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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Mon, Dec-30-2019, 08:05:11 PM   #1
ATB88
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Default Using secondary O2 voltage to diagnose cats

Praise be the return of the forum.

As documented here (http://www.m3forum.net/m3forum/showthread.php?t=605885) I've been trying to track down a poor fuel economy issue for quite a while now. My long term fuel trims are high (+7%-10%), and when I had the car on the dyno a couple months ago my AFR was very rich at the top end (10-11). The DME has no codes. I've looked at every other possible cause (intake leaks, fuel system etc. see linked thread) with no luck and I'm wondering at this point if it could be that my cats are soft failing?

Of course a backpressure test is one way to know, but the primary O2s are a real pain to get to and remove so I went looking for other easier ways of diagnosing cats first.

I've read that one way of knowing that you have bad cats is to look at the post-cat O2 sensor voltages -- they should stay constant around 0.8V (rich condition) whereas the primary O2 voltages oscillate between rich and lean conditions. Well, under most conditions, my post-cat O2 sensors are constant-ish as they should be, but in certain partial throttle conditions they will oscillate (like the primaries) for a few oscillations (between 1 and 3 peak to peak oscillations) before stabilizing at their normal rich values again. I'm wondering if this is enough to point to the cats as my problem, or is this transient oscillatory behavior normal and it would need to be constant oscillation of the secondary O2s that indicate a real problem?

(note: my primary O2s have recently been replaced, secondary O2s are original with 70k miles, no reason to expect either are faulty -- just trying to get a sense of whether the way the secondaries are behaving points to cat problems)

Last edited by ATB88; Mon, Dec-30-2019 at 08:15:54 PM.
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Old Mon, Dec-30-2019, 08:15:11 PM   #2
Drewster
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Default Re: Using secondary O2 voltage to diagnose cats

Could you document what you have and haven't tried so far? I would doubt a faulty cat would cause a stock tune to dip down into the teens for AFR. In order to be putting that much fuel in the first place, the DME had to have either the MAF or primary O2's, or the fueling table itself indicate it needed that much fuel (incorrectly). To answer your question, if pre and post are constant-ish (i.e. not following each other), I wouldn't suspect the catalyst.
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Old Mon, Dec-30-2019, 08:30:15 PM   #3
ATB88
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Default Re: Using secondary O2 voltage to diagnose cats

The secondary O2 voltages are sometimes semi-constant as they should be, but at other times they really are following the primary O2 voltages with full oscillations. Just not all the time, only under certain throttle conditions. Is that not enough to point at cat problems, even if it may not be a big enough problem to be the main issue I'm facing here? The DME has apparently been (erroneously) dumping extra fuel in for over 20k miles now, and my understanding is that this can rapidly accelerate catalyst wear.

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Originally Posted by Drewster View Post
Could you document what you have and haven't tried so far?
Sure, I meant for this thread to be just about secondary O2 sensor behavior since I already have a thread documenting my odyssey in diagnosing this issue, but a quick summary of what I've done/replaced as documented in my main thread:

-Most PCV related hoses, oil separator (OE BMW)
-Cleaned ICV, throttle bodies
-replaced brake booster (but not check valve) (OE BMW)
-intake filter (OE BMW)
-plugs & coils (OEM NGK & Eldor)
-injectors cleaned (RC fuel injection)
-fuel pump (OE BMW)
-fuel filter (OE BME)
-fuel pressure regulator (OE BMW)
-several MAFs (Bosch)
-Both engine bay TPS (OE BMW)
-Intake camshaft sensor (OE BMW)
-Both pre-cat O2s (Bosch)
-thermostat (OE BMW)
-re-timed VANOS (which was fully Beisan'd 15k miles ago, adaptations reset, passes VANOS test no problem)
-intake smoke tested to death by multiple mechanics, no leaks found
-compression looks great (176-179 across all 6)
-70k miles on motor, tune is stock US 6MT

dyno AFR graph for reference


Last edited by ATB88; Mon, Dec-30-2019 at 08:59:04 PM.
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Old Mon, Dec-30-2019, 10:51:03 PM   #4
Drewster
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Default Re: Using secondary O2 voltage to diagnose cats

Quote:
Originally Posted by ATB88 View Post
The secondary O2 voltages are sometimes semi-constant as they should be, but at other times they really are following the primary O2 voltages with full oscillations. Just not all the time, only under certain throttle conditions. Is that not enough to point at cat problems, even if it may not be a big enough problem to be the main issue I'm facing here? The DME has apparently been (erroneously) dumping extra fuel in for over 20k miles now, and my understanding is that this can rapidly accelerate catalyst wear.



Sure, I meant for this thread to be just about secondary O2 sensor behavior since I already have a thread documenting my odyssey in diagnosing this issue, but a quick summary of what I've done/replaced as documented in my main thread:

-Most PCV related hoses, oil separator (OE BMW)
-Cleaned ICV, throttle bodies
-replaced brake booster (but not check valve) (OE BMW)
-intake filter (OE BMW)
-plugs & coils (OEM NGK & Eldor)
-injectors cleaned (RC fuel injection)
-fuel pump (OE BMW)
-fuel filter (OE BME)
-fuel pressure regulator (OE BMW)
-several MAFs (Bosch)
-Both engine bay TPS (OE BMW)
-Intake camshaft sensor (OE BMW)
-Both pre-cat O2s (Bosch)
-thermostat (OE BMW)
-re-timed VANOS (which was fully Beisan'd 15k miles ago, adaptations reset, passes VANOS test no problem)
-intake smoke tested to death by multiple mechanics, no leaks found
-compression looks great (176-179 across all 6)
-70k miles on motor, tune is stock US 6MT

dyno AFR graph for reference
Yeah, it would be good to put that at the top of your 8-page odyssey to get people up to speed faster.

Unless your cats are glowing hot, etc. it really doesn't sound like there's an indication to look there. It certainly doesn't make sense that they would cause you to burn more fuel.

While you go through stuff, keep in mind that for modern cars it's pretty much
[Measure air in]->[Calculate required fuel]->[Apply injector duty cycle]->[Trim/ adapt based on O2 sensor reading]
The "calculate fuel" bit can get super involved with models and load calculations... but at the end of the day, a TPS or a cam position sensor is not going to change the efficiency of the engine or how the DME calculates airflow.

I assume this is also a stock engine/ tune/ nothing aftermarket? Have you tried re-flashing a stock calibration to be sure?

The good news is that your power levels are spot on, and the MPG is poor but not *that* poor.
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Old Mon, Dec-30-2019, 11:13:28 PM   #5
ATB88
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Default Re: Using secondary O2 voltage to diagnose cats

Quote:
Originally Posted by Drewster View Post
Yeah, it would be good to put that at the top of your 8-page odyssey to get people up to speed faster.

Unless your cats are glowing hot, etc. it really doesn't sound like there's an indication to look there. It certainly doesn't make sense that they would cause you to burn more fuel.

While you go through stuff, keep in mind that for modern cars it's pretty much
[Measure air in]->[Calculate required fuel]->[Apply injector duty cycle]->[Trim/ adapt based on O2 sensor reading]
The "calculate fuel" bit can get super involved with models and load calculations... but at the end of the day, a TPS or a cam position sensor is not going to change the efficiency of the engine or how the DME calculates airflow.

I assume this is also a stock engine/ tune/ nothing aftermarket? Have you tried re-flashing a stock calibration to be sure?

The good news is that your power levels are spot on, and the MPG is poor but not *that* poor.
Yeah agree, I'd been meaning to overhaul the first post of that thread to make it more readable.

Yep, stock everything. I flashed this factory tune onto the DME myself after doing my 6MT conversion. Could do it again for kicks I suppose, or maybe try an older factory US tune (current one is the most recent "march 2009" tune for MSS54).

While I agree that the power level's fine, it seems to be losing power after 7200 RPM when it should still be gaining, right? And I'm still getting this bad MPG (13-14 city) with the most gentle driving possible. That plus the super rich AFRs up top point to something weird going on and I really want it figured out before I go and start modding and tuning etc. Given everything I've tried and ruled out already, whatever the problem is is something strange and something that anyone would consider "unlikely", especially considering that the car's been to multiple mechanics who claim to know M3s well and still couldn't figure it out. If it's not the cats then **** me I'll probably never figure it out :/

Last edited by ATB88; Mon, Dec-30-2019 at 11:25:21 PM.
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Old Mon, Dec-30-2019, 11:29:17 PM   #6
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Default Re: Using secondary O2 voltage to diagnose cats

I suppose if the cats are failing, the EGTs could be getting really high, and the DME could be dumping fuel to cool them down... but I think that situation would throw an error. Maybe try logging your EGTs anyway.
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Old Mon, Dec-30-2019, 11:39:29 PM   #7
ATB88
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Default Re: Using secondary O2 voltage to diagnose cats

Quote:
Originally Posted by terraphantm View Post
I suppose if the cats are failing, the EGTs could be getting really high, and the DME could be dumping fuel to cool them down... but I think that situation would throw an error. Maybe try logging your EGTs anyway.
I didn't know our DMEs measured EGT, I'll take a look thanks!
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Old Tue, Dec-31-2019, 12:04:10 AM   #8
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Default Re: Using secondary O2 voltage to diagnose cats

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Originally Posted by ATB88 View Post
I didn't know our DMEs measured EGT, I'll take a look thanks!
Yeah, if you look at your headers, there are pre and post O2 sensors, and a slender metal thing - that's your EGT sensor.

It would be odd to see EGT impact LTFT, but if you unplug it and the problem goes away, you have a pretty quick brute force answer
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Old Fri, Jan-03-2020, 02:13:20 AM   #9
ATB88
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Default Re: Using secondary O2 voltage to diagnose cats

Couldn't get EGT to show up in Torque, so I busted out a laser IR thermometer, got under the car, and took a look at temps at inlet and outlet of cat. It was hard to get a great reading on cat inlet due to the geometry. The temps themselves seemed fine (between 300-470 F at idle, fully warmed up), but it seemed like the inlet temps were consistently higher than the outlet temps (by between 50-100 F). Apparently, it should be the other way around: outlet temps should be higher than inlet, and if that's not true then it's an indication of bad cats, according to many sources such as https://www.2carpros.com/articles/ho...ytic-converter

So, signs are pointing at possible cat problems, but admittedly not very strongly -- Secondary O2s only track the primaries in certain conditions, and it's hard to completely trust the temp test because it was hard to properly aim the thermometer at the cat inlet, so there certainly isn't any smoking gun. Backpressure test I guess is the only other alternative but if I work that hard to access the primary O2s I kind of want to have another set of cats on me to swap in if the backpressure test confirms the problem, while I have the heatshields off, the coolant reservoir drained, etc...

As for plausibility of cats causing fuel trim/consumption problems, does this sound like a reasonable hypothesis: if the cats are restricting flow of combustion gases then it's throwing off the primary O2 readings -- the buildup of gases which at the correct concentration might read slightly lean instead (incorrectly) read very lean, which in turn causes the DME to incorrectly ask for more fuel. Meanwhile, since it's asking for fuel which it didn't actually need, the car is actually running rich (as seen in my dyno AFR which is using a sniffer very far downstream from the cats where the buildup is)? would welcome feedback especially if this logic is flawed :]

Last edited by ATB88; Fri, Jan-03-2020 at 03:46:58 AM.
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Discussing Using secondary O2 voltage to diagnose cats in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)