'95 discovery v8 misfire

spratty · Post by **spratty** » Fri Feb 11, 2011 12:30 pm

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

DaveEFI · Post by **DaveEFI** » Fri Feb 11, 2011 1:09 pm

Unlikely it's done any damage.

The capacitor goes on the positive side of the coil. It has no effect on ignition performance - its purpose is to suppress ignition interference to the rest of the car electronics.

DEVONMAN · Post by **DEVONMAN** » Fri Feb 11, 2011 7:44 pm

Hi. With the coil connected the wrong way round, the spark at the plugs appears to jump from the outside to the central electrode instead of the other way round. Not as efficient but no damage done.
Regards Denis

TomLS · Post by **TomLS** » Fri Feb 11, 2011 8:14 pm

DEVONMAN wrote:Hi. With the coil connected the wrong way round, the spark at the plugs appears to jump from the outside to the central electrode instead of the other way round. Not as efficient but no damage done.
Regards Denis

I'm not disagreeing with you, but by that logic the X0,000 volt surge travels through the chassis and up the king lead? (I know it does any way, the the plug gap removes a lot of the power before the earth)

Wouldn't that cause all the earthing wires on the vehicle to arc the power away? (I don't know any that are 8mm silicone coated

)

I take it you don't have any delicate electrics onboard? Power spikes in the earth could cause a problem?

Like I say - I'm not mocking you, I'm just stupid

DEVONMAN · Post by **DEVONMAN** » Fri Feb 11, 2011 8:20 pm

TomLS wrote:
DEVONMAN wrote:Hi. With the coil connected the wrong way round, the spark at the plugs appears to jump from the outside to the central electrode instead of the other way round. Not as efficient but no damage done.
Regards Denis
I'm not disagreeing with you, but by that logic the X0,000 volt surge travels through the chassis and up the king lead? (I know it does any way, the the plug gap removes a lot of the power before the earth)

Wouldn't that cause all the earthing wires on the vehicle to arc the power away? (I don't know any that are 8mm silicone coated )

I take it you don't have any delicate electrics onboard? Power spikes in the earth could cause a problem?

Like I say - I'm not mocking you, I'm just stupid

In the old days when I was a lad

some coils were not marked and the only way to check if they were the correct way round was to look at the direction of the spark at the plugs. Them were the days

And added to that, we had +ve earth cars too and coils designed for +ve earth. But not much delicate electronics on board in those days.
Regards Denis

DEVONMAN · Post by **DEVONMAN** » Fri Feb 11, 2011 8:27 pm

Does a wasted spark coil pack fire one of the paired plugs in the opposite direction?
Anyone know?

Regards Denis

ChrisJC · Post by **ChrisJC** » Sun Feb 13, 2011 9:36 pm

If you wire the coil up backwards, it won't fry all the earth connections. All it will do is to generate a negative voltage across the plug, instead of a positivev one. So the spark jumps the other way.

I am pretty sure a wasted spark coil does generate one positive spark and one negative spark.

Chris.

spratty · Post by **spratty** » Sun Feb 13, 2011 9:39 pm

thanks for all the replies
since i've connected the coil back the right way, the misfire appears to have gone and the car is behaving itself like it should
looks like i havent done any more damage by not reading the symbols on the coil

spratty · Post by **spratty** » Sat Feb 26, 2011 6:42 pm

well, looks like i spoke too soon, it's still misbehaving.
i'm thinking now that both coils are past their best, so i'm going to replace it. i want to use a bosch coil, but i cant find one anywhere. the only thing i can find is this
does anyone know if this coil will work with my v8?
if not, can anyone direct me to somewhere i can get a decent one that will?

Thanks
Ian

ChrisJC · Post by **ChrisJC** » Sun Feb 27, 2011 8:34 am

Have you tried new leads?

Leads are critical on an LPG system.

Also, close the plug gaps so they are about 3/4 of the 'petrol' spec.

Chris.