Miss Fire

Hi guys

Can any one help with the next problem I have with my 1998 P38.
It has a very anoying miss fire through the rev range I have fitted new HT leads and new plugs and this sorted it for a short while, but its back. Test book has shown no problems and I have had nearly all the sensors checked.

Peter

is the air intake system airtight, if you has a single point lpg system with a gas mixer ring attached to the plenum then seal it tight with either silicone or some of that liquid metal stuff!!

did mine and changed plugs now no backfires whatso ever!

There is no LPG no mine Not realy sure what you mean there !!!!!

Peter

basically check everything is connected as it should be so at least the mixture is not to lean due to air ingress under vacuum of the engine pulling air in through a leaky air intake system!!!

i would suggest also that the plenum could be leaking at the seal towards the bottom or maybe a faulty coil????

there doesnt seam to be any leaks so far

coil pack, is either of your cats glowing at all?

ill check

Hi
Just checked the Cats and they are not glowing but i do get a rattle when its throtled hard sounds like it could be loose or broken bits in the cats may be,

sounds like collapsed cat.......does it rattle when nudged with your hand?

On the same note, i have black plugs and the same mis-firing through the rev range...changing to NGK plugs seems to have made it slightly better (although I could just be thinking that).....could I have the same issue? Mine is a 1995 HSE 4.6.

could be coil pack?

hmm....just read that elsewhere.....£180 off ebay......how can you be sure? Is there a simple test?

ok, the other fellas has a spark issue so does yours, change plugs, leads and in his case cps, now the only next place to look is coil pack. have a look when its dark and see if you can see any arcing when it misfires. probably best to do it where there are no light sources to interfere with your view and take a small mirror so you can have a good look!

you can also put a krypton type tuning meter on it and see which plug lead is at fault from the display and thus find out which plug/lead/coil is at fault.

its not guarenteed just what i would look at!!

i am supposed to be picking the car up tomorrow......bought off autotrader last week. Even the 'dealer' said he suspected a slight mis-fire. He has changed the plugs for me but is still not happy. Said I could chuck money at it and never find the problem.....I have a friend who is a mechanic (Honda and Volvo trained) who will look at it as soon as I have picked it up.....I am hoping it's just leads tho.......what would make plugs black anyway or are they supposed to be black? The car has only done 78,000 miles and is 14 years old (5,500 per year on average).

the plugs can go black very quickly. on one of mine that was in the pot where the liner had slipped it was half black and half normal, until it went totally tits up!!

i would ask the dealer to drop the price by £180 and get a new coil pack with the cash, they are easy to fit and that way it will at least iliminate the coil pack!

hmm....it may not be a mis-fire tho.....i am not used to the sound of a V8, but when it revs, it feels burbly and shakes the car slightly......

good example......rev it to say 3,000 revs, hold it there, and it doesn't sound/feel smooth

it should sit like a sowing machine and there should be no movement when the revs are held. if its misfiring then you will get a bit of shake. its hard to describe and hard to spot but you will know.

mine was still most of the time. didnt even know it was on when at idle but if it missed a beat you gould tell. if you listen to the exhaust closely you will be able to hear it miss.

funnily enough, it sounded like the exhaust was blowing....but wasn't......throaty noise coming from mid/front section underneath.....

dunno what that is, could it be blowing at the joints under the car, stuff a rag in the exhaust and hold it firm see if is blowing.