Bad O2 sensors or Catalytic Converter?

Hello,

Lately I have been suffering from pretty bad MPG in both my Tacoma and my Altima. I am hoping to at least solve the Tacoma problem here... After talking with a mechanic and talking about the problem, he and I concurred the problem might be the O2 sensors. My truck has about 120k miles. Even if they were not completely bad, I guess they were due for a change, so I went and ordered the Denso replacements that are same as OEM but dont come in the "little red box"...

Anyhow, fast forward to today. I have not installed the sensors because I have a feeling that they are not completely bad and I would be waisting $300.

Here is what is happening:

I am taking reading of voltage and temperature of the O2 sensors with my Ultragauge while I am driving.

Both sensors, Bank 1 and Bank 2 have similar readings. They are not aways the same, but they are usually close to one another, within 0.05 or 0.10 vot diference usually, until they both get to the same number. This tells me that one of the sensors might be going bad, since its taking a bit longer to get into the correct number. This is just me assuming.

I have no real way of knowing if what the Ultragauge measures is the Bank 1 sensor and Bank 2 sensor upstream or if its, 1 upstream and 1 downstream. If it was 1 upstream and 1 downstream that would mean my Catalytic Converter is bad right?

Anyone care to chime in and help me unravel this clusterfuck of a problem. I desperately need to fix my MPG problem but I do not want to waste money in parts that are working.

I am going to Seafoam the truck this weekend hopefully, dont know if that will do anything as my truck is very well maintained.

Thanks to anyone that chimes in.


As a side note:

- Spark Plugs should not be the problem, they have been changed twice and Ill prob change them again even though it should not be needed.
- Oil is Mobil 1 and gets changed often. I have a bad Valve Cover gasket, but that is going to be changed soon and should not affect this.
- Tire pressure is adequate.
- Gasoline was changed to 91 octane and used an octane booster to see if that was the problem. No change noticed.
- Transmission might need a bit of fluid. But it should not need much. I do find that in rare occassions I think it does not push into the adequate shift, but then again I feel the engine not working properly in those moments.

Edit:

Long ago I got a code, it was for a Bank Sensor problem, I think. I stupidly did not write the code down and the code has never shown up again. Dont know if this helps any.

 

what is your short and long term fuel trim readings

 

I honestly would need to check. I have not written that down and perhaps have overlooked it.

 

Well. I dont know if this helps any at all, I will do an extensive check tomorrow when I drive the truck, but in idle now the thing I noticed is that the STFT and LTFT for Bank 1 and Bank 2 was different. Honestly I do not know if they are supposed to be different. I only know that it is bad if they go into double digits, be it positive or negative. I can check that tomorrow while I drive it. In idle and revving it a bit, the most it got to was 7.91. I did not check how hard it dropped into negative, but it was not double digits.

 

I had to deal with this but it was one sensor I only had to replace but in your case you have 120k so it could be just the sensors.

 

What troubles me is that my Altima is doing the exact same thing, yet it only reached 70k miles now. I mean, perhaps the problems have nothing related to each other, but it made me wonder. I have not done any readings on my Altima yet though. So maybe this is a moot point.

 

Oh, I should note, and ill add to the OP.

Long ago I got a code, it was for a Bank Sensor problem, I think. I stupidly did not write the code down and the code has never shown up again. Dont know if this helps any.

 

I have been reading the Fuel Trims for a while. They seem like things are working normalish...

I did have 1 spike where it went to +10 in one side and then to -17. I did not have double digit numbers again though. I tried to recreate.

Voltage going into sensors seem fine, 1 is a bit sluggish though. Does not keep up to par as fast.

Going to do the red catalytic test this Monday. If they glow or sound I am going to strip them and put in an O2 simulator.

To annoy me even more... The Tacoma is now getting a bit of oil into one of the spark plug chambers. I need to change that Valve Cover gasket, I hope that fixes it. If not, it will be a pain to disassemble the engine to replace the other gaskets. The Altima has the same exact problem... What a pain.

Thanks to anyone that had input. Much appreciated.

 

If both cars are having the same problem at the same time, then it could be bad gas at your local station.

 

Are you speaking about the upstream wideband 02 sensors(A/F sensors)? At 120k miles, these sensors slow down and can affect fuel mileage. The only time you can be sure they have failed is when they throw codes for bank 1 or bank 2 AF sensors. They should be close to 3.5v at idle, and during high load when accelerating quickly, I would expect them to be around 2v. When coasting down a hill after previously accelerating, they should read around 4.99v during the fuel cut with foot off the gas. When they start to slow and get tired, they may read 4.5v or 4.6v during this deceleration during fuel cut.

Normal fuel trims are usually under +/-10, and the Toyota systems don't seem get really concerned unless the long term fuel trim is more than +/- 20 for a long time, then you will have lean or rich codes.

I don't know what your climate is right now, but it is cold where I live, and my fuel mileage has dropped because of it.

 

If you are getting oil in the spark plug tube, the first culprit is the spark plug tube seal on the valve cover, and the second culprit can be the tube starting to lift out of the head. They are just pressed in the head. Remove the valve cover, place a wood block or something non metallic on the spark plug tube, and give it a few small blows of the hammer to re seat it, then replace the tube seals on the valve cover along with the valve cover gasket, and then re install. Make sure the spark plug is installed in case any small pieces of wood fall into the spark plug tube. You can blow air down the tube after you install the valve cover.

 

Thank you for the responses.

Bad fuel, does not seem likely as initially I thought that was the culprit and I switched the Premium, and then different stations/brands. Curiously, apart from PUMA gas, the other stations tend to have very good fuel. In any case, I used Premium and added Octane Boosters to see if it was the problem. Problem persisted. As for the Altima, it is more an MPG thing, rather than loss of power.

Yes, I was speaking about both the upstream A/F sensors and the downstream ones which dont really do anything other than say if the catalytic is working. They seem to be working fine-ish.

As for the oil in spark plug tube, that s exactly what I plan on doing. Thank you very much for the little how-to as it will be useful when I di the repair.

 

Check here for more understanding O2 readings and fuel trim.

http://www.tacomaworld.com/forum/technical-chat/259164-understanding-o2-signals-fuel-trim.html

 

TY

 

Update:

Today I checked the cats. I drove the Tacoma hard-ish. Brought it up to 90 mph for a while, which is weird, since before it would not go up from 80 mph no matter what. Maybe its the lower temperature, it was 80 degrees tonight which is better than the 90s and 100s it usually is driven in.

In any case, when I got home, after a while of driving it, the cats were not glowing red. I smacked them with a rubber mallet and no big particles were heard. I did hear like a dust-like resonance. To me the sound is not like that of broken ceramic.

Any suggestions? Im at a loss, even though I will probably delete the cats anyway.

 

At a complete loss. Went and had the cats removed. All 4. Horrible sound... That was to be expected. No performance chance.

A mechanic suggested transmission and an additive named Metal-Lube for Auto Tranny. Used it... No change.

I just bought the sparks, not going to change them with Toyota ones this time, gonna go with iridium or platinum overcharge crap to see if there is a difference. They are not due, but I will try it nevertheless.

I seafoamed everything, nothing.

I am at wits end. Now I have a Tacoma with little power with a sound which I hate. It sounds like a freaking Civic. I am about to trade it in for another one... Which I'd rather not since this one is almost paid off and has a lot of mods.

Any help appreciated.

Side note. Now that I have no cats. Will glasspacks or a flowmaster resonator quiet the truck down substantially without affecting performance?... not that it has any currently...

Oh and my Tacoma sent a message to you guys...: "Braaaaaaap"

 

At this point I would use my fuel pressure gauge to check pressure and volume. I would tape the gauge to the windshield and watch the pressure under load and higher speed. I would also check the AF sensors with the data list to see what they are doing. Sometimes this stuff can to be hard to diagnose over the Internet. You may need to take it to someone.

 

Wanted to add some info.

Altima is better now. Changed fuel and it now loves to be used with a lead foot, even though I baby it. Problem here was fixed. Also found oil in one of spark plug tubes. I will fix that.

In the Tacoma I changed fuel. Nothing happened. I never got to changing the A/F and O2 sensors. Their readings were normal according to all the info given. I might take it to another shop after the tune-up this weekend.

I have thought about the fuel filter being clogged, I will check the fuel pressure. Hopefully it is this. I am honestly getting disheartened with this problem.

 

Any updates on this?