Deciphering O2 readings

[jackmech]

I have an O2 sensor bung in my exhaust and I'm using a heated 3 wire sensor. I was driving around today trying to see what was going on and I can't figure out what I'm reading. Fuel pressure was perfectly steady, although a little high (29.5 psi or so). I was reading the O2 sensor output with a voltmeter. At idle and full throttle the reading was what I would consider pretty good (around 0.7 to 0.8 volts). But what maintaining speed with the throttle just cracked and any mild acceleration things got really rich (around 0.1 to 0.2 volts). My next step it to get my injectors tested, but I'm curious if anyone has any opinions about my readings. It was quite odd to me that the readings were so varied from almost lean to rich.

[Jim Smolka]

Most O2 sensors have a narror range. A wide band O2 sensor may be in order. What type of Fuel management system do you have, stock?

[hardflex]

could be you have a bad mps, check the vacuum diaphram. you would get good readings when idling because it's on the idle curcuit, and full throttle because it's set for that. just off idle it would go rich, and would lessen until it was right at full throttle.

just a thought.

[Dave at Pelican Parts]

The three-wire sensor is a narrow-band one. You cannot trust it beyond "rich" or "lean". How far rich or how far lean the readings show depends almost entirely on the exact temperature of the sensor, and not very much on the actual mixture. I wish I had bookmarked the response curve graph that I had seen...

Rich is good under acceleration. You might not have a problem--it's tough to tell...

--DD

[John Rogers]

These two pictures might help. When I made the air/fuel meter from the PP tech article I found that the voltage really hopped around a lot, mainly due to the exhaust pulses from the cylinders into the collector. At high RPMs the meter (voltage) reads steadier due to less time between cylinder firing. Since you said that the voltage was low, it appears from the pictures I finally got to upload below, that the mixture is on the lean side, I.E. high voltage = rich and low voltage = lean. I might be reading that wrong though? Dave or someone can help me out here?

[John Rogers]

Here's one

[John Rogers]

Here's the other

[Dave at Pelican Parts]

There's also a graph I've seen that shows that first curve multiple times. Each one is at a slightly different temperature, and they differ quite a bit.

--DD

[jackmech]

From reading the O2 sensor info at work (NAPA Auto Parts) I found out that the heater on the O2 sensor is supposed to bring it up to operating temperature, so it shouldn't really need exhaust heat to operate fully. The O2 sensor is installed in the #4 exhaust runner.

I have the stock 1.7 D-Jet with a rebuilt MPS which was tested by Jeff Bowlsby and he found it to be fully functional.

[jackmech]

I took some pictures. The first one is just idling and reads around 0.1 volts and the second picture is me quickly alternating reving on and off the engine just to make the guage read higher. It's about the same reading I got while actually driving. Just reving the engine up to high RPMs didn't raise the reading (it was still down near 0.1 volts).

FYI, the guage is set to the first black scale with the numbers 0, 50, 100, 150, 200, 250 which relates to the voltages 0, 0.5, 1.0, 1.5, 2.0, 2.5 volts.



After taking these pictures I'm still very confused about what's going on with my car. It's confusing to think about.

[914timo]

jackmech, you can NOT use analog volt meter. The current is too low. You should use digital volt meter. That may be the problem.....

[Va914]

Can someone tell me what wire does what for a 3 wire O2 sensor?

I have one with no instructions on which wire is the signal wire.

Cheers.

[rockaria]

Originally posted by Va914
Can someone tell me what wire does what for a 3 wire O2 sensor?

I have one with no instructions on which wire is the signal wire.

Cheers.

I have a 3 wire O2 Sensor from CB and this is how it works:

White wire = 12v source
2nd White Wire = Ground
Black Wire = Signal

The manual to it is on the CB Performance site here:

Air/Fuel Ration Spec Sheet

I hope this helps,

[jackmech]

914timo, everything seems to be working fine. Readings are within range and such. Maybe I have a really good analog meter. =)

I'm thinking now that I'd better get a 5 wire, wide-band sensor for tuning engines. The narrow-band sensors just aren't suited for informative readings, they're more applicable for FI computers.

[Dave at Pelican Parts]

Wide-band is expensive. The sensors are tough to find nowadays (Honda just jacked the price for them through the roof and told its suppliers that they can't sell to anyone else!) and you need an interface board in order to get any readings out of them.

--DD