Engine Tuning

[alvav]

I have a 1991 CTSC and I just installed a GruppeM exhaust (no cats). I don't know if it's the lack of the cats or the o2 sensors screwing up, but there was some black smoke (running rich?). How do I check my A/F mixture? Also, is there any way to adjust? Thanks.
Leo

 

[nsxotic911]

Removing cats does indeed affect the 02 sensors since the CAT's are no longer , and due to the black smoke it is runnign very rich.
Without aftermarket engine management, it's hard to tune it out, but I've heard there are some tricks; don't know them though...
BTW: Have you tried resetting the ECU, seeing if it can re-learn? I'm surpirsed you don't see CEL.

 

[Daedalus]

Cats don't affect O2 readings. The O2s are upstream of the cats. On '95 and later NSX, the cats are monitored by an extra pair of O2 sensors, but these extra sensors don't affect A/F ratio either, they just measure cat efficiency.

You can backprobe the ECU harness connector to get voltage signals from the O2 sensors. Stock sensors will pretty much only say rich or lean, unlike wideband sensors, which tell you how rich and how lean. They should be alternating between rich and lean rapidly (above .45V then below .45V). Most likely they're not. You'll need either a pretty fancy digital voltmeter, or a cheap analog one to see the voltage movement.

 

[nsxotic911]

Cats don't affect O2 readings. ...

I guess I may have been mistaken. I see that there are 2 O2 sensors on the header, and 2 behind the cat (or on.. can't remember) but maybe only the headers is what measures A/F

If that's the case, then why does he get black smoke, which is often because of rich mixture..?

 

[Daedalus]

A little black smoke may be normal. It seems the car tends to run a bit rich at idle. It needs to be fully warmed up to be in closed loop mode. When cold all cars runs rich. A stuck thermostat could leave the car a bit too cool at idle. I believe the CTSC setup also raises the fuel pressure, but I'm not sure how and at what RPM. None of this should be any different before or after the new exhaust though.