O2 sensor and open loop and Vf

[SPD TRP]

When the O2 sensor is unplugged, does is go into open loop and Vf goes to zero, or does Vf just go to zero?

Been tackling some lean-rich issues.

Thinking O2 sensor is way out of whack.

Wayne

 

[jetjock]

When the sensor signal is lost the ECU defaults to slightly rich. Around .95 lambda. The sensor can be tested on or off the car. All you need is a meter and a propane torch for either test. Try not to set anything on fire

 

[SPD TRP]

When the O2 sensor is unplugged, does Vf go to zero?

 

[jetjock]

With the sensor disconnected the system is in open loop so Vf will be zero.

O2 sensor test on the car:

1) Warm the engine up.

2) Connect your meter to either Vf or Ox. Connect the other lead to battery negative. If Vf set the meter to read 5 volts and jumper T1 and E. If Ox set it to millivolts and leave the jumper out.

3) Run the engine at 2500 rpm. Watch for the meter to switch back and forth between 0 and 5 volts (Vf) or 0 and 800 mv (Ox) at least 8 times in 10 seconds.

The signal should average 2.5 volts (Vf) or 450 mv (Ox). That's stoich. If the meter has a Min/Max function use it. If not jot a note on your forehead to buy a better meter.

4) If the meter reads hard over rich (5 volts or 800+ mv) pull off the brake booster line. The sensor should swing lean (0 volts). If it does it's working and something in the system is driving it rich. If it reads hard over lean (0 volts) inject some propane into the booster line. You can also give it a shot of carb cleaner. If the sensor swings rich (5 volts on Vf or 800 + mv on Ox) it's working and something in the system is driving it lean.

Off the car:

1) Remove the sensor

2) Set voltmeter to mv. Connect the positive lead to the sensor wire. Connect the negative lead to the sensor body. Light the torch.

3) Heat the sensor tip. Don't melt anything but get it good and hot. Watch the meter. It should go to at least 800 mv (rich). Remove the flame. The meter should go to less than 200 mv (lean). Play the flame back and forth over the tip. Each time the meter should swing rich and lean within one second. The quicker the better. Finally, heat the tip for at least 2 minutes and be sure the meter stays at rich (800+ mv). Again, don't melt it.

Any sensor that passes these test is good enough to work in this engine. Put it back on the car and congratulate yourself.

 

[Burntz]

Or just go get a new one? lol. I think id rather pick one up and know for a fact that its good then do a bunch of tests on it.

 

[jetjock]

Maybe that's why you're on here asking for advice eh? Monkeys can swap parts, especially poor monkeys who got that way by needlessly replacing good ones. Cuts into money that can be spent on bananas. Nuff said.

The first test can be used to troubleshoot all kinds of other problems....

 

[Burntz]

If you get a universal one, they arnt too bad. If hes running rich, Id say check for codes, although i'm not sure if these supras will throw a lean bank sensor code like all obd II's, but id imagine since its a sensor, itd throw a code?

and Id just replace it

 

[Burntz]

I was reading through cygnus, and code 21 is o2 sensor, so maybe he should check for that?

I'm not trying to be an ass, just helpful lol

http://www.cygnusx1.net/Supra/TechTips/engine_error_codes.aspx

 

[pimptrizkit]

With the sensor disconnected the system is in open loop so Vf will be zero.

mine will be at zero on start up, and when it warms up it will bounce around 2v's even when my afr's are lean or rich. tho my stock o2 is only there for the heater wire, my lc-1 is hooked up but the maft pro is not reading analog out's right so i didn't think the ecu would either.

 

[jetjock]

I was reading through cygnus, and code 21 is o2 sensor, so maybe he should check for that? I'm not trying to be an ass, just helpful lol

It's all good. We're all here to be helpful. Just that I feel substitution is a poor method of troubleshooting. It can be expensive, frustrating and is something people who take pride in their diagnostic skills are loath to do. Another point is not everyone who reads this forum is a parts monkey and of those who are I'm betting few wish to remain so. The post was directed at them.

Our ECU is not as sophisticated as OBD-II but yes, there are three codes the sensor could set. The OP seemed unaware of them so I decide to skip it and go with something he could actually do. The problem is two of those codes won't necessarily indicate a bad sensor. The lean and rich codes only indicate the sensor stayed hard over. You're still tasked with determining if the sensor is faulty or simply doing it's job by reporting a lean or rich condition caused by some other problem in the engine.

That's where the techniques I mentioned are useful and they are in fact used by professional mechanics to resolve this very question. For example propane enrichment kits are common automotive test tools. The propane torch off the car test is an old school trick that won't always prove a sensor enough to satisfy OBD-II but is more than good enough for this car's archaic OBD-1.

 

[Nick M]

This is a tangent, but in my proffessional opinion, the Ford EEC-IV for OBD level 1 is far more flexible and easy to deal with. Well, as a whole, not including feedback control which all cars have.

 

[SPD TRP]

All right guys, there is something real strange going on here.

When I unplug my O2 sensor, Vf does not change and we stay in closed loop.

Wayne

 

[Nick M]

Your Vf is high when the sensor is unplugged?

 

[SPD TRP]

yes around 4.8 wich is 5v on my car.

shouldn't be going to zero when unplugged?

 

[Nick M]

Yes.

4) If the meter reads hard over rich (5 volts or 800+ mv) pull off the brake booster line. The sensor should swing lean (0 volts). If it does it's working and something in the system is driving it rich. If it reads hard over lean (0 volts) inject some propane into the booster line. You can also give it a shot of carb cleaner. If the sensor swings rich (5 volts on Vf or 800 + mv on Ox) it's working and something in the system is driving it lean.

 

[jetjock]

Careful. That statement refers to the emulated O2 signal taken from Vf when *not* in learned value mode. Vf outputs learned value only when T1 and E is unconnected. The test procedure written above for checking the O2 sensor is with the jumper installed. The meter may be connected to Vf but it's displaying O2 sensor activity. When referring to the Vf terminal it's important to remember what mode it's in.

 

[SPD TRP]

Yes.

I got that, but, whats driving Vf when O2 sensor is absent?

I can't help believing that my mixture woes, rich and lean, and the Vf signal present with no O2 signal would be related.

Does it go into some kind of "limp mode" when another sensor drops out and ecu ignores the O2 signal to keep it running?

I'm effen stumped.

My wife is getting close to having all of this car she wants, I spend to much time in garage. I am relentless and will not be defeated. Hopefully!!!



Thanks again all

Wayne

 

[Nick M]

The oxygen sensor is not used in open loop. It is feed back control to keep the catalytic converter in its best operating range.

http://www.mkiiitech.com/forums/viewtopic.php?t=873

That link is a good read about fail safe and back up modes. Just an FYI.

I had to reread the original post.

 

[Nick M]

Since you have piggy back control, and are scaling the AFM signal, the TCCS does not actually know what is going on. The piggy back is giving false information to change the fuel injection. But the TCCS does not know it is false, and assumes it to be true. This will effect open loop also if you change the settings.

edit: Are you having severe drivability problems? An oxygen sensor won't really give you any, aside for poor fuel economy when they get lazy.

 

[SPD TRP]

Careful. That statement refers to the emulated O2 signal taken from Vf when *not* in learned value mode. Vf outputs learned value only when T1 and E is unconnected. The test procedure written above for checking the O2 sensor is with the jumper installed. The meter may be connected to Vf but it's displaying O2 sensor activity. When referring to the Vf terminal it's important to remember what mode it's in.

I don't think the O2 sensor has any bearing on anything in my case, seeing how it causes little or no change in Vf or afr's when plugged or unplugged, I still get a Vf signal at the diag block with O2 input with its input or not.

I know it goes into open loop on initial cold start up because I checked it this morning, 0.00 until warmed up.

This has got me kinda perplexed, hoping you and others can help me.


Wayne