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Still Missfires!


jztoyn
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searched and searched.have tried almost everything..but the missfire in my 25det neo is still there. (can happen as low as 3psi and/or 3000rpm.)

specs..manual,full 3" stainless straight thru exaust,k&n pod,unknown 600x75 fmic,custom ff plenum,80mm tb,bosch 040 pump,profec b2

have just replaced the stock coilpacks with splitfire items and installed new plugs - BKR6EP 8 (0.8mm gap)and have tried two other boost controllers(turbo smart/gfb) both bleed valve.but have since gone back to the ebc.

car drives fine until positive boost comes on..then misses and backfires and will keep doing it until i close the throttle..

the only two things i can think of is afm and or ecu.but before i fork out 2k for aftermarket items thought i would just ask for ur suggestions..any help muchly appreciated.

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Put the ecu into diagnostic mode, see if there's any error codes.

Before cleaning the AFM (electrical contact cleaner or brake cleaner), attack the pod with compressed air, get the excess oil off.

Electrical tape on the coilpacks: http://www.skylinesaustralia.com/forums/Mi...500-t32356.html

regap the plugs to 1.0-1.1mm (might be missing at 0.8mm, might have to go back to 0.9mm or even the stock 1.0-1.1mm gap)

Compression test all the cylinders.

All this can do for free in your backyard if you have the bits.

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how can making the gap bigger solve the issue? if it isn't able to ignite the fuel at 0.8mm gap then going bigger won't help as the spark is stronger at a smaller gap.

taping the coils i don't think will help either as they are new coils, so if they are causing the issue then they must be faulty.

did you get the right part number coils? although if you got the wrong part number coils i don't think it would even run.

i would get it put on the dyno to see what is going on with afr's. could be something like the fpr playing up and dumping in fuel once it comes on boost. could also be an AFM issue.

what boost and ecu are you running? if you can, get a hold of a consult cable and a laptop to check for any fault codes. or you can do it by the flashing light but a consult program is much better as you can get someone to drive and make the problem occur while datalogging to look at hings like injectors and ignition timing and afm voltage when the problem occurs.

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A bigger spark plug gap will create a better spark because it requires more voltage to jump across the electrodes. so yeah open up your gap mate running it below standard spec wont help try goin to the maximum gap in your specs.

Allot of the old school V8 boys used to open up their gaps a fair way to get that better burn of the fuel air mix my old trade school teacher used to do it for all the HQ cop cars in at holden in geelong back in the day he said it shortens the life of your plugs but gave that little bit more poke. Will put more load on coil etc but if you go to max gap in your spec will be sweet.

happy gapping.

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a smaller gap actually gives a stronger spark as it is easy for it to jump the gap so less chance of it misfiring and arcing out somewhere because the gap is too big. a big gap gives a bigger spark but is more likely to misfire. if you look through the pages and pages of misfire threads on here you will find that making the gap smaller has helped in most if them. yes a bigger gap is better as you get a longer spark so better combustion, but if you are getting a misfire at 0.8mm going to a bigger gap won't help.

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Have you checked all your intercooler couplers and looked for vacuum leaks? This would be my first guess.

If you still have the stock BOV, is it still vented back into the intake? If it's not still vented back, it can cause the same effect as a cracked intercooler coupler.

Have you checked your oil level?

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a smaller gap actually gives a stronger spark as it is easy for it to jump the gap so less chance of it misfiring and arcing out somewhere because the gap is too big. a big gap gives a bigger spark but is more likely to misfire. if you look through the pages and pages of misfire threads on here you will find that making the gap smaller has helped in most if them. yes a bigger gap is better as you get a longer spark so better combustion, but if you are getting a misfire at 0.8mm going to a bigger gap won't help.

Sorry mate a bigger gap will will give you a stronger spark because of the voltage required to jump the gap. that gives you a bigger better hotter longer spark to completely burn your mixture. of course to big is always going to be a problem so best not to be rediculous about it. If your getting a missfire at a smaller gap could be the spark aint long enough to burn your mixture. thats if the spark gap is the problem. It could be any number of other things like a bung coil not making enough charge or a faulty ignition module. Gapping your plug correctly and then seeing if you have spark will let you know if the plug is fine or not and wether its somethin else in the ignition system that is the problem.

remember on forums mate everythings not gosbel. there are any number of people putting up things that are not quite legit. im not saying im the jesus of forum but i am a mechanic and and know a little about it.

all the best

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Sorry mate a bigger gap will will give you a stronger spark because of the voltage required to jump the gap. that gives you a bigger better hotter longer spark to completely burn your mixture. of course to big is always going to be a problem so best not to be rediculous about it. If your getting a missfire at a smaller gap could be the spark aint long enough to burn your mixture. thats if the spark gap is the problem. It could be any number of other things like a bung coil not making enough charge or a faulty ignition module. Gapping your plug correctly and then seeing if you have spark will let you know if the plug is fine or not and wether its somethin else in the ignition system that is the problem.

remember on forums mate everythings not gosbel. there are any number of people putting up things that are not quite legit. im not saying im the jesus of forum but i am a mechanic and and know a little about it.

all the best

Actually Mad082 is correct. You said "A bigger spark plug gap will create a better spark because it requires more voltage to jump across the electrodes" which is true. So if you suspect high tension problems, that means less voltage is available at the gap and therefore less ability to spark over a large gap. To compensate, you reduce the gap to make it easier for spark to occur. Its just Ohms law, where resistance is a function of distance.

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i would get it put on the dyno to see what is going on with afr's. could be something like the fpr playing up and dumping in fuel once it comes on boost. could also be an AFM issue.

IMO, you'd be wasting your time on the dyno.

If the car is misfiring, the fuel is not burning and using up oxygen, there fore, whether 10 litres of fuel is going down each cylinder per cycle, the car will read lean unfortunately.

So, plugs have been changed, and coils have been changed.

There is still a LOT of other things that can be done.

Clean AFM for one.

Check the temperature sender that goes to the ECU (The dash may read fine, but it comes from a different sender)

Check the CAS too. It could be jumping around internally a bit as it accelerates.

If you bring it on boost in 5th gear, does it do it exactly the same as in 1st gear?

I'd also be checking all hose clamps, and EVERY air line (Not just intercooler, but all rubber ones too)

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problem solved. turned out to be a lack of fuel pressure ( diagnosed by restricting the fuel return hose with a clamp)drove alot better and did not missfire until alot higher in the boost pressure/later rev range.so i removed the 6 month old bosch fuel pump and replaced with new.now the engine revs freely again.- aparantly it is not a good idea to wire these fuel pumps up direclty from the battery(through a relay of course) bypassing the factory relay,because it puts to much load on them(no priming ect)and they just burn out..anyone else heard this?i bought the vehicle like this(s13 converted)

Edited by jztoyn
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Another thing you may want to check is the Cam Angle Sensor. I am having the same issue on my 95 RB25DET and I just changed the CAS and the issue has seemed to go away. I still need to change the plugs and replace the MAF for other reasons as I just got the car. But if I were you I would take a look at the CAS. As mine would run perfect if I did not go more than half throttle or over .5 kpa boost. When I did it started missing like a fool. When I took it into the shop they said it could be either the coil packs, MAF, or CAS. Hope this helps.

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Actually Mad082 is correct. You said "A bigger spark plug gap will create a better spark because it requires more voltage to jump across the electrodes" which is true. So if you suspect high tension problems, that means less voltage is available at the gap and therefore less ability to spark over a large gap. To compensate, you reduce the gap to make it easier for spark to occur. Its just Ohms law, where resistance is a function of distance.

Oh thats cool i have no doubt that closing the gap will make it spark easier i get that it would make the spark plug work again by closing the gap but wouldn't we be wanting to work more towards fixing the problem of not having enough high tension spark and not just band aiding the situation. better off finding where the problem is and getting it to spark normal again instead of just goin the cheap fix and closin up the gaps. apparently its fixed now anyways.

enjoy

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