RE-SOLVED: Engine running way rich, 0V reading from all O2 sensors - BAD FUSE (1 Viewer)

Eicca

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This is for my Lexus but the motor is basically the same so I’m imploring you knowledgeable MUD guys for help on this one.

I used to have a bunch of vacuum leaks (positive fuel trim). I fixed all of those and the engine was happy for a few months with fuel trims sitting perfectly at 0 give or take 2%. Fuel economy was pretty good for a V8.

Lately my fuel economy sucks, my fuel trims are way rich (-26 to -32 on both banks). Replaced the MAF sensor, didn’t help. Cleaned it again, didn’t help. Started surging at idle today after I reset the ECU. Scan tool initially had readings from all O2 sensors but now there’s 0.000V from all four. Fuel system status still shows closed loop and fuel trims fluctuate with RPM.

Injectors are OEM and all recently replaced. Spark plugs are all fresh, coolant temp sensor is new and reading properly, all PM is up to date. I’m at a loss and really hoping there’s not an electrical problem somewhere.

Not sure what to check next. I have to travel Monday...
 

Eicca

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Which 4.7l 2UZ. VVT or non? Throttle cable or by wire? Miles?
Do you have tech stream?

It’s a 1UZ VVTi with semi-cable throttle. I do have Techstream and I’ll put that on it shortly. I’m going to replace my PCV valve in the mean time just to make sure all readings are baseline before doing further diagnostics.
 
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When you hook up tech stream. Go into engine date, active test. Change fuel injectors from 0 to - to +. You should see A/Fs and O2s react. If not you've likely a electrical issue wherein signal not getting to or being read by ECM.

You may have A/F sensors upstream. I know the 2UZ VVT does.
Not likely all A/Fs & O2s dead at same time.

Voltage is high in A/F when oxygen high (lean).
Voltage low in O2, when oxygen high (lean).

LT (Long Term) FT -26 to -36 is ECM setting a lean base-line for very rich condition.
 
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Eicca

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When you hook up tech stream. Go into engine date, active test. Change fuel injectors from 0 to - to +. You should see O2 react. If not you've likely a electrical issue wherein signal not getting to or being read by ECM.

Not likely all O2s dead at same time.

I had NO IDEA I could do that. Thank you very much, I will try and report back!
 
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2007 100 series
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070.JPG
 

Eicca

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Results are in. Thankfully the 0 reading from the O2 sensors seems to have been a glitch. I reset the computer and the data stream is working properly again.

The rich mixture appears to have been caused by the fact that I let the PCV valve vent to atmosphere and capped its vacuum line. Hooking it back up immediately put the fuel mixture back to normal.

How?

I hypothesize that by letting the PCV valve just vent away I created the exact opposite of a vacuum leak. The computer must account for the air volume of the entire engine, including the PCV system, when calculating AFR. By letting it vent, less air was getting into the combustion chamber than it expected to see.

My results are consistent: PCV vented to atmosphere = rich mixture at all RPM.
PCV hooked up = normal readings.

I’m glad this was an easy fix, and I learned something that a shop almost certainly wouldn’t have picked up. It goes to show, anything even remotely out of place on these computerized engines can throw everything out of whack. Baseline your car before diagnosing, folks!
 

Eicca

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Digging this back up. The running rich issue has returned and all four O2 sensors are simultaneously flatlining when it happens. When it’s running normal, upstream sensors will oscillate between 0.1 and 0.7 ish volts and the downstream sensors will hold relatively steady around 0.7 ish. Out of the blue all four will drop to 0.000 to 0.030 ish and the car will run like crap, backfire, read -35% fuel trim, etc.

It happens randomly, sometimes for prolonged periods, other times for just a few moments. But it’s getting worse.

This really worries me. I can imagine one of them konking out, but all four at the same time? This smells electrical to me and I don’t even know where to start.
 

Eicca

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Shot in the dark. What is your engine coolant temp running (ECT)!

188-192F. I graphed the data to see if it was an intermittent signal causing random open loop operation but it holds nice and steady.

I also graphed the O2 sensor voltages and all of them flatline when I hit the gas. It’s like the ECU isn’t getting enough power or something.

I consulted the EWD and all the sensors and heaters are powered directly from the ECU, and they all terminate at ground point EC, which is secure. The wiring at the ECU itself seems ok. There are some cuts and splices from the previous owners but they seem to be on wires related to the alarm system and body controls.
 

jerryb

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Are they true zero? Or .001+
It's entirely possible to see the bottom voltages ig the conditions were right.
 

Eicca

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Are they true zero? Or .001+
It's entirely possible to see the bottom voltages ig the conditions were right.

All four will drop into a range of 0.000-0.035 ish during a flatline event and this will last several seconds minimum. When they’re operating normally the upstream ones will oscillate from 0.1 to 0.8 with good consistency and the downstream ones will hang out between 0.4 and 0.7.

The graph function on my scan tool is super handy for seeing it happen. It’s like a heart rate failure. Big gaps in the natural rhythm at the worst possible times.

There is some flatline that occurs as expected during deceleration when the ECU switches to a temporary open loop, but these are acting up under load and at other random times like idling in the driveway.
 

jerryb

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is the drop before the engine reacts or after the engine reacts?

this happens at all throttle positions? how's that switch? iF it's closed does techstream say closed?
 

Eicca

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is the drop before the engine reacts or after the engine reacts?

this happens at all throttle positions? how's that switch? iF it's closed does techstream say closed?

Supposedly the system stays in closed loop but it happens in sync with the engine. And the engine feels terrible. Huge lag in power and even some backfiring.
 
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I'm no expert, when it comes to 02 operation and tech stream readings. But what I've noticed, when run logs on fuel trim in driving condition. Is to ignore all but when throttle held steady.

When I've had 4.7L that I've done everything to stop vacuum leaks and tune engine. This includes: spark plugs, having fuel injectors tested and rebuilt, intake manifold gaskets, all vacuum lines, etc. Everything done even compresses tested. Everything "except" coils (COP) R&R. But I still get heavy fuel smell while idling after warm up. I slap in a set of new Denso coils. Fuel smell goes away!

Few things I use to check coils, but they don't always show or happen:

1) While in tech stream. Hold foot on brake and start raising RPM. Watch miss-fires closely. Miss fire are most often coils if all else checks out.
2) Get on HWY. Starting around 35MPH floor it. Keep floored all the way up to 90 or 100 MPH. Be very alert to "any" hesitation through the power curve. I mean any. If not smooth as silk, you've issue.

In both cases of while miss-fire or hesitation and fuel smell. Coils cured all.
 

Eicca

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I’ll investigate the possibility of bad coils for sure.

The flatlining issue doesn’t happen only when giving it gas though, I’ll happen while idling, randomly while cruising, but it’s most easily reproducible when increasing throttle.

It also just occurred to me that lower voltage from O2s usually correlates with a leaner mixture right? But it’s running horribly rich. P0172 code and -35% long term trims when it flatlines. Seems completely backward.
 

jerryb

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I guess you've looked for any restrictions or big leaks? If pressing on the pedal doesn't get you more air it for sure gets you more fuel.
 

Eicca

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I guess you've looked for any restrictions or big leaks? If pressing on the pedal doesn't get you more air it for sure gets you more fuel.

Yessir, inspected the intake and vacuum lines multiple times.
 

jerryb

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Coils and plugs, gaskets. After spending a ton of time on everything other than the ecu, I would only then identify the 8 wires for in and out on the ecm.
It almost seems like a waste to backprobe them and watch the input voltage while driving/idling. Id do two at a time, rear two first.
But I think it's something else.
I would think techstream would show an output issue on the commanded afr or target I think is what it says. That's the output. The input is what the sensor returns to the ecm.

Should also erase any learning it's done over a previous session if trying to compare past and present.
I do not know how long long term is in ignition cycles.
 

jerryb

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I guess it wouldn't hurt to have one new sensor. You could then immediately return it to AutoZone or whatever.
 

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