Odd stumble between 4200-4500 RPM. Ideas??

[EM_S13]

This is our long on going SR swap project. Now w/ 91 octane in the tank, it runs just great except for 1 thing. Under hard acceleration when it reaches about 4200 RPM it stumbles a little: Acceleration slows and it slightly feels like driving over cobblestones, then at 4500 accel slams in hard and it pulls strong all the way to 8K.

Already checked and/or adjusted MAF, TPS, IAC, set timing, checked codes, O2 (its fucked btw), pipe couplers, no boost leaks, BOV not leaking, brand new HKS turbo so this time we know its good (went thru 2 bad used turbos previously).

Engine mods include GReddy big boy intake manifold, piping, intake, intercooler, BOV, HKS GT uprgrade turbo, Walbro 255. I'm probably going to recommend upgrading the injectors and a rising rate regulator after replacing the O2, think it could be an issue with the stock injectors or FPR?

Thanks a bunch guys,
Ace

[mtx450]

When my car had a bum o2, it stumbled like crazy. There is tons of info on this stuff.

[deesz]

it sounds like the maf. try taking out the maf spraying some electric cleaner on the needle and letting it sit for a while till drys.
do you have an aftermarket intake?

[usdm180sx]

Did you check the spark plugs?

[Skullavera]

Check out the engine harness grounds as well.

[mtx450]

it sounds like the maf. try taking out the maf spraying some electric cleaner on the needle and letting it sit for a while till drys.
do you have an aftermarket intake?

He said he checked that already in the first post.

Did you check the spark plugs?

This should be checked, as well as all the basic things that should be checked in this kind of situation. Sometimes, and most of the time, the problem is very obvious, but people just assume the worse and start naming things that usually don't break very often.

[SHIFT_*grind*]

What's your boost set at? With no fuel management/upgrades, that could be a problem if you're boosting higher than stock.

And on an unrelated note, I didn't see "built head" or even "rocker arm stoppers" on your list, so I'd be careful revving to 8K =P

[HalveBlue]

Could be a lot of things.

First thing to check is codes. Go from there.

Off the top of my head, issues with the following could all be culprits:

Knock Sensor
MAF
O2 Sensor
Sparkplugs
Injectors
Water temp sensor
Vaccuum leak
ECU

[EM_S13]

Yeah, i already checked to plugs and compression tested it and it all checked out. No vacuum leaks either, i've checked that all out too.
We have a UEGO on the way so i'll see if the problem persists after that.
It was running very poorly before when the turbo was shot and in that process i checked out all the Knock Sensor, MAF, O2 Sensor, Sparkplugs, Injectors, Water temp sensor, Vaccuum leak, ECU. Injectors are good but may possibly need a cleaning. Thing is its tough to see how they fire w/o x-ray glasses or firing them into a bucket & thats dangerous.

Its not a big stumble, the car runs good and smooth its just that very tiny window between 4200 & 4500 where it slows and then after 4500 it pulls HARD!

Boost is at 10psi & no fuel or engine management and yeah no built head either, the 8K revving was on accident a couple times. Remember when it said it pulls HARD above 4500? We dont take it above 6500 or 7... yet

Thanks for all the help so far guys!

Ace

[usdm180sx]

Perhaps it's the bum O2 sensor and the ecu compensating? It's hard to tell but here's a graph of my a/f:

http://i97.photobucket.com/albums/l232/usdmsilvia/s13_af_ratio.jpg

Notice the notches on the a/f. My ecu is also untuned so the ecu is compensating for the boost increase. Boost is set at 11 lbs. using an HKS actuator. Good luck and sorry I can't be more helpful.

[mtx450]

Sweet Equips bro!

[EM_S13]

/\/\ Hey THREADJACKER!!

Yeah thats what i was thinking since i unplugged the 02 and it didn't make any difference. Plus the one thats in there looks like an oil dipstick since our 2 previous turbos blew their guts everywhere. UEGO should be coming in today, lets hope that handles it!

[Slidin240Wayz]

It's not going to handle anything...just give you another number to work with.

You still have to fix the problem.

I think a tune is needed.

Carlos