Stock Blacktop SR, lacking low power, rough idle, lots of backfire @ low/off throttle

[BoroDrifter]

I hate to post these questions up as I am a decent mechanic and have no trouble diagnosing problems I run into, but this is just kicking me in the ass and I can't seem to find any one with the same problem!

Setup:
-Stock S13 Blacktop SR, 65k miles.
-Walbro 255
-Z32 fuel filter
-3" RSR downpipe
-HKS SPL intake
-Stock side mount
-NGK BKR7EIX-11 gaped to 0.30 <- changed plugs that night to iridium
-Mobil 1 10w30 high mileage at startup and switched to 10w40 high mileage before I drove it hard

Installed this motor as about 3 months ago and it ran like a champ! I ran an open down pipe and while it idled just a tad rough from time to time, that was the only cause for concern and I figured it was due to the fact that the AIV bung on the turbo elbow was open and there was no exhaust pressure from the open 3" DP.


I installed my exhaust with a 2" cat (I know, I know) I drove 8 hours to Nashville TN, ran an event with no problems at all. Lost a little power from the small cat, but nothing to worry about.

A week or two after the event, back home, it starts running a little funny. Rich symptoms, backfire when I let off the throttle or give it very very little throttle. Idle got worse. I figured the bung being open was causing the problem. I noticed that while it didn't run bad, no hesitation or misfires, it just didn't have the power it quite had before the idle got worse.

My exhaust randomly fell off one day (shitty bolts) and I left the cat on.

Another week or so and the back fire gets worse and so I deal with it for about a month chalking it all up the bung, open pipe and probably time for a plug change.

I decided to change plugs a week ago and seal up that bung.
I replaced the Japanese PFR6B 9, which were rather ashy and white, but generally just worn out, with NGK BKR7E-11 gaped at 0.28.

The idle was still rough and seemed to be deteriorating as time progressed (and it got colder out side) On warmer days, 65~75* it ran rather well, considering... But now it's around 40~50* and here I am, no power.

I removed the cat just to let off some restriction and boost rolls on just fine but no real change other than faster response from the turbo and its loud as shit, louedr than before when it ran fan.

MAFS
I swapped MAFs for two known working ones. No change, though if I unplug the maf, it idles fine, right at 900 and revs freely but only to 2500 as it should.

WIRING
I found the the o2 power wire was smashed and melted a bit at the +12 source (red/black wire) so I replaced that length.
I noticed also that the MAF wires were a little iffy with bad solders, so I replced them with new 14g wire. No definate change.

The idle now, as of today likes to fall to around 200rpm and every once in a while die. But intermittenly it will idel normal, right at 900 sometimes over 1000, but then it drops back down and starts sputtering. I even detect a slight misfire at idle.

Current Driving/Operation Conditions
Over all the car runs fine, just slower with a lot of back firing off the trottle. It doesn't hesitate in the traditional sense, no bucking, no sputtering or mising. Boost is responsive and solid. It pulls pretty hard once it gets past 4000 revs or so. The easier I go on the throttle, as in - I let boost build and fuel burn a bit- it makes better power, but it is just not right.

Other posible conditions to be ruled out
I have a vacuum manifold that feeds my recirc and wastegate with new lines.
The charcoal can and FPR come off the throttle body with no cracks or leaks.
The nipple on the intake elbow and the cold pipe are sealed.
The side mount should be free from leaks (audiably) though I have not removed it to test.
I recently cleaned the HKS filter.
The o-ring at the fuel pump/tank is new.
The coolant is full.

Recap of Symptoms
Rough/low idle with slight misfire, intermittenly idling fine
Slow acceleration under 4000 rpm, though still not perfect after.
Backfiring on decel and low/partial throttle.
Loss in MPG (normal 610 km/tank, current 480 km/tank)

I suppose next test would be TPS and then CAS.
But if any one has any input I would be glad to hear it.

Thanks and I will update as I try new things.

[MrFairlady]

Check the CAS.It can cause ALL types of issues.
Mine took a shit 1 day and the car cut off,...after inspection...the metal "wheel" inside was warped like a 12" record that sat nest to an open flame all day....and was Cracked in about 3 spots that were not visable @ all hardly.

No way to test it but Try another one if you can find one.

Wires on the o2 sensor should not be cut,...The wire from what i remember is like Steel or some shit.Very "Hard" wire.

when you wired the Maf...did you by chance use any left over KA wiring....I cut the whole Maf wire off the KA because its Shielded and ran it.My car has Been a Champion Since I installed it besides the CAS.

[BoroDrifter]

Thank you for a detailed response.

I just used good old 14g wire from autozone, that seems to run it just fine. I will test voltage stabilization once I get a new multimeter. As for the o2 wire, it was just the +12v wire so I honestly just trimmed back the burnt out strip as the wire was about 6 inches too long to start with. What had happened there was the battery moved out of its tie-down and smashed the wire causing a short.

I made a stupid mistake when I bought the new plugs. I looked today and they are V-power. I ran down to the parts store, grabbed some BKR6EIX-11 Iridiums and that cured the stumble on idle. Now it idles like a champ.

The hesitation may be simply a leaky manifold. I used a Mr.Gasket full copper exhaust manifold gasket which needs to be tightened after initial use. Well apparently it needs to be tightened more than once because a couple of the bolts where a little better than finger tight.

I still want to swap out the CAS to be safe, but that will come after I throughly tighten the manifold. Odd, the things you can over look when you get worked up, eh?

I'll note any changes.

[SoguRacing]

try cleaning your maf sensor. also and more than likely you do have a vacuum leak somewhere causing your AFR's to be off causing stumble.

[fliprayzin240sx]

Checked for vacuum leak? Pressurize the system, the best way to find leaks. If that doesnt do it, double check the timing like said already and I'd recommend changing the o2 sensor too.

[redline racer510]

check your coil

[BoroDrifter]

I'm going to leak down and/or compression test tomorrow or Monday. I smelled a little coolant today at the track.

[sxe]

check timing, and CAS like Mrfairlady mentioned, if neither change check coil, look for leaks, and if all else fails do a tear down and inspect everything...

[BoroDrifter]

try cleaning your maf sensor. also and more than likely you do have a vacuum leak somewhere causing your AFR's to be off causing stumble.I pulled the maf in cleaned it with some brake cleaner while I was changing plugs. The idle is 100%. Anything after seems...rough.

Checked for vacuum leak? Pressurize the system, the best way to find leaks. If that doesn't do it, double check the timing like said already and I'd recommend changing the o2 sensor too.I pulled the entire intake and intercooler set up off last night. I saw no cracks, holes or leaks of any kind. I checked all 4 vacuum lines I have and they are all solid.
The exhaust manifold was very loose with the exception of the outer two nuts.

How could timing fall out of place with out touching it? Just asking..I will check it though.

As far as o2 sensors, I have a spare "skinny" one that works, I hear there is no difference in funtion, just wondering if it is true or if I will be able to use it.

check your coil
I've never heard of a coil just going bad but I suppose anything is possible. I'll look into testing those.

Thanks for all your responses guys. I really appreciate not getting flamed. Like I said, being it is a stock motor, I just can't find a good reason why anything is screwed up. It's not like I have mismatched after market parts that the others can't keep up with. It just makes no sense.
Thanks again, and I will again post anythign I find.

I may pull the manifold and turbo off tomorrow and check for any problems.