sn95_331_GT_ yellow

Yeah active codes right now are P1132 + P1152 as well as P1000

Forgot to check the fuse, ill do that on lunch, otherwise start back tracking from there...

cliffyk

Those are "sensor indicates rich" codes for both banks. The output of a NB O² sensor indicating rich and not switching would be stuck at 0.5V or more, so the heater fuse being blown should not be the cause (cold O² sensors produce no output); but check it anyway.

The next question is, is the engine actually running rich? The best way to determine this is would be to monitor the exhaust with a wideband O² system, however is as rich as the PCM is reporting the exhaust should stink and fuel economy should be affected negatively.

What does the fuel pressure look like? A '96 has a return type fuel system and a bad fuel pressure regulator could be causing the rail pressure to be too high which would definitely cause a rich condition on both banks...

sn95_331_GT_ yellow

I have an AEM UEGO in the car, if anything it actually runs a little lean under closed loop, it fovors right around 14.5 and above, WOT isnt an issue right at 12.8-13.1

The car has longtubes so I dunno if the extensions might be causing an issue?

I have an adjustable FPR that is set at right around 40psi static, but like I said, the wideband says the car is running fine bit the ECM seems to disagree...

My wideband sensor is literally 10" further downstream than the front O2's so I believe it over them...

Open to suggestions master tech!!! Lol

cliffyk

I agree that the actual AFR seems to be OK, which leads to why does the PCM believe the mix is rich?

Does your adjustable FPR use a vacuum connection? If so is the 40 psi setting with the vacuum connected or disconnected and plugged--I suspect the latter but just checking...

What do the short term fuel trims look like?

One other issue is that with LT headers there is in the tune the O² Transport Delay table which defines the amount of time (in seconds) in which the PCM expects a commanded change in AFR to be reported by the O² sensors.

If the values in this table are not increased the PCM will generally throw DTCs P0133 and P0153, HO2S Sensor Circuit Slow Response. However I have seen other codes as well so this could be an issue. The values are usually increased by 10% to 15% as shown below:



The highlighted values have been increased by 11% to 14%, the plain text numbers are the stock values.

Dooney

OK Cliff,

so I checked out the fuse and it is good. I looked at the o2 extensions and they look good as well. Now there is something new that I noticed last night while looking at the live data again.

I started the car after clearing the KAM and monitored the o2's and short term fuel trim as the car was warming up. Once the car started to warm up the fuel trim actually started to read lean. (25%) So I thought this was weird because it is the complete opposite of what has been happening. So I gave it a little throttle and almost instantly the car switched to rich (-25%).

looking at the o2's while the car was running lean they went like this

o2s11 - 0.87v
o2s12 - 0.07v
o2s21 - 0.05v
o2s22 - 0.84v

once the car kicked over to rich the o2s read the following

o2s11 - 0.04v
o2s12 - 0.68v
o2s21 - 0.71v
o2s22 - 0.08v

Is there something wrong here? Why are opposite banks and opposite streams reading the same.

One more thing, am I reading this wrong in my research or are the o2's not supposed to switch voltage constantly? Those o2 readings were holding steady and not moving

thanks for the help

cliffyk

You are correct, in closed-loop mode the O² sensor outputs should switch between 0.2 V and 0.8 V or so, at a rate of 1 to 5 times per second. However this is not something the sensors do by themselves, but rather their response to the PCM's making the mix alternately rich and lean in its attempt to maintain an average AFR of 14.7:1.

Narrowband O² sensors are called that because the only meaningful output they can produce is across a very narrow band of Lambda (λ) values, this graph shows that:



You can see that the only information filled range is between the green lines, λ0.99 and λ1.01; or between AFR (gasoline) 14.55:1 and 14.85:1--and that is stretching things to the extreme. Outside of that narrow range all it can report is lean or rich--the NB output can only tell the PCM that the mix is lean or rich, but not by how much. Because of this the PCM, in closed-loop mode, constantly varies the injector pulse width, while monitoring the O² sensor output, to keep the sensor reported AFR at an average of 14.7:1.

Beyond all this, I have found that observing the O² outputs through the OBD2 port is pretty much a worthless exercise, as the diagnostic mode is limited to reading no more than 10 PIDs/second and that the timing of its sampling is not at all synchronized to the PCM's changing the injector pulse width--or anything, it's just random. Similarly the bank 1 and bank 2 sensors are not read simultaneously, making direct comparison of their values meaningless. Fuel trim monitoring via the OBD2 port works the same way.

Why the values do not seem to be continuously changing as you observe them is I suspect more of an issue with the OBD2 data reporting limitations and perhaps those of your monitoring tool. Though in general I have found the realtime monitoring of the fuel system, provided by most handheld tuners, to be largely worthless for diagnostic work.

To really make any use of the OBD2 sensor voltage readings you need to average them over a period of several seconds or more, as does the PCM, and look for the overall trend.

I generally read the voltage directly from the sensor, through a RC filter with a 4 to 5 second time constant so as to dampen the swings.

The sensors can be bench tested with a multimeter and a propane torch--here's how...

Dooney

Ok so judging by the graph the o2's shouldn't ever hit 1v or 0v's but I seen both of those numbers. Is this an issue?

cliffyk

No they actually do reach those extremes, I have even seen as high as 1.1V or a bit higher. These values are not shown on the graph because they are completely meaningless, especially an output of 0 V which could mean the sensor is defective or cold (see below) or indicative of one other property that can be confusing--that being that in a sufficiently rich mixture, say richer than 8.5:1, they will also produce no output--which if operating "in-spec" would mean a very lean mix.

Cold, which is anything below 660° F a PZ narrowband O² sensor will produce no voltage and "look" electrically like a very high value resistor. It is not until they reach 950° F that their output stabilises. The heaters keep them at 1100° to 1200° F. More about how they work here.

To further describe the "narrow band", the entire range of the graph's X-axis is just from λ 0.965 to λ 1.025, or 14.2:1 to 15.0:1 AFR...

Dooney

Ok well now I'm even more confused then before, everything is saying rich and I just threw a code P1151 (lack of HO2S21 Switches - Sensor Indicates Lean (see P2197)

cliffyk

The key phrase in that description is that the Sensor Indicates Lean; it does not say the mix IS lean, just that the sensor says it is. A dead or cold sensor will output 0.0 V which the PCM will interpret as a lean mix. An exhaust leak near the sensor (near the sensor, not necessarily before the sensor) will make it read lean as well, and it doesn't take much of a leak.

The PCM then does its damnedest to make the mix (which may well be 14.7:1) "normal" by adding fuel it, right up to +25% where it throws a DTC, until the O² sensor reports it is switching around 14.7:1--but the dead/cold sensor will just keep on reporting lean regardless of how rich the PCM makes the mix.

Do you have a custom tune, was it set up for the LT headers?