Lambda sensors after pre-cats????

[xain]

Hi all,

I've just accepted it for so long, but suddenly I got to thinking, what use is having a lambda sensor after a (pre) cat? Surely all the exhaust characteristics will have been changed by the cat (just look at the second O2 sensor after the cat on any modern family car... straight line readouts on the datalogger, as you'd hope)

I'd have thought that for the ECU to work out what's actually going on with the combustion, it would need to measure the oxygen in the raw exhaust. Otherwise some of the O2 will have been used up in making CO2 or added to by breaking down NO/NO2...

Somebody put me straight!

Cheers
Jon

[chassis 33]

Two off the cuff ideas here, dont know if they hold water though.

1. Theyre pre-cats and only in there to help while the dustbin gets up to temperature.

or

2. So long as all things remain the same it shouldn't matter, because the ECU will not know any different as it will all stay in proportion. The worst (theoretical) thing that can happen is the cats sometimes work and sometimes dont.

Regards
Iain

[Tobs]

Funnily enough I was wondering the same thing recently! I also thought that the O2 sensors needed to be positioned before the CAT to give an accurate reading!

I suppose the ECU could be mapped to take into account the effect of the precats (assuming they are all present and working!). If this is so, then removing the pre-cats may make more difference that was first thought!

Toby.

P.S. It’s now snowing quite heavily here in the midlands (Coventry) if it keeps up as it is I’m wondering if I’ll be able to get home – haven’t driven the Chim in the snow yet!

[chassis 33]

Tobs said:

P.S. It’s now snowing quite heavily here in the midlands (Coventry) if it keeps up as it is I’m wondering if I’ll be able to get home – haven’t driven the Chim in the snow yet!

Was quite heavy up here in Chester this morning, but it's all gone now. You'll be fine.

Regards
Iain