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Discussion Starter · #1 ·
Hi,
My car idles very lean at 16.5:1-18:1 after warming up fully. After starting it up and letting the WB O2 sensor warm up it idles at around 15:1 which is normal, but after driving it for a while it starts to idle at 16.5+ .
Last time i removed the exhaust manifold one of the studs got partially stripped (one of the middle ones). I didnt do anything about it hoping that the other studs will hold it tight enough.
My WB o2 is just before where the cat would be. Would an exhaust leak before the sensor cause it to read leaner A/F's? My theory is that once the manifold heats up fully, it deforms (expands) slightly causing it to leak near the stripped stud, and when its cold it seals fine. Does this sound reasonable?

What confuses me is that with any throttle applied (standing still or cruising) the AF's are OK near the 15:1 mark, but as soon as i close the throttle (in neutral or with clutch in) the AF's go to 16.5+++.

O2 sensor is almost new, ECU doesnt throw any codes. I was also thinking it could be a TPS problem but then the ECU would most likely throw a code.

Also, how difficult would it be to helicoil the exhaust studs with the head still in car? Is there enough room to do it properly?

Any ideas are greatly appreciated!
Thanks!
 

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Discussion Starter · #3 ·
Exhaust is a 3-inch turbo-back straight pipe (no resonators/cats)
 

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With the manifold off the head and the head still in the car you could easily helicoil the head for the studs. I have a leak around one of my studs also but my wideband doesnt read leaner than 15.3 at idle. It usually bounces from 14.3 to 15.3.
 
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