Odd Misfire
 

Well, so far ive had about 2 weeks trouble free Silvia ownership but as is the way with old cars the problems are starting to creep in. First it was the O/S Ball joint but ive got them on order so no biggie, then it was uneven/lumpy idle but thats oddly gone, now its a fecking pain of a misfire. Basically, under full throttle conditions, the car pulls like a train, no hesitation or misfires. But, try to drive normal and fml its a nitemare, made worse by the fact its an auto. If I drive under 3000 rpm it wont accelerate smothly, put it under any kind of load such as to speed up a few mph and it misfires like a pig, get it past 3000 rpm or give it more than half throttle and its absolutley fine.

So to sum up, anything under half trhottle and between 2000-3000rpm it misfires, anything over these and its spot on???
Its had new plugs, rotor arm, dizzy cap and the leads are decent magnecor ones. Any ideas guys?

Its the CA18ET engine fyi.

Try farthing the coil maybe ?

air flow meter maybe.

Im thinking along the same lines. As its only under very specific conditions, ie throttle possition and engine speed. As I said under boost it pulls fine which to me rules out a spark issue as surely when the cars being driven hard any weakness in the ignition side would show up more? I dunno. Thankfully I have a mate who also has an S12 and im gonna pop over to his later to borrow his afm off his car and see if that makes a difference. Also trying to source an afm just incase. Will keep you posted. :D

Well, changed the air flow meter over for my mates which was working fine in his car but it made no difference.
On the way home I noticed a few more details to the issue, when the car is missing/hesitating there is black smoke pouring out the exhaust but when the boost comes in all smoke goes away and the car pulls as normal. Also, the problem is significantly reduced when the car has full warmed up and has been driven for a while. I took the plugs out today and apart form being a bit sooty they're fine.

So now theres a vast number of things to start guessing. Gonna start with the cheapest bits and go from there.

black i think.is unburnt fuel from the misfire i believe maybe a compression test ?

try a different fuel reg too as they can go wrong and give you to much pressure.

Rite, wee update. Fine in the way to work thismornin but fuckin nitemare on the way home. Pulled the fault codes off the ecu and got code 22 which is fuel pump. So Im gonna investigate that area first, check wiring, relay etc before replacing the pump. The iregularity of the fault had me a bit perplexed, like I say it was spot on thismornin on the way to work but n the way home it would barely move. Symptoms consistent with my old xr3i just before the fuel pump threw in the towel so im pretty confident ive found the fault. Just need to be 100% before forking out £75 on a Walbro pump.

Related quiestion, are all the Walbro pumps the same? See some with specific fitting kits but closest to the S12 is an S13 DET kit. Would that do the job and are they hard to replace?

i usede a walbro for a pulsar, they are easy to fit.

Now heres a thing. I was out probing around today with the multi-meter and discovered one of the wires in the fuel pump loom had a pretty severe kink in it. Straitened the kink out and fired the ol girl up, fired up no prob (normally needs a bit of throttle and runs lumpy for a few mins) took it for a drive and it drove absolutley perfectly, no hiccups, lumpyness, misfires, nothing. Let it warm up and still fine. Now im very confused. I refuse to believe that the kink in the wire was the cause of all my problems. Only time will tell, need to take it to work and back tomorow so will see how it performs then.