Intermittent signal loss from all four 02 sensors

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eicca

Member
Messages
85
Location
Utah
Been chasing down a running rich issue for a while now and I made a scary breakthrough tonight. All four O2 sensors are randomly and simultaneously flatlining. 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.

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.
 
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361UZ

Member
Messages
167
Location
Canberra ACT
What model car? I would agree, it sounds like the supply to the sensors (or heaters?) is intermittent. If you can get the make/model info, you can find wiring diagrams or even what fuse supplies the sensors. Someone on tis forum will know how the sensors are wired.
I was chasing an intermittent problem on my ex-Crown 1UZ. Turned out to be in intermittent fuse.....
 

eicca

Member
Messages
85
Location
Utah
What model car? I would agree, it sounds like the supply to the sensors (or heaters?) is intermittent. If you can get the make/model info, you can find wiring diagrams or even what fuse supplies the sensors. Someone on tis forum will know how the sensors are wired.
I was chasing an intermittent problem on my ex-Crown 1UZ. Turned out to be in intermittent fuse.....

1998 GS400. I would love if someone could confirm what I found in the EWD. It looks like the O2 sensors and heaters are powered directly from the ECU and they all terminate at ground point EC. I checked all the connections and everything is solid.

I pulled the negative terminal to wipe the ECU and replaced the 25A EFI fuse. The old fuse looked a little discolored but read zero impedance.

My Techstream doesn’t refresh fast enough to effectively monitor multiple data points, so I’m relying on my basic OBD tool to watch graphs and numbers. The flatlining seems to occur when I hit the accelerator.

I’m typing this as I sit in my car waiting for the relearn cycle to finish. We’ll see what a fresh fuse and a test drive reveal.
 

Zuffen

Super Moderator
Staff member
Messages
5,515
Location
Sydney, Australia
I had a similar problem with my LS3 powered toy.

Accelerate hard and it would lose the Crank position by going hi or low impedence at is will.

Found a chaffed wire in the loom that was causing it.

Your problem sounds like a broken wire that pulls apart on acceleration or a bad ground.

I'd try putting meter (or test light) on the sensor wiring and start wriggling and pushing and pulling the loom. If it drops out you know roughly where the problem is.
 

eicca

Member
Messages
85
Location
Utah
I can't believe that I forgot to update this thread. It seems like replacing that fuse fixed not only the original problem but also a host of little things that I didn't like about my car's performance.

Not only is the ECU working properly again, but the transmission is shifting much more solidly. It used to be pretty slushy between first and second. I thought it was just wear. But now there's discernible shift shock, even at WOT. It also used to "cut out" when engine braking, like it would slip into neutral for a split second when the revs got down to 1200. Doesn't do that anymore. Downshift response is also much quicker.

I'm still a bit boggled that a seemingly-ok fuse could have done all of this, but over the past four years of owning this car I've pulled that fuse and reset the ECU dozens of times. I even cleaned the fuse blades just for good measure. Nothing has ever made a lick of difference, until I put in the new fuse. Now it's a whole new car.

It's running so well I gave it a Christmas oil change and upgraded to Mobil1 full synthetic.
 

361UZ

Member
Messages
167
Location
Canberra ACT
I can't believe that I forgot to update this thread. It seems like replacing that fuse fixed not only the original problem but also a host of little things that I didn't like about my car's performance.

Not only is the ECU working properly again, but the transmission is shifting much more solidly. It used to be pretty slushy between first and second. I thought it was just wear. But now there's discernible shift shock, even at WOT. It also used to "cut out" when engine braking, like it would slip into neutral for a split second when the revs got down to 1200. Doesn't do that anymore. Downshift response is also much quicker.

I'm still a bit boggled that a seemingly-ok fuse could have done all of this, but over the past four years of owning this car I've pulled that fuse and reset the ECU dozens of times. I even cleaned the fuse blades just for good measure. Nothing has ever made a lick of difference, until I put in the new fuse. Now it's a whole new car.

It's running so well I gave it a Christmas oil change and upgraded to Mobil1 full synthetic.
Good to hear, and great that you remembered to let us know the outcome.
As I said above, I have had "worn" fuses, especially the blade type. Not sure if its the link that goes high resistance under load, or the female socket that loses tension over the years. The Crown wiring I have in my Chev has (had...) a few like that, and as you say, I was surprised by what else worked batter than it did before, that I hadn't noticed.
 

eicca

Member
Messages
85
Location
Utah
Ok so it's starting to happen again. First symptom is the engine will fire really fast when cranking on cold start. When it's working properly it cranks for about 1 full second before firing. When the issue starts happening it fires after only a half second.

Transmission is starting to feel a little slushy again, and my fuel trims are creeping back into the negative double digits. O2 voltages seem to be oscillating more slowly but I haven't seen any flatlining yet. It's coming though, if this keeps up.

Now the fuse I put in that made a big difference was an old one, meaning a spare that had been sitting in that fuse box for the life of the car. Maybe it means that fuse is starting to go "stale" as well, or maybe the fuse block itself has corrosion in the contacts. The fuse blocks in the 2UZ Land Cruisers of the same era have been known to go crusty and cause some issues with the immobilizer. A new fuse block is cheap enough I'm willing to give it a shot. I plan to install that as well as dielectric grease on all the connections from the fuse block to the ECU.

And if that doesn't fix it, I don't know...
 

eicca

Member
Messages
85
Location
Utah
Further update: Spirited test drives today show the transmission is still working just fine. No slushy shifts when pushing hard. No O2 signal flatlining at all. Voltage readings oscillate as expected. Engine actually still feels plenty healthy, way better than it ever has. No misfires or stumbling or anything strange, no fuel smell from the exhaust.

Long term fuel trim still sitting at -20% on both banks at idle and I can't imagine why. It'll drop to -5.5% under full throttle, and change linearly with throttle position. I.e. half throttle results in about -11% long term fuel trim. Short term trims bounce around +/- 5% like normal.

Does this sound like failing O2 sensors? My only hang up is the fact that both banks are reporting identical numbers. If I had the funds I'd just replace them to be safe, but it'll be a while before my employment situation is settled. But it seems to me like bad O2 sensors will usually report numbers pretty out of whack with one another, no?

I suppose it's possible both upstream sensors could go bad perfectly in sync. I mean, I have seen a catastrophic oil loss cook only the driver's side half of a 2UZ, so anything is possible...

Other data points: Injectors, air filter, vacuum lines, intake gaskets, fuel pump, camshaft solenoids, MAF, all new in the last two years.

Hoping maybe @gloverman or @George_R can chime in, I'm really confused on this one.
 

ivan129

BlownZX
Messages
186
Location
Some where down under
Might sound like and obvious, but have you checked your grounds (earth straps) are not corroded. There will be a number of grounds straps, ECU grounds to engine, battery to engine, battery to body, transmission to body etc.
 

eicca

Member
Messages
85
Location
Utah
Might sound like and obvious, but have you checked your grounds (earth straps) are not corroded. There will be a number of grounds straps, ECU grounds to engine, battery to engine, battery to body, transmission to body etc.
I have, and the only one I can’t actually get to is the one that the ECU and O2 sensors ground through. It’s on the back of the block by the firewall. It’s labeled EC in the service manuals.

I can speak from experience though, when that ground does get bad the whole car goes bananas. It went bad on my parents’ Land Cruiser but thankfully that engine bay is bigger and I was able to get back there and fix it.

I’ll try again to get a wrench on it and at least make sure it’s tight...
 

eicca

Member
Messages
85
Location
Utah
UPDATE: I think I have traced the problem to the MAF plug. It seems like every time I mess with that plug, whether it's for cleaning the sensor, checking the air filter, or whatever, the fuel trims correct themselves for a short while and then gradually slide rich again.

That's the one common denominator I can come up with from all the times I've gotten the fuel trims to read normally for a bit. It seems to always be after messing with the MAF plug.

Any suggestions for cleaning/fixing the pins in the plug? Or maybe some high conductivity substance I can cram in there? Anything short of cutting the MAF plug off and wiring on a brand new one?
 

ivan129

BlownZX
Messages
186
Location
Some where down under
If the plug or pins are suspect, then replace them. Rule this out by fixing the issue and eliminating any doubt about it. If your problem reappears then look further.
 

eicca

Member
Messages
85
Location
Utah
So I replaced the MAF plug three weeks ago and the engine has run fabulously ever since. Fuel trims are all staying within 2% of idle, with .5-1.5% variance between upstream O2 sensor readings. Everything is working as expected and the car feels sooooo much better. The torque comes on nice and strong and the power curve feels like an actual curve instead of a bar graph like it used to.

So between the new EFI fuse and the new MAF plug, all engine issues seem to be solved and the car now runs far better than it ever has.
 

Lextreme

Administrator
Staff member
Messages
429
So I replaced the MAF plug three weeks ago and the engine has run fabulously ever since. Fuel trims are all staying within 2% of idle, with .5-1.5% variance between upstream O2 sensor readings. Everything is working as expected and the car feels sooooo much better. The torque comes on nice and strong and the power curve feels like an actual curve instead of a bar graph like it used to.

So between the new EFI fuse and the new MAF plug, all engine issues seem to be solved and the car now runs far better than it ever has.
It's a good feeling when it gets dialed in, isn't it?
 
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