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Discussion Starter #1
This morning I'm taking my wife to work so I can lug her 20lb Banana Split Cake up to the unit she works. On the way the SES light comes on. "Well, Crap!" I think. Take the cake up, get back to the car and drive home. No SES light. Hook up the Innova 3140- "No DTCs Stored"
hmmm

Weird, but I've seen weird before. I sit there, playing and decide to start the car up and check the live data. Everything looks okay. But then I see something funny. Going through the O2 sensor data one sensor, B1 post-cat, is reading 0.7v +-0.05v. All the others are bouncing up and down mostly between 0.2 and 0.8 volts like I would expect.

So would a dead O2 sensor cause a phantom code but not a real one?

Weird.
 

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Discussion Starter #2
Still weird, but my thinking was wrong.

So after consulting with friends on MyE28.com(Many of whom also have and work on M5s), I've learned a few things after installing a new sensor and readings staying the same. One, Post-cat o2 sensors do not dither. If the cat is doing it's job, they will not flucuate like the pre-cat sensors. Two, if this is right, I have a bad DS cat without throwing a code, or w/o the code staying any way.

I've suspected that I may have a "tune" and this may be another clue in that direction. The car came with the Hamann wing and ACS pedals so it's not a huge leap that it has a tune also. Shoot I'm surprised some of you don't know the car. Anyone know of an easy way to prove a tune is present other than going for a land speed record?

On the plus side, it's not an emergency so I have time to figure this out. More research/experimentation to follow.


dj
 

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Maybe the code is hiding elsewhere in the scanner. I am not familiar with yours. My a genisys hides things on me. I can only read cel setting codes on "read"codes. Pending codes gives me the codes it is thinking about setting and code history tells me about a light that went out.

The code that would likely come from a bad cat are the dreaded B2 andB3, the latter if you are right. Let me tell you those codes don't just blink and go away. They are mean and nasty. I think you will find that the post cat sensors do fluctuate on a VANOs motor. Has to do with the amount of exhaust gas recirculated.

Any code that could light the cel and then turn off unless present at the moment is not worth losing sleep over.
 

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dj: Please check to see if you have any o2 sensor bung leaks/cracks etc.

PS : Good to see you have checked out stuff on with diagnostic tool, and have come on here to ask questions about this issue. I am surprised there has not been much response YET !

I think you will find that the post cat sensors do fluctuate on a VANOs motor. Has to do with the amount of exhaust gas recirculated.
I like this angle of thought.

Please tell us more !
 

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dj: Please check to see if you have any o2 sensor bung leaks/cracks etc.

PS : Good to see you have checked out stuff on with diagnostic tool, and have come on here to ask questions about this issue. I am surprised there has not been much response YET !



I like this angle of thought.

Please tell us more !

Too early without enough real info, but My old 540 E34 had no egr. Really huge cats 3 stage. My cats dealt with the NOx issue. The small cats on the M5 suggest s62 is dealing with NOx emissions similar to most other manufacturers. If we look at how Honda deals with it on their v-tec engines I think we get a clue. Otherwise why vary the exhaust valve? By closing the exhaust valve before tdc they grab the exhaust gas(inert gas) to not allow an excess of nitrogen and oxygen (air) to convert. American engineers seem to love their EGR systems that return the gas to the air intake.

If I watch the posts on a ford van the numbers are pretty stable with its slow to respond EGR system. On my M with new post cats I see the number bouncing around which I attribute to the VANOs doing the job. It would be extremely fast to respond to correct the amount of unused air. If the vanos is doing the job it could respond on the next engine stroke. If it was doing that the O2 output would change accordingly. Egr or like my old E34 would have to do much more to adjust to unused air and by the time it got there it would be an average anyway. So a stable reading.
Just thinking out loud. I suspect that could be what is moving carbon into the SAI system. The pressure would change frequently in the exhaust part of the head. Because the two heads are connected by that stupid tube my guess is we have a active port moving exhaust gas back and forth between adjustments. They should have had two valves and two tubes.
 

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Discussion Starter #6
Many thanks for the replies. I was hoping for more input but I think that's my own fault for searching out the easy answers. ;) Seriously though, I try to avoid wasting people's time. On the other hand, I'm cetainly not a pro and do get things backwards occasionally and just need to check my thinking with those of you that know more.

I will probably get a Peake reader at some point but right now it's too specialized for my toolkit. Other than the non-OBD E28 528e we have two OBDI cars, one GM, One Ford; and two OBDII cars, the M5 and the 2500HD. I had to start with one tool that would do everything.

The good news is we just got back from a trip across state to see Mom, a 650 mile round trip, and the beast ran fine and returned 22mpg for the trip. No return of the mystery SES light. I'll be going out in a bit to check the O2 sensor bung for leaks and a recheck of the live data.

Honestly the non-dithering post-cat theory makes the most sense to me but I'll be experimenting today to try to wrap my head around what's been said. I'll hopefully have a better understanding this afternoon. Or maybe just a big headache.

Again, thanks for the ideas.

dj
 

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Many thanks for the replies. I was hoping for more input but I think that's my own fault for searching out the easy answers. ;) Seriously though, I try to avoid wasting people's time. On the other hand, I'm cetainly not a pro and do get things backwards occasionally and just need to check my thinking with those of you that know more.

I will probably get a Peake reader at some point but right now it's too specialized for my toolkit. Other than the non-OBD E28 528e we have two OBDI cars, one GM, One Ford; and two OBDII cars, the M5 and the 2500HD. I had to start with one tool that would do everything.

The good news is we just got back from a trip across state to see Mom, a 650 mile round trip, and the beast ran fine and returned 22mpg for the trip. No return of the mystery SES light. I'll be going out in a bit to check the O2 sensor bung for leaks and a recheck of the live data.

Honestly the non-dithering post-cat theory makes the most sense to me but I'll be experimenting today to try to wrap my head around what's been said. I'll hopefully have a better understanding this afternoon. Or maybe just a big headache.

Again, thanks for the ideas.

dj
I would be interested at what your readings are I know my car is not perfect and if the readings are not the same it may point to something. I have a 2500HD too. Does your scanner give hex codes or only OBD II codes? Try and look at the post cats on your 2500HD see how they respond. Assuming it is not a diesel like mine.
 

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Discussion Starter #8 (Edited)
I should learn to quit while I'm behind. The 2500HD is an '04 with the 8.1L Vortec and OBDI. I'm not sure how they managed that. Maybe it's the HD, so the truck is in a commercial class and not considered a "light truck"? Just guessing; didn't find anything on-line. Anyway there are pre-cat O2 sensors, but no post-cat.

The scanner shows P-codes plus live data- temps, fuel trims, load, timing, and o2 sensor readings. With the GM cars I can pull a code history, but not with the BMW.

I had left my wife's garage queen 2000 Z28 off the list since it was still under cover. So I brought it out and hooked the scanner up. Both post-cat sensors were steady at 0.7 while the pre-cats dithered back and forth like you would expect.

One thing I did notice was that my bank 2 LTFT is at 10%. It was at 10% when I replaced the vanos solenoido-rings, and afterward dropped to about 3%. Bank 1 is and was at 4%. Bank 2 would be the Cat that I believe to be dead. Also when I replaced my vanos o-rings, the first code that indicated a problem was "Intake Camshaft Position - Timing Over-Advanced". This all leads me back to the possibility that I have a tune that ignores the post-cat sensors. That's the only thing that makes sense to me since I'm not getting any codes.

Oh well. The car is drivable so I'm not in a panic. I'll get to the bottom of this eventually and I'll post the details here as I learn more.

I hope some of this makes sense,

dj

edit: I found documentation the 8.1L was OBDII exempt.
 

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8.1 oh ah drool that will have power to spare. I am thinking 8.1 gas trumps 6.6 diesel nice! There are times I think my truck will pull itself in half, yours definitely could.

Back to the real car Poor atomization of fuel can cause the trim to come up. Out of range codes can come and go as they please, o2adjusts trim to max short trim usually. I can't find those on my list of codes though. I know they are required under OBD II. Anyway point is how are your injectors ? Any chance they are dirty?

That is quite a list of mucho powered rides.
 

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Discussion Starter #10
Thanks re: the truck. It is my wife's DD and horse and dog hauler. All six pups can ride as it's a crew cab, Our mastiff, Dozer, is the Alpha and takes up a lot of room. It's also a Regency conversion and has almost every bell and whistle the M5 has. It doesn't even notice the horse trailer and I would imagine it could pull a house off it's foundation.
We're talking about matching plates- HZBEAST and HRBEAST now that Texas has gone to seven digits.

The Z is a rocket ship but it is singly the most uncomfortable car I have ever sat in and handles like a drag racer. Cindy loves it like one of her children tho. ??????

Back to the M :D When I bought the car last June, I wanted to learn every detail about it the way I know the E28s. Only I thought it would take a bit longer. The car has about 113k now so injectors are a possibility. I've been planning an under-plenum maintenance, going through everything and replacing things as needed. That could be my next step. If bank 2 is overfueling, that could have killed the cat so I'm back to what I was doing before this- replacing items that are not up to "as new" performance. I'm thinking about, along with getting a Peake, buying something that can give me more info like INPA or working with Evolve or another tuner to see more of what is going on. I keep feeling that I'm missing something. That something might just be the first 100k miles though.

One thing I was told by a friend sticks with me and I've read it here too. "It may be a $16,000 car now but it will always have $80,000 car maintenance. I knew that going in and still plan on keeping this one forever.

Thanks for sharing your comments.

dj
 

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Interesting I guess I am going to dig deeper into my vano's. I have my drivers cat acting like yours, and have had it out to test, it works. Question for after 100miles where the ltft goes. I have been thinking that LTFT has an effect on how the vanos is controlled on the exhaust side.
 
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