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04 STI silver\silver
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217 Posts
Discussion Starter #1
nothing but blank stares on the other board..

After doing an ECU reset the other day to clear a CEL that has been there for quite a while the car started running a little funny. Idle was ok if a little high maybe 800 RPM’s and driving was good to unless I went to WOT it runs incredibly lean (A/F meter to front O2 sensor). If I ease on it is usually good no matter where I am in the RPM range but after I say 60 to 75% throttle and above it seems to stop sending fuel I guess drops right off the scale if I ease up it goes back to bouncing back and forth between rich and lean. Little background I have a MY99 with a Cobb intake, borla headers random tech cat and Stromsung midpipe w/ dual-tip. The last ECU reset I did and had a similar problem but it cleared up after 2 days this time although I am only driving the car one a day it’s lasted about 4 so far. Any Ideas? The intake was working well even with out the S-AFC which I still have (not hooked up right now since it wasn't necessary).


Fuel filter didnt help that much and plug number 3 looked a little ashy. I am going to check the fuel pump and MAF sensor tomorrow.
 

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98 RS (Busted), 04 350z
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3 Posts
From what I've heard, it's not abnormal for a MY99 to run lean with an intake, you should also take the reading from your stock front o2 sensor with a grain of salt -- it's not exactly a precision instrument. When you say "drops off the scale" above 75%, you mean it drop off on the rich side? You also descibe the car kicking into open loop operation which is normal.

What you seem to be descibing is a lean throttle tip-in condition which just means your ECU hasn't worked out acceleration enrichment values since the reset. Drive around doing quick changes from low to high throttle and see if the ECU doesn't catch up to speed.

I wouldn't worry too much though :)
 

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140 Posts
Does the Cobb intake use the stock air plenium (big black box), or is it just a tube? I think the MY99 will run lean if you don't use the plenium.

Also, I think that the stock sensors are not the appropriate sensors to use for the A/F gauge because they are not wide band (?) I would probably try to invest in a dedicated wide band sensor for the gauge, and weld that bad boy into the borla header. That's what I did with mine.

I got a CEL once after the MAF replacement, but it went away after a little while. Car always ran fine. I had the exact same exhaust setup that you do, and was running NGK plugs and the Ganzflow Phase II MY99 intake that retained the stock plenium. Never had any problems with hesitation or running lean. I had the AFC hooked up, but never had to make any adjustments.

Also, if you have the Autometer A/F gauge, it is not made to take readings at WOT. If you want it to, you have to get it modified. One such place that does it is:

http://www.gadgetseller.com/gauges/index.htm

Hope this helps a little.
 

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Premium Member
04 STI silver\silver
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217 Posts
Discussion Starter #4
Well it’s going into the shop tomorrow @ 9am to get looked at. Hopefully it’s the MAF sensor since I haven’t had it changed over yet. I never had any problems with the intake before all it is a Cobb intake cut after the filter w/ space enough for the MAF works WAY better than the Ganzflow top end increased tremendously. I let you know whats up. interesting on the O2 sensors but isnt the widebands crazy expensive?
 

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I got mine for a couple hundred bucks. Can't remember where though. Good luck with the hunt. My car, unfortunately, is going back to completely stock, so I can have some warranty security.
 
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