Turbo O2 Sensor Unusual Findings, Problems - Lexus IS Forum
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post #1 of 9 (permalink) Old 08-05-2018, 07:53 PM Thread Starter
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Turbo O2 Sensor Unusual Findings, Problems

Story time.

I truly hope someone on here points out some glaring, obvious oversight I have made, because I've been wracking my brain on this for the last week.

I turbocharged my IS300 about 4 years ago. Since that time, I've been running the FIC piggyback. I joined B1 and B2 primary O2 sensors together and have been enjoying CEL-free boosting ever since. Occasionally though, due either to the stock ECU's constant 'testing', temperature fluctuations, or some unknown gremlin, my B1 and B2 fuel trims would get into a funky dance and start driving themselves in opposite directions. It is most prevalent around seasonal temperature changes; once i get my summer map dialed in and an overnight ECU reset, it's generally fine for months.

Anyways, I decided to try an experiment. I've seen one or two IS members on here (with turbos) report they've avoided these issues entirely by running primary O2s in each bank off single cylinders. I also know the NA guys that run Megan Headers have their primary O2's in a single runner for B1 and B2. Armed with this knowledge, I pulled out my turbo and exhaust manifold, welded bungs in Runner #1 and Runner #5 (due to the fitment / sensor location). I purchased two brand new Denso oxygen sensors and put it all back together. What followed was a mind-numbing diagnostic round-about...

At first-start up, during the cold-start open loop, things are fine. ~14.7 AFRs on my wideband (post turbo) and a steady idle. As soon as the fuel status switches to closed loop, O2 signals reported that Bank 1 went full lean, Bank 2 full rich. The short term fuel trims (STFT) tried to compensate by sending +19.5% fuel to bank 1 and -19.5% to bank 2 in an attempt to correct the issue. OK - this is fairly expected of the trims, but how likely is it that there would be such polar opposite issues on cylinders 1 and 5?

At first, I thought it was the tune doing something funky. I reloaded a rich map in the FIC and started the car. As expected, the O2s for Bank 1 and 2 reported 0.9V (rich). I slowly backed off the fuel in the FIC and as soon as it got within range where the ECU could reduce ST fuel trims, the O2 voltage in bank 1 would crash full lean and bank 2 would go back to full rich -- regardless of whatever I did in the FIC after that point.

I then thought the O2 sensors were backwards. (corroborated by countless online posts in other forums and blogs) But no... I poured over the electrical schematics and documents I've amassed over the years -- FIC, ECU, repair manual, diagnostic manual, sensor pinouts - EVERYTHING!!! it all checks out. So w/ the level of misfire I'm seeing, it had to be something mechanical.. right? I mean, full-lean could mean a total misfire (remember O2 sensors measure oxygen, not fuel, so even if it was dumping loads fuel due to no-spark, it would say full-lean).

I'll make this short;
- swapped plugs
- swapped coils
- ohmed coils
- ohmed wires
- reset ECU
- verified wiring to ignitor
- disconnected FIC, quadruple checked wiring
- verified connections of new O2 sensors (3x)
- swapped O2 sensors
- reset ECU
- swapped O2 sensor for B1 with known good
- reset ECU
- scoped injector pulses
- swapped injectors #1 and #5 -- this made the condition flip, coincidentally it would seem. Now B1 was rich and B2 was full lean. AHA!
- reset ECU

New injectors purchased. Deatschwerks 550cc. In they went.. AANNNDDDD.. NNOOOOOOOOOOOOooooooOOOOOOOOO!!!!!

The same problem. B1 full lean, B2 full rich. It went back! You can imagine my frustration. Now take that x10 and that brings us to today.

- compression test (All good)
- fuel pressure test (50psi, good)
- checked cam timing
- poured over my notes to verify valve clearances (the engine only has 12k on it)
- ohmed cam sensor
- ohmed crank sensor
- CHECKED O2 WIRING @ ECU AGAIN. WTF?!?!
- reset ECU
- ohmed MAF (and IAT)
- adjusted TPS
- smoke tested for vacuum leaks
- propane tested for vacuum leaks
- reset ECU
- reinstalled air-assist hose
- checked PCV system, all good there too
- checked gm/s MAF rate in OBD
- checked coolant temp sensor
- checked all ground connections at ECU, and ohmed all O2 wires at ECU
- reset ECU
- scoped O2 voltages
- put NEW spark plugs in #1 and #5
- checked for exhaust leaks
- RECHECKED O2 WIRING AGAIN!!!!!!!

I even ran the car without any O2 sensors to verify there was no misfire until the ECU commanded it. It ran PERFECT in open loop.

So, just as I was about to give up, and wire the O2 sensors back together as it was before, I figured.. what the heck.. Let's try a test.

I plugged the Bank 2 oxygen sensor into the Bank 1 connector... and Bank 1 sensor into the Bank 2 connector. And it RUNS PERFECT!!!!!!

Keep in mind (2002+).. Bank 1 Sensor 1 is D28 (E6-28) on the ECU, and Bank 2 Sensor 1 is C28 (E5-28) at the ECU.
I have Bank 1 Sensor 1 (in runner #1) wired to the documented terminal for Bank 2 Sensor 1 C28 (E5-28)
and Bank 2 Sensor 1 (in runner #5) wired to the documented terminal for Bank 1 Sensor 1 D28 (E6-28)

Fuel trims are perfect. Car idles in closed loop very smooth.. And my head has never hurt more.

So as a test, I decided to UNPLUG B1S1 terminal (technically the sensor now physically in bank 2). The theory is, if I disconnect it, the ECU will see Bank 1 go full lean and increase STFT to Bank 1 to enrichen it. I should therefore see Bank 2 O2 voltage go very rich on the OBD scan tool, because I still have a sensor plugged into it (physically in Runner #1 (bank 1) but connected to Bank 2 terminal at the ECU).

THAT DID NOT HAPPEN. I disconnected the sensor.. Bank 1 fell to 0v. B1 STFT increased to +19.5%. But the sensor physically installed in runner #1 stayed perfectly stoich. And the Bank 2 STFTs continued to monitor, adjust, and maintain stoich.

I even took a video because I was losing my goddamned mind.
Is it possible that ALL the documentation from Lexus/Toyota is wrong? People that do the B1+B2 O2 merge never notice it because they are connected, it doesn't matter which connector you use.
is B1 at the ECU is actually B2???
even the OBD PIDs are wrong?!
Or maybe just the PID association in the android Torque app?
Or this is a super coincidence?
or did I totally misinterpret this test?!

how can this be explained? Did I miss a TSB somewhere? That would mean anyone reporting any number of too rich/too lean or B1/B2 O2 codes would inevitably be looking at the wrong sensor? I feel like repair techs would've found this out years ago. Or do I have some crazy one-off whacky ECU?



https://youtu.be/6qxVjuxSqww

I'm quite interested to hear what people think about this. or maybe I'm just a dunce and missed something super obvious. and total and sincere embarrassment will ensue.

Supraforms: jong
1986.5 NA-T Supra (Sold)

2003 IS300 NA-T

http://www.chippernut.com/

Last edited by jong1; 08-05-2018 at 08:02 PM.
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post #2 of 9 (permalink) Old 08-06-2018, 11:17 AM
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I've been chasing a closed loop lean idle condition for months now as I prep for my NA-T install. I have the o2 sensors merged so both b1s1 and b2s1 read from the B2S1 sensor that is in the stock B2S1 location. Heater and signal wires are merged at the ECU based on the widely available schematics here. I'm totally stumped and I'm ready to just install both O2 sensors and unmerge the wires because I'm at a total loss. My fuel trims look perfect, but in closed loop the car is pushing very lean, like 20:1 (based on my aem wideband plugged into the B1S1 manifold location) and stumbling at idle. I'm trying to understand what you've described above...I don't understand why you would need to do anything with the oxygen sensor wires at all since you have both primary o2 sensors installed. It sounds like you don't have anything merged anymore and you're just flipping the signal wire between bank 1 and bank 2?

2004 IS300 8N6 automatic - F&F Coilovers, Eibach sway bars, DIY Intake hardpipe, FR-S LSD swap, buddy club driver seat, Aftermarket HID's and LED Tail lights, Trunk strut mod
.....Soon to be NA-T with CX Racing hard parts, Garrett 60-1 turbo, Tial 50mm BOV, Tial MVR wastegate and FIC 6 piggyback

Last edited by VegasIS3; 08-06-2018 at 11:25 AM.
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post #3 of 9 (permalink) Old 08-06-2018, 11:55 AM Thread Starter
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If you're going to merge the O2 signals from one O2 sensor, you need to put the sensor post-turbo. Otherwise you'll end up with the banks deviating and going super-rich or super-lean. The stock computer is like 2 computers in one; When it goes into closed loop, if it's receiving feedback from only 1 half the engine, it doesn't know what to do w/ the other half. It's an unstable system.

I would be interested if you wired up two primary O2s pre-turbo, if you experience the same issue I came across. I'm going to drive the car around tomorrow and put a load on it to see if I still get stable fuel-trims / O2 readings. But basically I thought I was putting the car back to "stock" O2 configuration, more or less, but it actually didn't work out that way. I had to flip the Bank 1 and Bank 2 O2 sensors then it went back to stock. Like the engine is backwards or something. lol.

Supraforms: jong
1986.5 NA-T Supra (Sold)

2003 IS300 NA-T

http://www.chippernut.com/
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post #4 of 9 (permalink) Old 08-06-2018, 01:13 PM Thread Starter
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Here's a good thread:

https://www.my.is/forums/f114/splici...ensors-426664/

Supraforms: jong
1986.5 NA-T Supra (Sold)

2003 IS300 NA-T

http://www.chippernut.com/
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post #5 of 9 (permalink) Old 08-06-2018, 07:12 PM
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What kind of engine management are you running?
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post #6 of 9 (permalink) Old 08-06-2018, 09:40 PM Thread Starter
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AEM FIC-6. But I've disconnected the FIC from the O2 sensors entirely (some argue the IS does not respond to closed loop O2 manipulation anyways). I've also verified all the above wiring with the FIC harness completely removed.

Supraforms: jong
1986.5 NA-T Supra (Sold)

2003 IS300 NA-T

http://www.chippernut.com/
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post #7 of 9 (permalink) Old 08-07-2018, 11:03 AM
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I've got an FIC too and with the bypass installed I still get intermittent lean conditions. I shut the car off and then restart it and the lean condition goes away for awhile and then comes back. I'm starting to think it's related to the hot weather here in Las Vegas that is heating up the fuel in the return line. Currently I'm using a rubber hose for the return, but I'm going to rerun a braided hose and ensure it's away from anything hot.

2004 IS300 8N6 automatic - F&F Coilovers, Eibach sway bars, DIY Intake hardpipe, FR-S LSD swap, buddy club driver seat, Aftermarket HID's and LED Tail lights, Trunk strut mod
.....Soon to be NA-T with CX Racing hard parts, Garrett 60-1 turbo, Tial 50mm BOV, Tial MVR wastegate and FIC 6 piggyback
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post #8 of 9 (permalink) Old 08-07-2018, 08:01 PM Thread Starter
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Update: I drove it tonight and it feels better than it ever has. Fuel trims stayed perfect, fluctuating around zero. Did some very light boosting (I have to adjust my new map). It no longer has the light-load fuel map hole that I've been battling for years. At tip-in it would miss and when barely moving in 1st gear (light throttle, light load), at like 1800 rpms, it would lean-out for a second which was super frustrating. Now it just goes!! The engine sounds a lot healthier too, rock solid idle, purrs like a kitten.

Quote:
Originally Posted by VegasIS3 View Post
I've got an FIC too and with the bypass installed I still get intermittent lean conditions. I shut the car off and then restart it and the lean condition goes away for awhile and then comes back. I'm starting to think it's related to the hot weather here in Las Vegas that is heating up the fuel in the return line. Currently I'm using a rubber hose for the return, but I'm going to rerun a braided hose and ensure it's away from anything hot.
Which bypass are you referring to?
I wonder if it's the air-intake sensor on the MAF getting hotter and the FIC can't keep up. (although fuel trims should adjust for this). Have you tried uploading a richer fuel map?
How are your O2's configured? Perhaps your O2 is overheating or just worn out?
I kinda doubt it's the fuel return, wouldn't the fuel just cool off as soon as it's back in the tank?

Supraforms: jong
1986.5 NA-T Supra (Sold)

2003 IS300 NA-T

http://www.chippernut.com/
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post #9 of 9 (permalink) Old 09-21-2018, 07:18 AM Thread Starter
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Update:

It's been nearly two months on my 'backwards' setup, and the car is driving perfect. Fuel trims are better than they ever have been. I'm patiently awaiting the seasonal temp change so I can see how it responds.

Supraforms: jong
1986.5 NA-T Supra (Sold)

2003 IS300 NA-T

http://www.chippernut.com/
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