multiple o2 codes

[Bob Ayers]

A lean code means the HEGO sensor output at the PCM reads less than 0.45V as the PCM commands a rich cycle. The HEGO sensors on many of these vehicles ground through the exhaust Y-pipe, through a ground strap, to the body (most -- and perhaps all -- of the OBD-I HEGO sensors are 3-wire types). The PCM has its own ground directly to the battery. If one of these corrodes, you get an extra voltage drop from that. This means the PCM thinks the HEGO sensor measurement is lower than it really is. If it's enough (and I've seen it), you get a lean code.
.

Mike, a high resistance ground on the O2 sensor will cause a voltage drop, which will increase the sensor output voltage making the PCM think it's richer than it actually is, not leaner........A high resistance in the PCM ground will have the opposite effect.

 

[zedx]

so, is there supposed to be a ground wire from the exhaust to frame, I don't remember seeing one when I did the transmission this weekend.

 

[MAKG]

Mike, a high resistance ground on the O2 sensor will cause a voltage drop, which will increase the sensor output voltage making the PCM think it's richer than it actually is, not leaner........A high resistance in the PCM ground will have the opposite effect.

Bob, I know you're the EE, but I just don't see that.

Modelling both grounds as resistors, they are just in series between the HEGO sensor and the PCM, and just add. The HEGO sensor is essentially a battery (unlike most other sensors), and the PCM a voltage divider. There is no power provided by the PCM.

Are you modelling the HEGO sensor as a resistor? It isn't.

 

[MAKG]

so, is there supposed to be a ground wire from the exhaust to frame, I don't remember seeing one when I did the transmission this weekend.

On the RBV's I've seen, it's a strap (not a wire) from the exhaust Y-pipe on the passenger side, to the body behind the wheel well. It's VERY easy to miss if you aren't looking for it, though it's hard to imagine how if you had the Y-pipe yanked for a transmission R/R.

Note that you don't need it -- and it might not be there -- if you have 4-wire oxygen sensors. Those ground through the wiring harness.

 

[Bob Ayers]

Bob, I know you're the EE, but I just don't see that.

Modelling both grounds as resistors, they are just in series between the HEGO sensor and the PCM, and just add. The HEGO sensor is essentially a battery (unlike most other sensors), and the PCM a voltage divider. There is no power provided by the PCM.

Are you modelling the HEGO sensor as a resistor? It isn't.

Nope, I'm modeling the HEGO sensor as a voltage source.......Mike, take a voltage source, and tie the negative side through a resistor to GROUND.
With a current flow, there will be a positive voltage at the resistor/voltage
source node. So, the voltage source, and the IR voltage drop will be additive,
raising the voltage source voltage. So, this will make the PCM think the HEGO is "RICHER", not "LEANER".....

 

[MAKG]

I see what you're getting at.

But you've made a sign error.

Connect a voltage source with the "-" through a resistor to ground, and current will flow INTO the device's negative side; i.e. voltage is HIGHER at "ground" than at the negative side of the voltage source. Consider the following simpler example:

Suppose I get a battery and run a resistor from the negative terminal to ground. I also get a wire and short the positive directly to ground. Which way does current flow in the resistor?

The "ground" discussed in this example is a misnomer. It's just a wire, electrically.

 

[Bob Ayers]

I see what you're getting at.

But you've made a sign error.

Connect a voltage source with the "-" through a resistor to ground, and current will flow INTO the device's negative side; i.e. voltage is HIGHER at "ground" than at the negative side of the voltage source. Consider the following simpler example:

Suppose I get a battery and run a resistor from the negative terminal to ground. I also get a wire and short the positive directly to ground. Which way does current flow in the resistor?

The "ground" discussed in this example is a misnomer. It's just a wire, electrically.

Nope, no sign error......check this our Mike:

The voltage across Rload = V (voltage source) + IRdrop

 

[Bob Ayers]

Nope, no sign error......check this our Mike:

The voltage across Rload = V (voltage source) + IRdrop

Mike, my bad, the current would have to be in the opposite direction to get
the IR drop polarity that I have shown!!!!

 

[mazdamama]

my truck is having similair problems and i was wondering anyone could give some advice.
After recently replacing my clutch on my 94 4.0 i started getting codes, immediately after reassembly. at first it statred out with 172, 173, 176, and 177, which are too rich and too lean, on bank1 and bank2. i did remove the exhaust and 02 sensors to replace my clutch, but i have 4 wire o2 sensors, so could this still be a ground issue, or maybe a problem at one of the o2 connectors? anyway later on it started giving codes 335,332, and 336, which are all codes for low egr flow or egr circuit voltage out of range. i also got a throttle position sensor voltage code as well as 411, which is unable to control rpm during low rpm self test. So i have so many codes i dont know where to begin, but i replaced the egr sensor and valve, but that didnt do anything, and i replaced the o2 sensor, but that didnt do anything either. I also traced the wires on the wire diagram and found out that the throttle position sensor and the egr sensor are connected, so maybe they are both exhibiting a related voltage problem. if anyone has any ideas could you let me know.
im starting to think that is some wierd electric problem but im not certain.
even though the truck is throwing like 10 codes it still drives kind of allright. its only getting about 15 mpg, and it is rough at idle, bogging then surging, occasionally dying, and the truck doesnt have a hell of a lot of power uphill.

 

[MAKG]

Mike, my bad, the current would have to be in the opposite direction to get
the IR drop polarity that I have shown!!!!

Oh man, we've all done THAT at some time or another.

I'm busy arguing with a subcontractor right now that we should take specs from a set of tests, rather than formulas on paper, for just that reason. It's not exactly textbook, but it means the difference between finding errors like this and writing them in stone...

 

[Bryan22]

10psi when turn the key on, 28psi at start up while on high idle, after idle dropped it went to 28.5psi

did you try testing the pressure with the FPR unplugged? That would let you know if it's bad or not. seems like not though. When's the last time you changed the fuel filter? some people will say this is dangerous and im an idiot, so do this test at your own risk.

pull the inlet line off of the fuel filter and have someone turn the key on for just a second. there should be enough pressure that you wont hardly be able to hold a finger over the line to stop the flow. If you can stop the flow with your finger i'd say it's probably the fuel pump.

 

[Bryan22]

my truck is having similair problems and i was wondering anyone could give some advice.
After recently replacing my clutch on my 94 4.0 i started getting codes, immediately after reassembly. at first it statred out with 172, 173, 176, and 177, which are too rich and too lean, on bank1 and bank2. i did remove the exhaust and 02 sensors to replace my clutch, but i have 4 wire o2 sensors, so could this still be a ground issue, or maybe a problem at one of the o2 connectors? anyway later on it started giving codes 335,332, and 336, which are all codes for low egr flow or egr circuit voltage out of range. i also got a throttle position sensor voltage code as well as 411, which is unable to control rpm during low rpm self test. So i have so many codes i dont know where to begin, but i replaced the egr sensor and valve, but that didnt do anything, and i replaced the o2 sensor, but that didnt do anything either. I also traced the wires on the wire diagram and found out that the throttle position sensor and the egr sensor are connected, so maybe they are both exhibiting a related voltage problem. if anyone has any ideas could you let me know.
im starting to think that is some wierd electric problem but im not certain.
even though the truck is throwing like 10 codes it still drives kind of allright. its only getting about 15 mpg, and it is rough at idle, bogging then surging, occasionally dying, and the truck doesnt have a hell of a lot of power uphill.

Those EGR codes are a BIG hint. A failing EGR system will cause all types of poor running conditions. check the system out. Which 'egr sensor' did you change? was it the DPFE? Changing parts isn't always the answer ! you might want to start by simply checking all your grounds cleaning and reataching them. One time a bad ground was causing an intermitent no-start on my truck, was scratching my head trying to figure it out until one day I got it running and it died when i slammed the hood! haha there goes the light bulb!

 

[zedx]

the bed is coming off tomorrow so I can do the electrical test at the pump and change it if needed.

after I checked the fuel pressure yesterday it started running worse than before.

 

[zedx]

Ok, got the bed off and checked the voltage to the fuel pump. doing it with just the key turned to the run position it would go within spec for about a second or so then down to 6.85v. then I did the jumper at the VIP connector and retested and got within spec reading continuously. also redid the pressure test got 13psi with the jumper.

the chiltons is really vague for this and could they have printed the electrical diagrams any smaller? lol

 

[Bryan22]

that's still way too low man. Koeo should be 35-45 PSI. did you check the fuel filter? I had one that got plugged bad due to the filter in the tank not being on. truck wouldn't even run. sounds to me like the fuel pump is taking a dive though.