So I decided to treat the Disco to a bath today as we FINALLY had a nice day in the midst of this arctic winter.

I’ve a slight stumble on idle for a good long while and just figured I might have another minor vacuum leak or maybe some plugs or whatever. I’ve been struggling with the MAF for a good while and used to get codes and then got a new one and then back to the original after cleaning and blah blah blah. I went back and read some of the threads I’ve either started or commented on and I’ve been talking about this for some time. I’ve not had a MAF centric code for quite a while now.

As I pulled up to pay for the wash, BEFORE going in, the idle got really bad and then driving home it almost wanted to stall and the CEL even started blinking for a sec. It eventually cleaned up and was fine.

After getting home and clearing the codes, I started it and watched the O2 sensors on the Ultragauge focusing on bank 1. As a quick recap to some research I did a while back, the O2 sensors cover each bank of cylinders with one pre and post cat. .45V is 14:1 air to fuel and anything below is lean and above is rich and the pre cat O2 sensors provide the value to adjust the mixture.

From the research, the pre and post values should kinda mirror each other, with some lag of course. In other words, the pre cat O2 sensor will tell the ECM to change the mixture, and then the post cat should verify the change was made and provide the appropriate mirrored value. Now I could be wrong or interpreting this incorrectly, so please correct me if I’m wrong.

I mention all of that because in bank 1 the values seem to dance as normal from below .45v to above .45v. However, the post cat O2 sensor seems to stay consistently above .45 and not really mirroring the pre cat. Bank 2 seems to more closely mirror pre and post cat for the O2 sensors- not perfectly, but better than bank 1.

So, my question is, should I be looking O2 sensors?

I posted this pic in another thread some time back and I can’t recall which is bank 1 and bank 2, but you can see where there was a rough idle on one bank. Could that be plugs, wires, or the coil? I did not hook up the Nanocom tonight to see if I had any rough running conditions, but I bet I would have seen similar.



Other variables:
- I’ve done the PCV mod
- I just cleaned the IACV a week or so ago
- For this latest tank of fuel I put in a couple cans of sea foam and I’m a scoshe over 100 miles in on this tank
- My economy has not changed and is what I’d expect a bit 13 (lift, bigger tires, roof rack) and prior to the rack I could get 15 or so too
- I’ve not read the MAF values from that page I set up on the UG and honestly can’t recall what those values should be if is healthy
- My fuel trim values (short and long term) are WAY better than my other thread and I think are in good shape fluctuating from negative to positive with long term being under 10 for each bank
- I’ve reset the adaptive values a couple times since I started digging into this- but did not when I cleared the codes today
- I’ve also got P0501, but I don’t think that is related at all and have been researching resolution to that

Again, does this point to the bank 1 O2 sensor (I’d just place them all) being ready to go?

 

As you say, the post cat sensors only monitor the cat, I’m pretty sure they don’t do anything to affect the fuel trim, so I don’t think replacing the post cat sensors will have any effect on how your engine runs.

 

Agreed. With the code being pre cat, though, and assuming my logic was correct, would the values I am seeing with the O2 sensors pre and post car not mirroring each other denote an issue there?

 

have you swapped the pre-cat O2 sensors from one side to the other to see if the readings follow the O2 sensor?

 

How long has it been since the front sensors were replaced? I bought a pair of Walker's from Rock Auto and have had no problems, and honestly given how easy they are to replace and how bad the fuel economy is on the D2 it doesn't take long for a pair of front o2 sensors to pay for themselves even if it "only" nets a 5-10% fuel economy gain. O2 Sensors read "lean" as they age so over time they dump more and more fuel into the system. It's important that you replace the o2 sensors in pairs, but not necessarily all 4 at once. If the back ones aren't acting up then I would ignore them. The only purpose they serve is to throw an engine code if your cats aren't doing their job, they don't give any actual feedback to the engine management, only the fronts do that.

 

The forum acts stupid from time to time and won’t post what I’ve typed? Anyone know about that?

Anyway, you guys are pretty good. So it looks like I should be looking to replace at least the pre cat ones?

To answer the question about when they were replaced, for all I know, they are original and it’s at 176k miles on the clock. Doubtful, but still.

Also, am I correct in the sensors should mirror each other pre and post cat?

 

If the 02 sensors are of unknown age I would definitely replace them, the fronts at the very least. As far as the cat sensor readings they should not be mirroring each other. The upstream o2 sensors will constantly “zig zag” between a hair too lean and a hair too rich as the computer attempts to get the air fuel mixture perfect. The downstream sensors will initially upon cold startup mirror the upstream sensor because the cat isn’t hot enough to light off and start catalyzing pollutants. However, once operating temperature has been reached the downstream should show a constant voltage, as any excess oxygen in the exhaust is “consumed” in the operation of the catalytic converter. Technically when the engine is running lean the catalytic converter is breaking apart oxides of nitrogen (NOx) in to nitrogen and storing the oxygen and when the engine runs rich the excess oxygen is released to convert carbon monoxide and unburnt hydrocarbons in to carbon dioxide and water (for all the crap they get, cats are wonders of chemistry).

If the upstream and downstream reading are the mirroring each other at operating temperature it means the catalytic converter isn’t doing its job quite right and will throw a code.

 

I wish there was a “love” choice for the last post. THAT was what I was looking for.

So if I’m following your logic, would I be correct in saying if the post cat O2 sensors are reading around .75v or so (assuming operating temp) then the cat isn’t doing so hot? (No pun intended) Doesn’t that denote it’s still a bit rich? Or is the higher reading a function of the pre cat O2 sensor failing? Make sense?

 

In your case I would get back to first principles for trying to troubleshoot this. Here would be my initial approach:
1) A P0134 code means (and this comes right out of the RAVE, page 18-2-32) "O2 Sensor no activity detected, bank 1 sensor 1" which refers to the pre cat o2 sensor odd bank of cylinders, which are the cylinders on the RIGHT hand side of the engine when looking at it from the front (drivers side in USA).
2) Replace your front o2 sensors. Get an oxygen sensor socket and a long extension if you don't have one and the fronts are surprisingly accessible and easy to replace. They are of unknown age on an old vehicle plus the code, taken at face value, indicates the sensor is malfunctioning so it the right thing to do anyway (for the reasons on my previous post). Clean the contacts and check the wiring while you are in there. Clear the ECU faults and see if it comes back, if not-hooray, problem solved! If it does come back I would then:
3) Check for vacuum/intake leaks because it's free, some times this code can be thrown because excess air is finding it's way into the intake track behind the MAF causing a mismatch between how much air the ECU thinks its getting (via the MAF) and how much it's actually getting. The excess air causes the engine to run lean, which is then picked up by the front o2 sensor and the computer throws more fuel into the mix to compensate. This shows up as a positive long term fuel trim value, but since you reported that your long term trim levels were within 10% I don't think that's what's happening here (but it's free, and probably a good thing to routinely check on an old truck anyway).