well Sounds like it is setup pretty well too.

To check, can you not just cable tie the ECU to the wipers and go for a drive? I am assuming it didn't backfire under heavy load, rather under coasting or light throttle?

I am suspicious, as you say it is on petrol and LPG, that once you sort the electrics the problem will go away.

Good luck

[ian_s]

To check, can you not just cable tie the ECU to the wipers and go for a drive? I am assuming it didn't backfire under heavy load, rather under coasting or light throttle?

unfortunately it's fitted right at the front of the car next to the coil, and some of the cables are only just long enough for it to reach there. moving it anywhere isnt possible.

i did see on the tinley tech website that you can get a remote viewer,Link

that might be an option if this continues much longer.

and yes, it goes mostly while cruising along at steady speeds, tho it has just done it again to me in a carpark.

i've very rarely had it blow while switching over from petrol to gas, and that was only when i was running low on gas i learnt that lesson quite a while ago

[Pwyll]

Right, once you have the cap changed, I'd try and known good ignition amp, it's the only other thing that can cause this sort of misfires.

Be careful with the amplifiers, it is perfectly possible for a brand new one to be faulty. Go on, ask me how I know!

Apparently this is becoming more and more common with factored electrical/electronic parts. to make it worse, once you open that little bag that it comes in you can't take it back. Factors are fearful, rightly so, of pikey types putting their duff unit in the new bag and asking for their money back.

My Dad works in the motor trade and his boss has told him to stop using factors for this type of thing and go stright to the main dealer.

Just my £0.02, obviously.

[ian_s]

well, fingers crossed at the moment, since i changed the wiring back to being the right way round on the coil, it's been running fine. no misfire, no bangs.

i've only done about 40 miles though, so it may not be fixed yet, we will just wait and see.

Ian