Engine bogs only AFTER MIL is cleared

[RobH]

Guys,

The O2 sensor that is working rapidly changes around 0.8 volts up and down like I think it is supposed to.

The O2 sensor that I think is bad, the P0140 code, SOMETIMES moves but as you said it is slower than the other O2 sensor. But once the MIL pops up the voltage STAYS at 0.445 volts.

So what you said about "faster" reacting O2 sensor is what I believe to be the front one and is working fine. The other "slower" acting O2 sensor, when it does work, jives with it being the rear one.

So again, as some have said here, I thought the rear O2 sensor had nothing to do with fuel delivery. So I'm back to WHY does this car bog after resetting the MIL? I can't figure it out either.

[Schmieder]

I don't believe a rear O2 sensor would cause the drivability issues. There must be something else going on that is also giving the PCM to believe the rear o2 is bad. Or, as I would believe, you have more then 1 issue.

To test your rear o2 sensor, remove it and get a volt meter. Test the signal leads, not the heater leads. Get a propane torch and run the flame over the O2 sensor tip. The low oxygen environment from the torch will cause the sensor to read near 1 volt. Remove the torch and watch how the sensor adjusts. Reapply the torch and remove all the while watching the voltmeter or have a buddy help. If you fail to get near 1 volt and/or the sensor doesn't respond to changes, the sensor is bad.

Also, find the purple/white and tan/white wires and test for complete circuit between the sensor and the PCM. The pins are CLR 47 and 48. CLR is the bottom connector to the PCM if you didn't know

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Did you clean your MAF recently. I'm thinking your car is somehow going into a speed density mode and that allows it to run much better. But I would think you will throw a bad MAF code, but sometimes they can go bad w/o a code. Try clearing your codes and unplug the MAF and see if it drives like it does when the SES comes on.

I would be concerned about a short since you already had a critter chewing wires. Sometimes shorts can be very easily over looked.

To test for a short circuit, disconnect the negative terminal on your battery and get your volt meter. Complete the circuit with the volt meter (negative wire to negative batt terminal). You should get a low millivolt reading which is normal batt draw. I think my car was around 30mV and you essentially have the same engine. If you get anything much higher, you have a short circuit. If that is the case, unplug one fuse at a time until the short is gone. A short circuit can cause all kinds of funny symptoms.

EDIT: If your rear O2 is truley going bad due to age, the front O2 isn't that far behind and is also probably dying but hasn't thrown a code yet. Try testing the front o2 sensor the same as the rear o2 sensor. A bad or going bad front O2 can cause all kinds of issues. That is the O2 that controls fueling, and has a strong say in what the AFR will be commanded.

Sometimes a series of small issues can add up to a large issue that is very hard to pin point. Like a dirty air filter + clogged fuel filter + old O2 + old spark cables + clogged oil filter + dirty MAF + ect....you get the point. A general fix up of those items should be in order if it hasn't been done in a while.

[Weatheralls Auto]

1, P0140 Sensor 2 Insufficient activity.
If the rear sensor not swinging above 500 and below 400 this Dtc sets.
Once sets, PCM goes into default table, will run better.
Yes, Yes, Yes, The rear sensor affect fuel consumption and performance quality.
The Pcm monitors this and adjusts fuel accordingly, so can lean or richen mixture to do this, if the Sensor is faulty or Ckt inop, you will have drivability concerns.
Not only that, but the PCM will actually TRY to make the Engine run "crappy" just to see what the sensor will do, (testing its sensors capabilities) and if it does not react accordingly, will set DTC P0140.

2, P0140 Rear Dtc is fairly easy to diagnose, if it swings at all (slowly but to 400 or 500) then the Ckts are ok and you need a sensor. (just pulling sensor out and reinstall will help ground connection and sensor may work better)
If sensor is dead stuck at 440 to 460 then either the sensor is still defective or you have an open Ckt.(Tan/White Purple/White)

3, The sensors are very sensitive, you may be able to test the way Shmieder said, however, you will need a new one anyways as it will be contaminated and poisoned by the time you are done.

[DefEddie]

The rear 02 sensors do not affect fueling,in GM vehicles they are used for like less than 1% of long term fueling.
Deleting them completely(through pcm tuning)will only cause goofy transient fueling that isn't even worth messing with most times and most people (tuners included) haven't a clue it's even there/affected.
I know this from factory driveability training and because i've tuned out these sensor's on no less than 100 GM vehicles.

I'm with schmeider,you either have something going on in the circuit that is causing more than just a faulty rear 02 would cause or two different issues.

A bad rear 02 sensor shouldn't run crappy and then smooth out when it defaults to open loop (the speed density mode schmeider speaks of).

As long as you do nothing dumb,checking that 02 sensor should not contaminate it.
Just don't touch it or clean it,and if you reinstall use antiseize compound.

Do the circuit checks outlined by the above two posters,as mentioned though if you see it moving on the scanner the circuit is likely good. (or intermittent while somehow holding a reading).

[Schmieder]

Like the other posters said, testing the rear o2 carefully.

The torch should be fine as long as you don't do anything else to the sensor, i.e. touch it, add water, wipe it, set it down in dirt, ect.

But don't over torch it. Just enough to get the readings then stop. I seriously suggest checking for a short due to the torn wires. If your batt isn't draining, it may be a short when ignition switch is in the run position. In that case, check any pink wires that are exposed. I believe all ignition/run voltage feed wires are pink in our models.

Planethax, I was unaware of rear o2 effecting GM LTFT but you know how it goes, learn something new everyday, thanks.

please excuse the brief off topic question
So, I should be ok tuning out the Rear O2 via HPT (3-no error reported)?

[Weatheralls Auto]

The rear Catalyst monitoring HO2s is NOT just for setting DTC P0140.
I CAN and DOes effect fuel trims, however, not near as much as the Front one.

Even with Tuning, all the fuel MAPS are Not know.

I have seen first hand the effects of a poisoned Cat Mon sensor has, including on a Dyno Emission tester.

[Weatheralls Auto]

Example, quote from GMSI for 96 Olds Ciera 3100

The catalyst monitor sensors operate the same as the fuel control sensors. Although the HO2S 2 sensors' main function is catalyst monitoring, it also plays a limited role in fuel control. If the sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time, the PCM will make a slight adjustment to fuel trim to ensure that fuel delivery is correct for catalyst monitoring.

The first thing to check, is whether the HO2S is swinging at all between 400 and 500 or just sitting still between 440 and 460

[DefEddie]

Originally Posted by GMSI
The catalyst monitor sensors operate the same as the fuel control sensors.
(This means it is the same thing,same part and doing the same job-it's just for a different reason)
Although the HO2S 2 sensors' main function is catalyst monitoring(the different reason), it also plays a limited role in fuel control(Less than 1% of long term fueling).
If the sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time(it needs to stay constant in other words).
the PCM will make a slight adjustment to fuel trim to ensure that fuel delivery is correct for catalyst monitoring. (it adds fuel to check the switching difference between front and rear).

I've already explained this and posted up the facts from the monitor on a previous page.
It does not affect fueling as in your actual fueling for driveability.

I have not seen a catylyst monitor cause an engine to run poorly except in circumstances where it was another issue and the monitor was a symptom of the cause.
Doesn't mean it doesn't happen,just that I haven't seen it.

And you should NOT see the cat monitor swing back and forth between anything.
If it is switching constantly than it is reading other than 14.7,which means the cats are not doing their job.

The fuel adding and switching of the rear sensor will only happen while the pcm is running the catalyst monitor to check the sensor and cat.

Reread my previous posts,I outlines what the monitor does directly from a factory powertrain/emission control class.

[Weatheralls Auto]

.......

And you should NOT see the cat monitor swing back and forth between anything.
If it is switching constantly than it is reading other than 14.7,which means the cats are not doing their job.............

Unfortunately, if that were the case, you would NOT have the DTC P0140.

It DOES SWING!!!! not as much as the Front but it DOES and MUST or Dtc P0140 is set.

Due to the better efficiency of the Catalysts and EMS, the DTC criteria has been shortened (example, 99 2200sfi = Dtc sets if HO2S 2 remains between 425 mV and 460 mV for 125 seconds. 96 3100, Dtc sets if HO2S 2 remains between 400 mV and 500 mV for 90 seconds.)

In a perfect world the Rear HO2s would sit @ 450mv and not move, but realistically is does, and if it doesn't it sets a Dtc P0140.

I do not claim to have exact figure, but the Cat Mon HO2S can effect more than 1% fueling.

[Schmieder]

Well, for the sake of the original poster, they need to check the rear o2 functionality. And if it were me, I'de check the front o2 if the rear o2 fails, I know they don't die at the same time, but if one is dead, the other should be on its way out soon.

How about your antifreeze levels? Are there bubbles in the radiator? Getting antifreeze into a cylinder can kill an o2 sensor fast and THAT may be your first symptom of a bad gasket, a dead O2. Pray your head gaskets are ok.

If the o2 checks out fine, I would then check the system for a short.

If no short, then clear the SES codes and unplug the MAF and see if that is the cause.

I would suggest a clogged cat but I would think that would be a permanent symptom. Unless fueling AFR can effect drivability w/ a clogged cat.

Now, it is possible the front O2 is malfunctioning and the PCM hasn't noticed it yet. In which case the front O2 is commanding a much richer/leaner fuel mix then it should. This would definately cause a drivability issue until the PCM goes into Limp Mode or use the VE tables instead of the O2 sensor for fueling. In fact, correct me if I am wrong but, doesn't the vehicle run extra rich if the front O2 sensor fails? I know all eyes are on the rear o2, but that o2 may be dead enough to throw a code where as the front is getting close but not quite.

Would a bad rear o2 throw a car into limp mode? VE tables?

[RobH]

I know the rear O2 is stuck at 0.445 volts and does not move.

I hear what you are saying about the front O2 also. Probably needs replaced.

I will test the MAP and probably end up replacing the rear O2 and see what happens.

After I get my tax return back. . ..

Thanks for the input.

Rob

[DefEddie]

Planethrax,I've been wrong before.
But at my dealership,rear 02's flatline unless something is wrong with the cat or fueling.
A p0420/p0430 and/or switching rear sensors are enough for me to condemn a cat for warranty.
That's my experience as a driveability tech and from factory obdii training.


My suggestion to original poster is to have someone look at it that will recognize what is going on. An issue like that should be pretty easy to spot on Pids.
Does your scanner support mode 6 data?
If it does check mode 6,if it is a catmon issue than the transitions will be in the log.
Also check freeze frame data,it should have captured info at the time of the flag.

[Weatheralls Auto]

If its stuck at .455, then you have an open ckt, either in sensor itself or wireing from pcm to sensor.

[Schmieder]

If its stuck at .455, then you have an open ckt, either in sensor itself or wireing from pcm to sensor.

If it is an open circuit, it should be the tan/white or purple/white wire. Or it may be a pink wire that supplies the voltage to the sensor, iirc, all ignition/run voltage feeds are pink.

Hey Planethax, If the sensor is stuck at .445V, does that mean it is receiving no voltage? In that case I would check the pink wire and the black wire. A bad ground would cause no current as well.

hehe, I just realized how stupid that question was....no voltage would mean the circuit reads 0v. lmfao, I need more coffee.

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That would make the most sense since the OP already said he had critters chewing wires.

I bet there is some wire insulation missing near the tear, or the OP didn't fully insulate the rewire job. OR, the squirrel had dessert around another area of the wire.



OP, I would reinspect the wires before anything else. If your luck is anything like mine, trying to fix something that isn't broken will surely break it, lol.

[Weatheralls Auto]

The PCM sends the initial voltage, if there is an open ckt on the Tan/Whit purple/white there will be NO change in the voltage so the PCM will still see the .445V

To verify I am correct, unplug you HO2S and scan the data, it will read 450mv.

The Pink feed wire is for the Heater only.