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Discussion Starter #1
Just did both air-fuel sensors and both O2 sensors on my 2008 today. I didn't find much in write-ups for this work so I figured it was either super easy and nobody needed it or not many have done it. Either way, here is what I did and a problem I ran into. Nothing earth shattering but maybe it will help someone considering doing on their own.

Symptoms leading up to the replacement: I was having some off idle hesitation up to about 2500 RPM or about 35MPH but it wasn't 100% of the time and nothing I tried could make it happen or make it stop. I posted a thread in the engine performance section about the issues I was having and got a lot of good advice. I cleaned the throttle body and bought a blue tooth OBD2 reader to get a better look at what was going on. I didn't have any codes but something wasn't right. After cleaning the throttle body (off engine) and taking a test drive with the OBD2 reader reporting on the Air/Fuel ratio commanded and the Air/Fuel ratio monitored nothing jumped out at me right away but I did notice that sometimes there was a pretty big Delta between the two and they were not really closely matched all the time. I was still running the OEM sensors and in the other thread had learned I was well past what was an expected life for all of them. I decided to go ahead and swap them all out in effort to restore the lost performance.

Specailty tools I used:
OBD2 Bluetooth
Torque Pro App
Lisle Dual Drive O2 offset wrench
Lisle O2 thread chase
I used Denso A/F ($108) and Denso O2 ($48) sensors from Rock Auto

Normal tools I used:
I didn’t keep a comprehensive list, I replaced all 4 sensors from underneath and I did use several 1/2" extensions with the Lisle wrench on the A/F sensors, 3' extension with U joint on the passenger side exhaust bracket upper 14mm bolts (from engine compartment), 14mm ratcheting box end on the driver’s side exhaust bracket upper bolts, 22mm box end wrench for the O2 sensors and 3/4" ratcheting box end wrench on the Lisle thread chase. The wiring harness plugs are set free by way of a standard screw driver with a narrow head and about a 3-4" length.

Drivers side A/F sensor:
The FSM alludes to removing a bracket with 3 bolts and by the picture in the FSM I wasn't sure what it was. Once under the driver’s side I saw what was in the way - a big bracket with two bolts on the engine block side and a single bolt on an exhaust coupling. Removing these bolts took a lot of effort and a cheater bar, which had either a bunch of torque from factory or had just taken a hard set over the years/miles.

Once the bracket was out of the way I was able to use a short/narrow standard screw driver to release the wiring harness from the sensor pig tail. I used the O2 wrench with 1/2" drive extension as a handle and a short cheater bar to get the A/F sensor to break free. Once the sensor was broken free it unscrewed by hand pretty easy. I ran the chase in/out the threads with little to no resistance. I coated the new Denso A/F sensor threads with their provided copper colored anti-seize and ran the sensor into the threads by hand until lightly seated. Once it was seated I used the Lisle O2 wrench to seat the sensor with a reasonable amount of torque.

Passenger side A/F sensor : The exhaust bracket was a PITA to remove. I could only reach the two upper bolts with a 3' extension and a U joint. Even then it took a cheater bar to pop each one free. The bottom bolt came out without issue. Getting the wiring harness free took the narrow standard screw driver but by the time I had my arm up where it needed to be I couldn’t see the plug so I had to do it by feel. Took me a couple tries to unplug the sensor. Removing the A/F took the Lisle O2 wrench but this time with a 3/8 socket wrench in the top side of the O2 wrench and a cheater bar on the socket wrench handle. Once it popped free, it was easy to un screw by hand. I ran the O2 thread chase in/out with little to no resistance, applied the copper color anti seize and installed the A/F sensor.

Drivers side O2 sensor : The sensor unplugs same as the A/F sensor with the short narrow standard screw driver. I used a 22mm 12 point box end to unscrew the sensor (the pig tail on the sensor fits through the box). The O2 sensor was tough to break free and just as tough to unscrew to remove. A couple times I stopped to make sure I wasn’t accidentally tightening it by accident. I even pondered if it might be reverse threaded for some reason. In the end it came out with a lot of effort. The threads on the old sensor looked like crap and I was worried it might have galled the threads and I was right. The threads in the exhaust pipe were in bad shape. It took a couple starts to get the O2 thread chase started square but once started I would turn it in about 1/4 turn and then back it off 1/2 turn or more and run it back in another 1/4. I pulled it out to clean and relube several times. It took a while but I eventually got the threads re-cut and cleaned up. I was able to install the new sensor with the copper anti seize without drama.

Passenger side O2 sensor: After all the drama on the driver’s side I was a little hesitant to pull this sensor out. I was able to unplug the pig tail and pop the sensor free without drama. This one unscrewed by hand once popped free and the thread chase ran in/out with little resistance. I coated the threads of the new sensor with the copper anti-seize and installed it by hand and set the final torque with the 22mm box end wrench before plugging the pig tail back in.

I re connected the negative side of the battery and started the FJ. It fired right up to a pretty fast idle, like it would on a very cold day (it was about 75F today). The initial idle seemed to have a little roughness to it but there were no apparent exhaust leaks or dash warning lights. I shut off the engine, plugged in the OBD2 scanner and started the engine. I ran through the scanner readings and everything seemed to be ok with no codes. I took a couple runs around the block and everything ran great. I took a longer test drive after cleaning up and its very noticeably better. Throttle response is back to what I remember and everything is smooth at idle. The initial idle roughness I felt at first start went away fast and everything settled in during this longer test drive.

I am really happy I bought the O2 thread chase! Both Lisle tools were great quality (forged) and I am pleased with both the pruchases. I almost skipped the chase but I would have been SOL today without it. I have no idea what happened with the threads in the drivers side O2 sensor, maybe I somehow messed it up while unscrewing it or maybe it got jacked up when it was installed on the assembly line. I'll probably never know but I am damn happy I had that thread chase and was able to recover from that situation.
 

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Awesome!

Post some photos of the tools you referenced, and of the parts you removed (probably too late for photos of the new parts).


N
 

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Discussion Starter #5
What is thread chase? Never heard of it. Thank-you.
I linked to the thread chase in the section for the specialty tools I used. A thread chase is like a thread tap but it's not designed to cut new threads rather its used to clean up existing threads and fix minor issues in the threads. In my case, I ended up using it more like a tap and it felt like it was cutting new threads quite a bit. Having some basic knowledge and experience with taps helped me to not over use the tool and risk breaking it off. The technique I used was to run the clean and lubed chase in about 1/4 turn and then back off at least a half turn or more. Backing off allows the chase to clean the cutting threads into the debris channels. After a full turn or two you want to back the chase all the way out and clean it off and re-lube the threads. It is slow going but you dont want to force it or go too fast - too much heat/friction will make things go from bad to worse quickly.

Normally when using a thread chase you will just find a rough spot or two in the threads and that is what the chase helps to cut/reform. The other three sensor holes had very little need for the chase other than a spot or two that was easy to turn through with little effort. I mostly used the chase to verify the threads were in good shape and to help apply a coat of the anti-seize to the threads since that is what I lubed up the chase with for the three good holes.

Considering the price was so cheap and it saved the day for me, I am very happy with the purchase. I can't say that everyone will need it for the sensor, heck I may end up with the only FJ that has the problem.
 

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Discussion Starter #6
Just a quick update for anyone that finds this thread while doing research for their own troubleshooting.

After changing all 4 O2 sensors and resetting the ECU the overall performance did improve significantly. Even with the improvements, I could still detect some minor hesitation in off idle part throttle acceleration at times. It was not always consistent and same as before I could not make it happen or make it go away when it did happen. I still wasn't getting any codes thrown so I wasn't sure where the issue was occurring. Overall everything felt like it was about 90-95% back to normal but something still wasn't quite right at times.

I decided to replace the MAF sensor with a new Denso unit from Rock Auto ($65). I had previously cleaned the MAF several times (all parts, the match head and the recessed little wires) but I was willing to accept that at 188K maybe the MAF just wasn't up to snuff any longer. I replaced the MAF and reset the computer a couple nights ago and things are now back to 100% normal. I no longer have any hesitations or flat spots in power delivery while accelerating from off idle with part throttle.

The first 1/4 tank of gas after the MAF replacement was showing signs of poorer than normal MPG but I am chalking that up to the computer settling in and me making several big stabs at the throttle while driving in effort to see how things responded. I have topped off the tank and reset the trip meter to make a new MPG calculation and so far its all looking good. I will keep the thread updated if this ends up not being the end of the issues but so far its all now back to normal and no negative signs or symptoms.

So far the biggest learning experience for me has been that the sensors can be degraded enough that its really obvious something isn't right but not degraded enough to throw a code. I had always believed that the computer would complain soon as things drifted out of a narrow window of optimal performance. I now can see that just isn't the case.
 

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Thanks for the write up. Like others, I am interested in knowing how your MPG improved with the changed out sensors. What was your total cost and do you have part numbers?
 

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Discussion Starter #9
Quick update, everything ran well and good after doing this work but a week ago I started getting nearly the same sorts of hesitation issues. This time they were a lot more consistent and happened almost all the time. I ended up replacing the fuel pump and filter with the kit from UDR and this fixed the issue this time around. I made a new thread for the fuel pump replacement here.

Knowing what I know now - I wouldn't wait for ECU codes to be thrown (which was my old stance) before replacing the sensors or fuel pump/filter because I never did get a code when things were obviously degraded.
 

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do you think that your O2 sensors and MAF issues have anything to do with the latest on the DUSTGATE being discussed for other Toyotas (especially the 1GD-FTV). I'm being told by the mechanic that I need to replace the O2 sensors (but I've only done 88000). It keeps throwing the codes after starting a couple of times after the reset! Any suggestions?
 

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do you think that your O2 sensors and MAF issues have anything to do with the latest on the DUSTGATE being discussed for other Toyotas (especially the 1GD-FTV). I'm being told by the mechanic that I need to replace the O2 sensors (but I've only done 88000). It keeps throwing the codes after starting a couple of times after the reset! Any suggestions?
A little more clarity would be helpful:
1. What year model is your FJ?
2. What are the specific OBD-II codes that are being logged?
3. 88,000 miles, or kilometers?

(There are actually four sensors in the exhaust system, two air-fuel and two oxygen.)
 

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This is a great write up. I have a '07 TRD standard transmission and the exhaust is getting soft so I am replacing with stainless/performance exhaust and wondered if you had any experience with O2 Sensors for "higher-end" exhaust systems? Does it matter enough to not use the OEM spec sensors? Any suggestions would be greatly appreciated.
 

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Discussion Starter #14
This is a great write up. I have a '07 TRD standard transmission and the exhaust is getting soft so I am replacing with stainless/performance exhaust and wondered if you had any experience with O2 Sensors for "higher-end" exhaust systems? Does it matter enough to not use the OEM spec sensors? Any suggestions would be greatly appreciated.
I have only ever used the OEM exhaust and so far with 11 years and 210K+ miles its working just fine. Same with the sensors, OEM used Denso and thats what I replaced them with and everything seems to be working great. I think the supercharger guys might be doing some 'higher end' stuff but I dont really know. Underdog Racing would be the place to check out for the super charger stuff.
 
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