O2 Sensors

[RTS]

O.K., this might be a topic for the "Off Topic" section, but it's engine related, just not my boat engine, so I'll give it a shot here.

How often should properly functioning O2 sensors switch from 1V to 0V in the course of 10 seconds at 2K RPM. The Service Manual for the car I'm working on says "at least 5 times in 10 seconds" and according the the self diagnostic light in the car I'm getting about 1 time per second. I seem to think I read somewhere it should be quicker.

I'm getting an intermitant rough idle (ides good, then idles bad 10-15 second cylcles), and in the diagnostic mode, the rough idle goes away when the self diagnostic light is on, but goes away when it's off (or vice-versa, I can't remember right now). This happens the same way on both the right and left O2 sensor. It's the same on/off idle as when I'm not in diagnostic mode.

To my untrained nose, the car seems to be running rich.

Funny thing is, when I disconnect both O2 sensor harnesses, the car idles a little rough, but at least it's consistent (ECM not trying to adjust the fuel flow, I'm thinking)

Would you thing I have bad O2 sensors, or would you suspect the problem runs deeper than that?

P.S. car is a '95 300ZX Non-turbo with 83000 miles, and O2 sensors don't look to have been changed anytime in the recent past...

Edited March 4, 2006 by rts

[MoonDawg]

O.K., this might be a topic for the "Off Topic" section, but it's engine related, just not my boat engine, so I'll give it a shot here.

How often should properly functioning O2 sensors switch from 1V to 0V in the course of 10 seconds at 2K RPM. The Service Manual for the car I'm working on says "at least 5 times in 10 seconds" and according the the self diagnostic light in the car I'm getting about 1 time per second. I seem to think I read somewhere it should be quicker.

I'm getting an intermitant rough idle (ides good, then idles bad 10-15 second cylcles), and in the diagnostic mode, the rough idle goes away when the self diagnostic light is on, but goes away when it's off. This happens the same way on both the right and left O2 sensor. It's the same on/off idle as when I'm not in diagnostic mode.

To my untrained nose, the car seems to be running rich.

Funny thing is, when I disconnect both O2 sensor harnesses, the car idles a little rough, but at least it's consistent (ECM not trying to adjust the fuel flow, I'm thinking)

Would you thing I have bad O2 sensors, or would you suspect the problem runs deeper than that?

P.S. car is a '95 300ZX Non-turbo with 83000 miles, and O2 sensors don't look to have been changed anytime in the recent past...

I have read that the sensors last about 80K miles as long as you don't abuse them with fuel additives so your O2 sensors are at the end of their life expectancy. They are not that expensive but are hard to change since they seize into the threaded bung. Usually you need an Oxy/Acetaline torch to heat them until they are glowing read and then remove them.

[RTS]

Some more research on the web tells me I should be looking for 5-7 on/off cycles a second from the sensor in a MFI vehicle (carb systems should give about 1 on/off cycle per second, TBI 2-3)

Does this sound right to any of you in the know?

I'm thinking MoonDawg is right, and that the sensors are at/very near the end of their life.

It's going to be loads of laughs changing those things out. There's not quite the room in that engine compartment as there is on my Monsoon

[Pistol Pete]

I'll try to help but, take what I say with a grain of salt because I'm not a Nissan tech. (even though I own two of them) and, I can't see the car.

1) Do you actually have a check engine light or are you just noticing the intermittent rough idle?

2) If you have an engine light, what is the code?

3) Do you have 3, 4, or 5 wires coming out of the o2 sensors? There's a huge difference in the way they operate and info. you get on the web is just too generic.

4) What type of meter are you using to read the voltage? An analog meter is going to be waaaay to slow to read the voltage swing. You need a digital meter.

5) In my experience, I can ususally find a bad or "lazy" o2 sensor at idle. And, you say the engine gets rough at idle so why not test everything when the problem exists? That 2K rpm test is convoluted.

6) In my experience, normal "old school" 3 wire o2 sensors would swing from 0-.6 volts at idle. They will sweep about once a second. High voltage = lots of fuel (rich condition) going by the sensor and vice versa. You are leaving the wire connected when you read it with your meter, right?

7)I don't know what you guys are talking about as far as "normal" life expectency. My Nissan Sentra is 12 yrs. old and has 126K on it with the original o2 sensor (and clutch by the way) and, it spent the first two years of its life in Boston dealing with road salt. My other Nissan (2001 Pathfinder with 90K) has all three out of four of its o2 sensors original. I mistakenly replaced one because I thought it was bad but, it ended up needing a software re-flash. In my 12 years of turning wrenches at a dealer, I never said to a customer "Well Mr. So and So, your car has 80K on it, you know those sensors are gonna go bad somet time soon. Do you want me to go ahead and replace all of them while you're here today?"

8) Which brings me to my final thought. If you don't know what you're doing, don't guess and throw parts at your car. These things are more complicated than you know. There's a reason the dealer gets ~$100/hr. to work on them. $100 worth of quality diagnostic time is worth more than a $100 o2 sensor that you can't even get at and then end up finding out it wasn't the problem anyway. For all we know, you've got an intake leak or an E.G.R. problem.

[RTS]

Thanks Pete,

They are 3-wire (heated) O2 sensors. I'm using a digital multimeter in conjuction with the O2 self diagnostic mode built into the ECM.

At idle, the O2 sensors are going from rich to lean and back over the course of 1-2 minutes, not seconds. I use the ECU to put the Check Engine light into the O2 sensor diagnostic mode, you can check each side individually, and thats where I'm getting the rich lean info. I confirmed the voltage at the harness to be cycling at this rate (0.95 Volts to about 0.05 Volts)

When I'm at high voltage - rough idle, At low voltage, idle is quite smooth.

I have no intake leaks, and I just replaced the Idle Air Unit, which made a huge improvement, and it got me to about 95% of the idle quality I remember these cars having.

The sensors are not throwing a check engine code, because according to the Service Manual, they will not as long as they go Rich - Lean 5 times in ten seconds at 2K RPM. They certainly are not doing that at idle.

I checked continuity between both harness connectors and the ECM connector. All 6 (3 each side) showed 0 Ohms

I'm getting a code 55 - All clear.

Edited March 4, 2006 by rts

[Pistol Pete]

I don't know what this 'self-diagnostic mode' is. I'm not that familiar with Nissan engine systems. But, it would occur to me that I'd want to be looking at the o2 readings with the car under 'normal' conditions.

I'm also not sure why you're on the o2 sensors. It seems to me that if the idle gets rough when the sensors are reading high (rich cond.), maybe the sensors are doing their job and reading correctly. Especially if there is no fault code. I don't know about this 'no code if it swings so many times'. It would seem to me that the car's computer will notice any irregularities and throw a code way before you'll see them on your meter under your shady tree. (no offense)

Anyway, good luck, I hope you find the cause. I think I'd be looking for reasons why the idle gets rough under this rich condition. I.E. P.C.V. problem, E.G.R. problem, air pump, variable intake, M.A.F., M.A.P., E.C.T., Baro sensor, banana in tail pipe?

[MoonDawg]

rts,

This link has some good stuff on the science behind the O2 sensor.

[MoonDawg]

If you want to get deep into the science you can see why the O2 sensor has a life expectancy.

The sensor is always reacting to O2 even when the engine is off. When O2 enters the sensor a chemical reaction takes place that causes the lead anode to oxidize.

2Pb + O2 --> 2PbO

When all of the lead is oxidized away the sensor fails.