Low Load Missfire

[mad082]

i have had this issue for a while and it has just gotten worse. it used to be that at idle it had a slight pop. now it is at idle, or cruising at very low throttle. odesn't really matter what rpm it is. i can make it do it at 4000rpm if i go along at a steady speed and just keep the throttle in the 1 spot. you will feel it be a bit sluggish, then it will clear, then it will do it again a few seconds later. if you floor it it will sometimes have an ever so slight hesitation, then it will rev out, and you can go up through the gears without any problems. it only seems to be if you go from cruise to WOT quickly.

i thought it may have been the afm, so i cleaned it last night. didn't help. the spark plugs aren't that old (about 4000k's, and the missfire has been happening for about 2000k's), but going to replace them anyway (they are currently BCPR6ES gapped at 0.8mm). coils are taped up.

its an r33 rb25det manual. i'm running stock bov, turbo back exhaust, $22 boost controller, pod, fmic, remapped ecu (but i stuck the stock ecu in and it still does it). it doesn't seem to do it until the o2 sensor comes up to temp and the signal starts to bounce (i have a air/fuel gauge - i like flashy lights). the o2 sensor is only a few months old as well, but might try my old sensor again.

any other ideas as to what it may be. can't find any vacuum leaks, but going to have another look tonight. might have to clean the aac valve again (it was also only done a few months ago).

[Busky2k]

Certainly sounds like a fueling problem. Maybe its running too lean? Whats your O2 readings when it does it? Is it still cycling or does it go lean?

[mad082]

i was thinking too lean or 2 rich, but more towards too rich. i can't remember what the gause was doing last night. all i do remember is that as the o2 sensor warmed up it was heading towards rich. and i know that a quick blip of the throttle would send it to rich.

[mad082]

i'm also taking a bike pump home tonight to try and pump air into the manifold to see if i can find a leak. its a bit easier to hear a leak when the motor isn't running.

[skylinegtts]

ive got this problem onli that it happens the opposite way around. it happens when i start driving the car when its cold and stops doing it after it comes to operating temp, i.e after the temp gauge has gone to half way up.

any idea what it could be? or if the two problems are related?

[Busky2k]

I don't think it'd be an issue of too rich as misfire normally happens when its lean. You'd be blowing black smoke and it'd still won't misfire.

Edited October 18, 2006 by Busky2k

[mad082]

well i disconnected the o2 sensor and it seems to be fine. the o2 sensor is only a few months old, so i might put the old 1 in and see what it does then.

[moonus]

u prolly running too lean.. get it on a dyno, maybe its the pump dying or fuel filter..does it detonate at high revs?

[Busky2k]

Usually when you disconnect the O2 sensor the engine will run rich. So yeah sound likes a leaness problem.

[Godzilla32]

Id look at those taped up coils.. If they're cracked throw them away and buy some new ones.. IMO the most common cause for misfire under load is ignition. Check for arc marks along the coil tubes and hairline cracks on the coil bodies. Probably a good idea to check ignition timing too.

[mad082]

i doubt it is the fuel pump. i had a wideband o2 sensor on it a few weeks back and the afr's are 12.0 through the middle of the range, then fattened up to 11.5 at top end. that was at 11psi. and the fact that when i disconnected the o2 sensor it ran fine.

it doesn't ping up top. vacuum is at about -21 on the aftermarket gauge, so i don't think there is a vacuum leak. i'm going to check the tps tonight and see if that is out of whack. and check the IAC valve (although i don't really want to since it is hard to get to). might also get a new fuel filter. maybe i got a bad batch of fuel and clogged the filter up.

as for the coils, i only taped them up to be on the safe side. there were no cracks in them when i taped them. and since i fixed the problem by disconnecting the o2 sensor, that means they aren't the cause. and it isn't under heavy load that it misses. its at idle, or at extremely light throttle.

Edited October 18, 2006 by mad082

[Rolls]

I had this problem on my old rb20det. First thing in the morning itd miss and overfuel cronic, around 1/4 throttle it be like a rev limiter poping and backfiring like crazy, full throttle was fine, itd rev out without an issue but 1/4 was f**ked. It wouldnt idle proplerly then magically itd fix itself and be fine after about 10mins. Some days itd be fine as well for no good reason. Replaced fuel filters, ran injector cleaner, cleaned out the AAC and the throttle body to no avail, it did make it run smoother when warm though. I think it was just massively overfueling though as you could almost see a cloud of black smoke when it was happening, itd make your eyes sting and you would feel dizzy sitting at the lights.

I changed plugs, coils all spark related stuff but it didnt fix the problem in the slightest, replaced O2 sensor and a few other things. Never solved it and ended up selling the car for other reasons. A few people suggested temp sensor and that the ECU could have just been buggered.

Edited October 18, 2006 by Rolls

[mad082]

well i know it isn't the ecu because i have tried another ecu in it, and it still did it.

i took notice of what the o2 sensor was doing last night. it was pretty much just staying at the 1 voltage and not moving much. it would slowly lean out slightly (not quite into the lean section of the gauge) then when the hunt would start it would bounce from where it was to rich.

the problem is that it will run fine 90% of the time, then ocassionally do this. so that leads me to rule out vacuum leak, etc, since that would make it do it all the time.

[R32Abuser]

Do u know for sure that its not the AFM. Just because u cleaned it doesnt mean that that has ruled it out...Mine went cactus the other day and had symptoms very close to yours,even though i cleaned it

. Mite pay to check the voltage on the AFM and see whether it is itermitantly cutting in and out as mine was. Drop Gary (Sydney Kid) a line and see if he knows the voltages for a 25 AFM and what its all supposed to read. Only other thing i could suggest is ur o2 sensor, check ur coilpacks one by one, check the connections for these, or

[mad082]

i'm going to stick my old o2 sensor back in tonight, since unplugging it last night helped the problem. the only reason i got a new o2 sensor to start with was that i was chasing better fuel economy. and it made no difference. the economy still sucks.

and if the problem is still there i'm going to borrow a mates afm on the weekend and see if that does anything.

[mad082]

what are the possible causes to make a car run lean at idle?

vacuum leak is always the most obvious, but what others?

and they would have to be something that isn't always there. a vacuum leak should make it do it all the time.

[Wheezy]

ive got this problem onli that it happens the opposite way around. it happens when i start driving the car when its cold and stops doing it after it comes to operating temp, i.e after the temp gauge has gone to half way up.

any idea what it could be? or if the two problems are related?

Ditto

[mad082]

that could be moisture in the coilpacks.

[moonus]

if u can unbolt ur cat check that, it mite be melted!

[Rolls]

if u can unbolt ur cat check that, it mite be melted!

I doubt that would affect low load, full load it would.