Sudden A/F change...running leaner.

[fstlane88]

Noticed it tonight for the first time, on my PLX WB. Happens off idle between 1k and 2k RPM, A/F goes in the mid 16's, and then with more throttle/RPM richens up and drops into the 14's, 15's. But, when cruising I also noticed it bounces around in the high 15's to low 16's.

Motor doesn't run any different, just like I said noticed the change tonight. That's what has me worried, things don't just change without a reason.

I am running MKIV 550's with an aftermarket ECU that is supposed to be tuned for them, but it runs rich under WOT, so still needs some fine tuning. Could it be O2? I'm running my WB 02 in place of the factory unit, and all has been fine until tonight. Didn't notice any vacuum leaks, and like I said just noticed it tonight.

Any help appreciated guys, and am I worried over nothing....

Also, forgot the mention, fuel pressure ~33 psi with vacuum line off.

 

[tissimo]

Thats odd that it just started happeneing tonight. Look for an exhaust leak giving false readings on the o2 sensor.

 

[fstlane88]

Thanks for the reply Tissimo, wanted to ask where I should be looking? Manifold area, downpipe, I'm assuming before the 02 correct? Could a leak anywhere in the system cause it?

 

[JASONA70]

anywhere before the 02 sensor