O2 sensor

[Christian Hack]

Along a similar line I have an LM3914 based mixture meter on my Nissan RB30ET.
Last night when I was lowish on fuel (still around 12-15 litres left) I noticed that
my mixture meter was not going all the way to the rich end of the scale. It would
go to somewhere around the 3 last LED (about 0.8V) but not all the way. At the
lean end of the scale it would go all the way. At a constant 110km/h it was
moving up and exactly like normal except that it didn't reach the end of the
rich scale. 

Then I looked at the ECU (on Nissans they call it an ECCS). The ECCS has two 
LEDs to flash fault codes when in diag mode. In normal mode they flash together
to show the mixture is right. However only the green LED flashed which means
that the mixture was lean (from memory - but possibly it's rich).

The coolant temp seemed a little higher than normal too. Is this a probable side
effect of a slightly lean mixture?

So why did the ECU think the mixture was lean when the O2 sensor was putting
out a signal toggling through the magical 0.5V point? 

Filling up with fuel made the problem go away (the ECU said the mixture was right
and the LM3914 meter was bouncing all the way to the end of the scale). So I'm 
guessing my fuel pump is on the way out. Plus the car always seems to run better 
on a full tank (even when you're talking an extra 40-50 kg of weight). Any Aussies
know where I can pick up a cheap VL Turbo fuel pump? Anything else that would
cause this problem besides the fuel pump?

Thanks
Christian

> 
> From: "David A. Cooley" <n5xmt at bellsouth.net>
> Date: Mon, 15 Mar 1999 07:41:30 -0500
> Subject: Re: O2 sensor
> 
> Closed loop depends on more than the O2 sensor activity... it depends on
> time since start on some models, coolant temp, throttle position AND O2
> sensor activity.  When the O2 sensor is hot, it should be bouncing from
> lean to rich (low V to High V) several times a second... At idle it should
> be sending a voltage (But it can ONLY be monitored by a Digital meter with
> several Meg-Ohms per volt input impedance...an analog meter across it will
> ruin it!)
> 
> 
> At 08:23 PM 3/14/99 -0700, you wrote:
> >I have a question about how to tell when the computer is in closed 
> >loop mode when a person doesn't have a scan tool.
> >I built one of the O2 sensor gauges using the LM3914 chip.  What 
> >should I be looking for to tell when closed loop is on?  From what I 
> >have read, I should see the gauge fluctuating several times a 
> >second (or as close as the eye can tell).  Also, with the O2 sensor 
> >disconnected from the computer, the O2 sensor does not send a 
> >voltage during idle even though it is a heated sensor.  I find this 
> >interesting.  
> 

Christian Hack
christianh at edmi.com.au
EDMI Pty Ltd
Ph : (07) 3888 3066
FAX : (07) 3888 3583