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Will exhaust leak cause lean WB O2 reading

3K views 11 replies 5 participants last post by  tekmode 
#1 ·
I've had two dyno runs on separate dynojet systems and they both show an A/F ratio of 13-14 below 5500 rpm. A normal reading should be 11.5-12.5 The ratio does fall below 12 right before redline at ~6200 rpm. The sensor was placed in the tailpipe during the dynojet run and prior cars showed correct A/F numbers. My car has an AEM intake, Hotshot header, and custom cat-back exhaust.

Does anyone know if a header flange or collector to cat pipe leak would cause a leaner O2 reading? I don't really think I'm pulling in more air than the qr25 stock mappings can handle. I have an Autometer A/F guage that I will hook-up this weekend to double check the readings at the primary O2 sensor. Don't know, not too many gurus around for this engine yet. Appreciate any help.
 
#7 ·
Yea, I don't have it in my sig, but I have relocated the KS and silenced it. After the relocation, the dyno curve and numbers are nearly identical. Something else is robbing my low end torque and some top end power. The joys of upgrading continue!

cortrim1 said:
how hot was it when the car was tested. also if you have a header you should have the k/s relocated because the extra vibration added by the header trips the sensor causing the ecu to dump fuel.:D
 
#8 ·
ck03specv said:
Yea, I don't have it in my sig, but I have relocated the KS and silenced it. After the relocation, the dyno curve and numbers are nearly identical. Something else is robbing my low end torque and some top end power. The joys of upgrading continue!

Is there any way u can tell yourself if there is a leak, do u hear anything?

Possible exhaust piping?
 
#9 ·
Hotshot said a leak would not cause an issue unless a large enough leak exists on cylinder two. That's where the O2 sensor port is. This is not the case because the ecu would see the false lean and add fuel, causing a richer mixture to appear at the tailpipe O2 sensor.

Otherwise, I have a leak on intake side after MAF or my scavenging is so effective that it's overrunning the ecu mapping. If indeed the scavenging is that good, I may be purchasing an s-afc-ii in the near future. I have not heard of anyone else with this issue though. One thing is for sure, I'm activating my knock sensor tonight due to the lean conditions.

So, the hunt continues.
 
#11 ·
Don't know about easily, we're still only talking about 20 HP or so at the upper rpm range. After I start, my upshifts land me back in 130+ HP range. Similar to your car.

Besides, your in denial, your a good drag racer! ;) One of the times I raced you your freakin response time was .165 seconds! Jeez, how can I compete against that.........


tekmode said:
no wonder you weren't running very fast at kennedale. You should've easily caught me after the 60'
 
#12 ·
ck03specv said:
Don't know about easily, we're still only talking about 20 HP or so at the upper rpm range. After I start, my upshifts land me back in 130+ HP range. Similar to your car.

Besides, your in denial, your a good drag racer! ;) One of the times I raced you your freakin response time was .165 seconds! Jeez, how can I compete against that.........
I just have good reaction and launching. It's not that hard. Just took some practice. :D
 
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