Re: [alfa] Re: alfa-digest V9 #1174

[webb.p@xxxxxxxxxxx]

> Question about O-2 sensor, exhaust, cat.----
>     Have an 84 Spider, with minor engine mods. (enlarged ports, higher perf. 
> cams, 10:1 pistons/liners, and headers) done about 10 years ago.  Had some 
> repair on the headers, and my mechanic noted the cat. is all clogged up.  Also, 
> the mechanic had disconnected the O-2 sensor 10 yeas ago after the engine mods. 
>  The car never passes  "official"   NJ inspection here in NJ.  
>     I wonder why the O-2 sensor disconnect was done, as now I want to be able 
> to pass inspections here in NJ.  Mechanic says get a new cat., and he is 
> going to make an exhaust tube connecting the headers to the cat., and locate the 
> O2 sensor in this new tube. However he says he has to experiment to decide 
> exactly where to place the sensor  (i.e., how distant from the engine) to obtain 
> the correct air/fuel mix.
>     My quesion is, wil this work,andr will the ECU be able to  manage the 
> right air/fuel mixture at all engine loads and rpm's?

The O2 sensor should be as close to the exhaust port as possible.   The 'correct' (should be indicated) AFR is constant throughout the exhaust prior to the CAT.  What's usually at issue is the EGT.  Non-heated sensors need to be much closer to the port to maintain heat to report correctly.   If you're running lean and your EGT goes above 1500degF, you could damage the sensor.

If you felt extremely frisky, you could go WB02.   The WB02 controllers from Tech Edge run about $100-200 and the LSU 6066 Bosch sensor is about $50 nowadays from Volkswagen.   The WB02 sensor puts out 0-5v true AFR.  The controller outputs 0-1v to input to your ECU.  WB02 records true, realtime AFR not a slow reacting guestimate the NB02 does.   The controllers usually have either lights or displays to indicate AFR which makes tuning your modifications much much easier.

>      Also, I never figured out why he disconnected the O-2 sensor after he 
> made the engine mods 10 years ago?

Disconnecting the O2 sensor forces the ECU into open-loop mode with no feedback.   NB02 sensors are usually VERY slow and mostly inaccurate.   The usual failure mode of a bad NB02 sensor is slow response to the feedback loop in the ECU, making the car run badly.

L-Jet runs open-loop up to 160degF and when the WOT switch is triggred.  It's active at cruise low-load at temp.   It's possible the 'mechanic' saw it running lean with the O2 connected due to the extra air and figured open-loop was always rich.   You can compensate for the extra air by tweaking the AFM flapper or adding an adjustable FPR, or rising-rate FPR at 1 atmosphere reference (making sure you don't get one designed for forced induction).   This is a trick of the aftermarket boost crowd that can't adjust their AFR for the boost, so they increase the rail pressure based on manifold pressure to add extra fuel under boost.   Stops things from melting most of the time.

HTH

-Peter
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided