Downstream o2 - Fuel trim or no?

[Sidewinder CC]

If you can view a graph of the O2s on a scan tool output, you would see the upstream with a spiked pattern; Very quick and extreme switching from lean to rich (0v to.9v) as the ECM adjusts fuel injector pulsewidth. Looking at the downstream you should see even, smooth swings that look fairly flat. Usually some where between .3v to .6v. If the down stream waveform becomes steeper and spiked like that of the upstream, the CAT is loosing it's ability to clean the exhaust.

The downstream 02 is used to "fine tune" fuel mixture to keep the CAT inside this target window of .3v to .6v If the ECM sees the D/S voltage trending too high, i.e. staying more towards .6v, it will lean the mixture to set the trend more neutral, and vice-verse.

The ECM will set a CEL for cat efficiency when the average stays too high, and it cannot compensate by adjusting mixture, or the waveform becomes too spiked and starts reacting quickly to fuel mixture changes.


All that being said, a downstream O2 will not, typically, affect driveability in the least, including fuel efficiency.

If you addressing me here. I understand all that.

My scanner is a simple scanner, without graphing.

 

[Sidewinder CC]

Don't need one.

With OBD-II, an ELM327 adapter (Bluetooth) and Torque Pro (five bucks) will give you the same sort of display, per HEGO sensor, and you can watch them all at once. Direct HEGO signals, LTFT, STFT - you can put them all up on the screen and see what's going on.

MUCH cheaper than an O-scope, and you can watch it while driving (or with someone else driving, or...)

Oh - and you can datalog, so you can examine it later. Nice plus (can't do that with an O-scope.)

Don't need one says he.

Don't want one says I.

 

[gradon]

Just an FYI: for Iphone users that aren't jailbroken, buy the ELM327 wireless(not bluetooth) and buy the $6 Enginelink app(wish we could use Torque).

 

[Keekleberrys]

Don't need one says he.

Don't want one says I.

Really my only point was serial data has an extremely slow reporting rate and it is not as effective for diagnosis of some sensors because of that. Especially with snap throttle type stuff you would be doing to test for a slow O2 sensor, it is very slow to respond, and considering o2 sensor switching is in milliseconds you are not getting anywhere close to the whole picture.

I have an ELM327 wifi as well and I like it, but it doesn't give you the whole picture. Even a fancy scanner wouldn't. I realize a lab scope for a DIY'er is not really reasonable due to cost, but that's the only way to get an accurate picture of what I was talking about in my original post which is why I brought it up.

 

[5-90]

Really my only point was serial data has an extremely slow reporting rate and it is not as effective for diagnosis of some sensors because of that. Especially with snap throttle type stuff you would be doing to test for a slow O2 sensor, it is very slow to respond, and considering o2 sensor switching is in milliseconds you are not getting anywhere close to the whole picture.

I have an ELM327 wifi as well and I like it, but it doesn't give you the whole picture. Even a fancy scanner wouldn't. I realize a lab scope for a DIY'er is not really reasonable due to cost, but that's the only way to get an accurate picture of what I was talking about in my original post which is why I brought it up.

Your right about Serial TX/RX of data - and I think OBD-II DP are running about about 27k-30kBAUD.

But, you're also right about hobbyists not needing the capabilities of a full-on 5MHz lab scope, or anything like that. For most people, 27-30kBAUD serial data and realtime interpretation is plenty.

 

[blistovmhz]

A question for blistovmhz: Have you checked the individual injectors for leakage and the like? I lose track of what's been done here, but if pulling an injector line causes a problem, I'd check for dribbling and other injector faults.

Yup. Pulled all the injectors and checked them all for leaks. They all look good. Discovered that pulling the power to the #6 injector, or the spark lead to #6 cylinder, causes the smoke to come back at any temperature. This does not occur with any other cylinder.
Swapped the #6 injector and plug to #5 cylinder, and pulled power and spark to both 5 and 6. 5 works fine and pulling power to 6 (even with known good injectors and spark) causes smoke.

So, at this point, my money is on cracked block. Compression test looked perfect and the head has maybe 20,000k on it so I'm doubting it'll be a valve seal.
Any other idea's before I haul it away to storage till 2015?

I suppose maybe I should ask if anyone knows of a decent machinist who won't rape me for the price of 5 engines to do a magnaflux, and probably bore/hone some cylinders if I find a new short block?

 

[altrocker1]

Re: Re: Hesitation issues

Wrong thread

 

[blistovmhz]

oh yea! This thread.
Turns out my block is cracked. Finally confirmed! There's a tiny crack near the rear top of the #6 cylinder. When the water pressure gets high enough, a tiny trickle comes through. The only way I can confirm is that my new #6 injector is rusty, and my coolant is still very slowly disappearing, but isn't landing on the ground. If I run the coolant at very low pressure by manually depressurising at the cap (I've got a valve to manually depressurize now), it doesn't leak at all and I get substantially more power until it pressures up again.

 

[yossarian19]

The downstream o2 sensors monitor CAT efficiency, and the PCM will throw a code when out-of-spec results are received.

To my knowledge--and if anyone can post up a reference to any make, model, etc. that is the exception, please do--NO downstream o2 sensors are used for fuel trim.

I don't know a ton of detail but I am quite sure that as of 2005, Volvo was giving the downstream sensors "increasing" authority over fuel trim. I have to suspect that other manufacturers, even Americans, would have followed suit. The downstream "used" to be a dummy sensor and always was on XJs, but times are a-changing.

EDIT: Further reading on my part suggests that yes, the downstream sensors are getting more authority from more manufacturers as time goes on. It's still a damn sight less than the other 02s get. I wouldn't expect to see a MPG hit from a rear 02, but increased emissions - yes - and this is (I infer) why they have any fuel trim authority at all.