Bad O2 Sensor? Cat? What is this telling me?

[BuffaloXplorer]

In an effort to tack down the source(s) of my horrid gas mileage (can't even call it "Fuel Economy" without frowning) - i'm looking at the o2 sensors (which i replaced within the last 1.5 years). Here's a graph of the post-cat "monitor" sensor:

I've read on here that it should not fluctuate like the graph indicates. The sensors at bank 1 and 2 look eerily similar to this one - although they should fluctuate. What is this telling me? Bad sensor? Bad cat? Ultimately, what has to be done to determine a bad cat (backflow test?)?

If it is the cat, can i replace the OEM cats with a high-flow aftermarket single unit? Not looking at "high-flow" as a performance upgrade, as the cat offers less resistance than the muffler, but as a cheap(er) option to a Ford dual unit. Anyone know of a cheap retailer (i'm guessing Summit is an option).

 

[Eneurb]

Looking at that pattern, your convertor is doing nothing, the signal should be an almost flat line when the convertor is up to burn temperature. Be careful though, because the failed convertor could be a result of the bad fuel mileage, not the cause. What is your fuel pressure?? What are your long fuel trims?? Don't replace the catalytic untill you are sure that the engine isn't dumping fuel into the exhaust, if gasoline is getting into the cat, it will fail again.

 

[Glacier991]

Yeah, a dead cat.. those look like pre cat sensor readings. (I do note the crossover seems a tad slow). Eneurb is right about finding out what killed the cat... I too would be curious to see LT trim no's.... bet they are close to 20%....

 

[BuffaloXplorer]

believe it or not the LTFT rarely crosses the +/- 10 threshold at the front sensors...not sure at the rear but i'm going to set the carchip to monitor the FT at the three sensors. The fuel pressure is fine and holds at 33 or so...the injectors hold steady for at least 1/2 hour, so they're not leaky. My coolant temp is good.

 

[Eneurb]

Fuel is measured at bank one and bank two. There is no distinction between upstream and downstream oxygen sensors. An ideally running engine would have the the sum of long and short term fuel trims equal close to zero. If your long fuel trim reading is -10%, that means that the oxygen sensors are reading too much fuel, and the PCM is removing 10% fuel, and vice versa with +10%.

 

[BuffaloXplorer]

Here's all three sensor voltage back in November - no big changes since then:

Bank 1, Sensor 1:

Bank 2, Sensor 1:

Bank 1, Sensor 2 (Monitor):

Here's LT Fuel Trim on a different trip - it was 33 minutes of stop-n-go driving.

LTFT @ Bank 1:

LTFT @ Bank 2:

With CarChip, i can monitor any 4 of the following parameters at a time (plus speed):

RPMs, Throttle Position (%), Coolant Temp, Engine Load (%), Air Flow Rate (lbm/s), Intake Air Temp, Timing Advance (degrees), STFT B1, STFT B2, LTFT B1, LTFT B2, O2 Sensor Voltage B1-S1, B2-S1, B1-S2, Battery Voltage;

What would the best combo that'd be best utilized to diagnose my problems?

 

[Eneurb]

It's rule of thumb that if O2S11 and O2S12 are mirror images of each other, the catalytic convertor is doing nothing. I can't see what the time intervals are on your recording. LFTs are tables that use many blocks of info, meaning that there will be a different LFT reading at idle, than at part throttle, with or without A/C on and so forth. I would perfer to know what LFT and SFT is at idle. The ups and downs that you are seeing is different blocks for different driving conditions.

 

[sabatosh]

The sensors look good, they are running at a full resistance, you detect problems with the sensors when they don't fluxuate much. That is pretty normal tolerances for oxy sensors in general in my limited opinion. I don't think the oxy sensors are the problem, you aren't stalling out or having real bad power problems, so I doubt the cat is clogged, you don't hear any sound like marbles, so I doubt it's decintegrated. The peaks and valleys in the graph are just relative to the engine and wether it's on an exhaust stroke or not.

Have you scanned the engine for any other codes? Does the engine feel like it's missing or shaking? Are you having any driveability problems other than bad gas milage? Another big possibility is that you are having torque converter failure. Classic symptoms of torque converter failure which can happen for many reasons, is bad gas mpg as much as 50% or more.

 

[Glacier991]

Some of the other O2 gurus help me but do I see some indication of a possible sticky injector?

I am still trying to figure out what killed the cat.. I have no doubt it is non functional from the chart readings. 10% FT doesn't impress me especially as a cat killer...this is a puzzler....

(and for those who think computers in cars ain't a good thing, how about those o2 charts! LOTS of info there.... less so for the LT Trim for some of the reasons that Eneurb mentioned but still... great stuff to use in diagnosing... )

I'm still thinking the response time is a tad slow, maybe that's me. Can you graph injector pulse width for us to look at ?

 

[BuffaloXplorer]

Have you scanned the engine for any other codes? Does the engine feel like it's missing or shaking? Are you having any driveability problems other than bad gas milage? Another big possibility is that you are having torque converter failure. Classic symptoms of torque converter failure which can happen for many reasons, is bad gas mpg as much as 50% or more.

Nope, no other codes - i recently cleared a p0402 - EGR excessive Flow - and the code has not come back. I replaced the egr, dpfe AND evr to manage that.

No shaking/shuddering.

My tranny&TC were rebuilt last april and have noticed no problems there.

Some of the other O2 gurus help me but do I see some indication of a possible sticky injector?

I am still trying to figure out what killed the cat.. I have no doubt it is non functional from the chart readings. 10% FT doesn't impress me especially as a cat killer...this is a puzzler....

(and for those who think computers in cars ain't a good thing, how about those o2 charts! LOTS of info there.... less so for the LT Trim for some of the reasons that Eneurb mentioned but still... great stuff to use in diagnosing... )

I'm still thinking the response time is a tad slow, maybe that's me. Can you graph injector pulse width for us to look at ?

Unfortunately, i can't get injector/fuel system stats with CarChip - I've tested the fpr (holds fine at 33psi) and the injectors hold pressure for quite a while. It may be possible that they're sticking (i do run gas treatment/seafoam on occasion). Sticky injectors would correspond to the FT stats, as it seems that most of the time i'm running lean(?), but i'm not sure.

ENEURB/Glacier: Here's LT & STFT at idle during the same week as the above charts - this was a morning remote-start action for 15min of idle. It had not been driven - this is a cold-start idle. I'll reprogram CarChip so that i can get more current data as my situation has gottem much worse. Would you rather see 15 min of Idle after it has warmed up?

Bank 1 Short & Long Term at Idle

Bank 2 ST/FT at Idle

 

[BuffaloXplorer]

I just realized that these graphs were derived from data acquired at 2am, so, those FT graphs were not from a morning cold start, but from a late evening slightly-cold start. I probably started the X from the bar and forgot about it.

I forgot to add that, for these readings, i have it set to pull readings every 5 seconds. The timeframe for each set depends on just how long i drove for that particular trip.

 

[BuffaloXplorer]

Until i can pull current ST/LTFT warm idle readings, here's some from Nov. involving 2 min. of stop-n-go driving, 10 min. of warm idle, and a couple more mins. stop-n-go driving.

Corresponding Speed Graph:

ST & LT FTs

 

[sabatosh]

If you are getting such fuel mileage then you probably have a fuel leak somewhere. A lean condition without performance problems doesn't constitute much. Sometimes fuel leaks will only happen at high speeds or going up hills etc. It can be difficult to know that it is leaking, but keep checking.

 

[Glacier991]

I see an odd short fuel trim in excess of 10%, appears to have occurred at idle. Still I am not seeing any cat killers here. Still I'd solve my mileage issues before replacing the cat.

 

[BuffaloXplorer]

Unfortunately, aside from a leak, i really have NO clue where to go from here. Not sure what an aftermarker cat costs installed, but that may be my best option at this point. If nothing i have shows excess fuel being used, maybe the cat IS the culprit just by age??

Anyone know offhand what size piping is just before & after the cats? I'm looking at Magnaflow high-flow cats on eBay and not sure if i need 2.5, 2.25" - i'd likely go with 2.5" b/c i've got 2.5 from the cats back

 

[Glacier991]

By now a dealer would just have replaced the cat... it's a small gamble, but you may be right... others on this board are way smarter than me, but I just do not see anything that should have killed the cat, based on your current readings. Still... bad fuel mileage/dead cat all sounds vaguely haunting. Go for it.... replace the cat and let's see.....I am like you. I really have no idea where to go from here.

btw... usually we beg for actual codes from Board members, with you we get X-y Graph plots!!! Thank you very much !

 

[Eneurb]

How long do you idle your engine while warming it up?? That could be a major contributor to poor fuel mileage.

 

[sabatosh]

Check the coolant temperature sensor, this can make it run rich and it won't throw a code.

 

[Glacier991]

Good point, but wouldn't that show up as open loop? Those graphs are clearly closed loop to my eye (always learning)...

 

[Eneurb]

Check the coolant temperature sensor, this can make it run rich and it won't throw a code.

Actually, OBDII has a code for insufficient engine temperature for closed loop operation. I can't remember what the code is, but that is the definition of it.