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Would appreciate some help interpreting my fuel trims

9K views 63 replies 13 participants last post by  Sailor24 
#1 ·
Hello everyone,

I recently completed the vanos solenoid boards maintenance, installed new VW MAFs the PO included with the car, and replaced the spark plugs. I completed all of these items within about 300 miles of driving. Everything was fine for a week or two, but the fuel mileage began to worsen. On a very cold morning I got an SES light and have since been averaging about 12.9 mpg over mixed driving. Yesterday I got INPA working and I pulled the codes.

I found 16 codes, but I'm not sure if the PO ever cleared them. So I documented them in the stickied Peake Reader thread and then cleared them. Now I'm going to drive the car until something comes back. In the meantime I checked the fuel trims and took the below picture.

Note* This is what they showed with the car off as I wasn't sure if I was supposed to check these with the car running or not.

Can anyone help me interpret what this is showing? I want to know if something is obviously wrong with the car. Right now it seems to be running fine if just a little down on power. Today the car actually ran better after I cleared the codes with a better behaving throttle after a cold start.

As always thanks for the help.
 

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#3 ·
Ok I'll post here, I just try to use the stickies when applicable.

All of the following readings were taken in the driveway with the car idling after I arrived home from work. About half way through the drive home the SES light came on again. I got the following two codes:

143 - E box fan
178 - Catalyst system efficiency, Cyl #1-4
 

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#4 ·
The only thing out of range is your additive adaptations. Not sure what that means but with your poor gas mileage I suspect something is amiss. Perhaps it is just car readapting to VANOS work and new MAFS. I would give it some miles.
Regarding the cat code, have you replaced O2 sensors? If not I would do that. If they are not old, you may have a fouled cat from running rich for a prolonged time. Try an Italian tuneup.
 
#5 ·
Are the STFTs showing a 33% and 24% increase in fuel to get the car to run right? I just want to make sure I'm interpreting the outputs correctly.

Furthermore I read in a few places that people have experienced issues when using the VW MAFs, especially if there are other underlying issues when they're installed. I reinstalled the BMW MAFs last night and started the car to see if there was an improvement. It stalled immediately after starting and threw a new code 41 (MAF cyl. 1-4). The ones from before did not return. I cleared the code and started it again. It ran fine and I did an italian tuneup on the highway a few times. With the BMW MAFs back in I saw a little over 17 mpg after the hard drive with no SES. After I got to work this morning I saw 17.9 mpg on the OBC, but the SES light returned with the same code 41.

I think I have a few issues, one is that the pre-cat O2 sensors should be replaced. They were actually replaced by the PO 26k miles ago, but the car had a bad thermostat when I got it. I have no idea how long he was driving it around with the rich condition that comes along with the bad thermostat. I'm going to replace them as PM. I'm just glad my catalytic converter code went away, but I am still concerned about them. I'm going to do a carbon treatment this weekend and an oil change I think. The price for a cat on FCP is eye-watering ouich

The other issue is that in the stack of records I got with the car there's no evidence that the CPS sensors were ever replaced. They may be original, and I've read that when they are off the condition may manifest itself in codes for other things like my MAF code. The car has been idling fine on startup over the last week of ~20°F mornings though, so I'm dubious that it's the MAFs.

I just usually try not to fix the car by throwing sensors at it mindlessly, but this may have to be an exception. I would think, though, that if these two items needed replacing the car wouldn't seemingly run so well.
 
#6 ·
It is clear you need things to be given at least 300 miles min 500 miles for the full effect. From plugging in the old mafs and the code they gave it is clear they are bad. They will have changed all sorts of adaptions. Time and miles will be required to change things back to where they should be.
Fuel trims will make immediate changes but it takes lots of miles for ICV adaptions to change back. The fuel trims will adjust as those adjust. On any given day your fuel trims could be different depending on how the DME is adjusting other things.
The point being, what your trims say may be moot until everything has settled. This further goes to suggest you need to change how you do things. You can't just keep changing things. Change one thing do 300 miles min then change the next thing, even if parts might be bad. Changing many things in fast sequence requires a full adaption reset but causes so many other issues it really is not recommended. Because after something like that you need to watch a bunch of different sensors for the next 50 miles to decide how thing are.

If the VW MAFs are good they will work fine if other things are fine, they will add about 5% to your fuel trims which is well within the window for the DME.
I would suspect you will have to change your CPS but let the car settle and in that time pull one and see if it is the old design or new then decide if you are going to buy 4 or 2, then two again. I do one side at a time that way if one turns out to be new design I don't order it for the other side just reuse it.
 
#7 ·
Ok I didn't realize that it took that much time to adapt. I probably put around 200 miles on the car with the VW MAFs. I'm not sure why, but swapping them for the old units has increased my mileage considerably, and the car no longer reeks like unburned fuel. I feel the VW units were a disaster, but like you said others have had good experiences. Embarrassingly enough, I solved the MAF code issue I got with the old MAFs. The one on the left if facing the engine was unplugged. I'm not sure how I managed that, but I did. What's interesting is the car ran better with only the right one hooked up. The throttle was much smoother on cold start, and the power delivery more linear up to around 30% throttle. Have you ever heard of something like this? I reset the trip computer and will drive for 500 miles before I do anything else.

In any event, I can tell something isn't quite right despite the fact that my codes have gone away. Weather permitting I'll pull a CPS to see what I've got in there this weekend.

Cheers
 
#9 ·
I have the "original" BMW MAFs that we're in the car when I got it. I'm not sure if they were ever replaced by either of the two previous owners. The person I purchased the car from included two new VW MAFs with the car but never installed them. I assumed there was a reason he got them so I put them in as preventative maintenance. After about 200 miles of driving the mileage issue started and I began to worry about the cats. That's when I switched back to the BMW ones that were installed when I got the car.
 
#10 ·
AAs are not STFT, lambda integrator is. AA is something like a medium load fuel trim.

You need to have clean MAFS and O2 sensors. CPS are not consumables but there is an old style and a new style.

I would toss the old MAFS and run the VWs. Clear codes and drive the miles to let the ecu adapt. Recheck fuel trims after 300 miles. The other issue is VANOS. I know you did the maintenance but without a DIS VANOS test you really don't know how good of a job you did. But I would leave that alone for now until you put down some miles.
 
#11 ·
What are AAs?

I have DIS and just attempted to test the vanos at lunch. After driving around I got the unpleasant surprise that my DIS can't connect to the car (Fault 200.159)... so that's great. I'll be battling that later on tonight.

Anyway, I am very hesitant to put the VW MAFs back in the car. This last time the car was practically spewing fuel out of the exhaust, I started getting codes for the cats etc., and I was getting 12.9 mpg. This issue is bugging me, but pales in comparison to having to potentially having to replace a cat. Either way I am 90 miles into the old MAFs and am going to wait 300-400 miles before I do anything else.

To summarize where I'm at:

1) I have about 90 miles on the car since I put the old MAFs back in.
2) The throttle is very non-linear/wild when cold, but somewhat settles out when warm.
3) The performance is not as good as it was with one of the MAFs unplugged and the car running on a table. Power was stronger and smoother.
4) I don't know the condition of the pre or post cat O2 sensors and both are important for proper operation.
5) I have a shadow code for the E box fan. I found during some research, Sailor, that you mentioned they wired in that fan in parallel with the fuel injectors... I ordered a fan from FCP and will replace it tomorrow or Saturday.
6) I don't know what version of CPS I've got, but I have a feeling they are original.
7) I recently did the vanos board maintenance. I can't run the test right now, but I saw somewhere that sailor mentioned it's not necessarily that useful. I feel I did a good job there but it would be nice to run the test anyway. It's worth noting that this driveability issue was exactly the same before I performed the procedure.
8) I also haven't treated the car for carbon, but am planning to do so soon.

So, I think the best thing to do now is:
1) Replace the E box fan.
2) Keep driving to 300-400 miles.
3) Pull a CPS to see what version's in there.
4) Order new CPS two at a time if I've got the old ones.
5) Wiat
5) Drive 500 miles.

What confuses me is that the car ran so well with the left MAF unplugged. Would running the car on those tables cause the car to mask over other issues like bad CPS? It seems to me that unless the MAF was the only faulty item, having them unplugged and the car running off a table would not eliminate the issue.

As always I sincerely appreciate the help. In the meantime I'm searching and consuming older threads.
 
#12 ·
Please look up the EPA spec for mileage on the M5. I don't remember for sure but 12.9 is what it should be. While things are adapting they can go a little rich. It is your car and you are the one with in front of you so likely your feelings will be correct.

What mileage are you getting with the other MAFs? How are you measuring your MPG? If you are using the OBC have you adjusted it for accuracy might be test 19 but look it up in the thread for the tests.
 
#13 ·
IIRC the EPA ratings were 13/21 later revised to 12/20 but I was getting 12 mpg combined with the VW MAFs whereas now I am getting 17 mpg combined with the old MAFs. It's possible that I just wasn't aware of how long it takes the computer to adjust and didn't give them a chance. I could see how if my current MAFs are bad the adaptations could be near their limit thus throwing the car way off with the new ones. I'd be willing to try them again, but I did get two codes for the cats which really concerned me. I could also have overreacted to that though.

I have been using the OBC to monitor the mileage. I haven't heard of the mileage adjustment. I'll look that up.
 
#14 ·
We had this guy Paul who took his mileage that way and it causes many problems because of how it is read. Don't do it. Fill up the car write down your mileage then drive and fill up again note the mileage and do the math repeat. If your trims are not in the middle then something weird happens to the OBC readout. I can't explain what goes wrong but something does go horribly wrong.

That said what is your split on highway to city? To actually get 17 you would have to be 90% highway for miles time wise that maybe 50/50. If the miles of each were 50% then I would expect you to be in the bottom 1/3 of the range 12-15. Depends on how you drive. I am bad on gas and only get 12-13 liters per 100km on the highway and as high as 20 l/100km when stuck in town. I have no idea how that converts to US gallons and miles.

It is hard to know what you are smelling or how you feel it is rich and you might be 100% right, I am just throwing things out there for you to decide.
 
#19 ·
Honestly reading what you guys are saying I could just be having a youth moment where I was being presumptuous. If the OBC isn't a good indication of what I'm getting than my assumptions about the fuel economy are probably baseless. I will say that there was a faint smell coming from the end of the car that seemed like unburned fuel, and it did throw a code for the cat which freaked me out. That being said, I didn't give the car 500 let alone 300 miles to adapt. I am now up to about 160 miles since I put the old MAFs back in. I am going to monitor the fuel mileage with a calculator as you suggested so I've got a true baseline for when I make the next move.

Which MAFS were you using for the analog readings on post #3? The HLM of 16 is good and tells me those MAFS are OK. I had a reading over 20 with old dirty MAFS, dropped below 20 with new ones.
The MAFs used for those readings were the VW MAFs. So now there's some more evidence they might not be an issue after all. What does HLM stand for?

Imo...

Genuine mafs, new cps and pre o2 sensors along with an adaptations reset should restore performance assuming you don't have off spec cam timing and or Vanos solenoid issues. Let it readapt and re check the knock adaptation values after some time. If these are high then the dme is making adjustments to prevent knock = less torque, possibly caused by carbon build up in the combustion chambers.

Fwiw I had an ebox fan fault and it didn't make any difference to performance after replacing it.
Which adaptation is for knock?

Ebox fan failures have many faces. Some have no symptoms because the fan is two speed. If the high speed fails there is no issue, but if the fan loses low speed or both speeds then there are issues with trims and idle and many other things. It is very well documented on the board. Why has not been well defined but none the less some ebox failures come with many symptoms.
That's exactly what I remember reading, from another one of your posts no less. The fan will be here today so I can at least rule that out quickly.


I'm thinking that at this point, knowing what I know now about the necessary mileage for the adaptations and the fuel readings on the OBC, I think I agree with the suggestions that I put the VW MAFs back in once I get to 300 miles again. This time I will have calculated the mileage myself and I will be able to more accurately gauge the change in economy.
 
#16 ·
Imo...

Genuine mafs, new cps and pre o2 sensors along with an adaptations reset should restore performance assuming you don't have off spec cam timing and or Vanos solenoid issues. Let it readapt and re check the knock adaptation values after some time. If these are high then the dme is making adjustments to prevent knock = less torque, possibly caused by carbon build up in the combustion chambers.

Fwiw i had an ebox fan fault and it didn't make any difference to performance after replacing it.
 
#18 ·
Ebox fan failures have many faces. Some have no symptoms because the fan is two speed. If the high speed fails there is no issue, but if the fan loses low speed or both speeds then there are issues with trims and idle and many other things. It is very well documented on the board. Why has not been well defined but none the less some ebox failures come with many symptoms.
 
#21 ·
I managed to get DIS to connect to the car last night which was awesome. I'm going to have to familiarize myself with the program. We got some snow here and it looks like we'll be getting more on Tuesday, so unfortunately that means I won't be able to drive the car for a week or so. I'll report back once I hit the 300 mile mark.
 
#22 · (Edited)
The snow finally melted away and I was able to complete the 300 miles. The performance is right back to where I started. The car surges at low RPM and generally feels slow and nonlinear. I’ve done the following now:

1) I was able to do a VANOS test with DIS and neither side passed. I took pictures of the results. I pulled the covers today after work and both were spotless underneath. FWIW I'm still not sure if I have the old CPS so it could be meaningless. If legitimate, could this be contributing to the problem?

2) I replaced the E-box fan and haven't seen an improvement. Good to have out of the way.

3) A few days ago I got an SES light and the codes were B2, B3, and AE. B2 and B3 are CAT efficiency codes and AE is air fuel efficiency cyl. 1-4. B2 and B3 suggest that the post CAT O2s are getting readings that aren't acceptable, and AE is telling me the car has adjusted the STFTs to their limits. I cleared the codes after I replaced the E-box fan and none of them returned for the last 50 or so miles.

4) Today having hit 300 miles I put the VW MAFs back in the car. The car feels a lot better with these units. It feels much more on the boil, and the car is definitely faster. However, seven miles into a drive the SES light came on. It was the B2 cat efficiency code again. The STFTs went up from 32,23 to 35,26 on this drive.

5) The upper oil separator hoses are very soft. I pulled the plenum cover off and took some photos. There isn’t a lot of oil in the plenum. The end of the hose has some oil in it but it didn’t seem to spill over into the plenum. I checked the intakes and air boxes and there wasn’t any oil in either. My oil cap isn’t leaking either.

I saw in an old thread that oil going into the engine can lower the octane of the fuel mixture. I was thinking that maybe oil making it into the plenum could cause the car to compensate for detonation and run rich, but it doesn’t look like I have badly working oil separators. I also don’t believe the VW MAFs are bad. The car runs much better with them installed.

The next thing to do is check to see what CPS I have, and I’m going to try and put some mileage on the car in the next few days.
 

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#24 · (Edited)
He has done that. Plus bank 1 where you are having fuel trim issues, only fails the VANOS test by a simdge. I don't think that is causing your problems. If you are not getting AF code also, you may have a bank issue like air leak or dirty injectors. Again, you keep saying STFT when it is your AAs that are out, not lambda integrators.
I would like to see a video of analog 2, about 30 seconds at idle, then keep filming while you accelerate evenly up to 3000 rpms and hold it for another 30 seconds.

And that amount of oil in the plenum is normal.
 
#25 · (Edited)
I would agree you need to do the noids again. Sometimes it takes people two or three tries to get them clean.

The oil looks like it might be significant but it is hard to tell because I see both things that are telltale signs. Stains is a good sign because it means the oil is drying out so the oil is limited. But I also see that the plenum looks like it is glistening, that is a bad sign. If there is a coat of oil on everything then you may have a return issue IE the tubes, but more than likely it means you have a crankcase air leak. Air leaks in the case cause the codes you are getting but normally we would see AF with the AE. A large volume of oil will cause the B2/3 codes.

My guess is you have multiple problems all from a lack of previous maintenance. These cars adapt very well so often it is the forth problem that breaks the camels back.

I would clean the noids again but use a compressor this time if you did not last time and listen closely to how each clicks. The sharpness of the close is the subtle sound to listen for, so hold it open for a sec then let it close and listen intently then. This is likely not the issue but you must have the VANOs working well to find and solve the other issues.

I am guessing until you describe the plenum condition better. I think you likely have a crankcase leak which allows un-metered air in and screws the MAF reading. Because you have one bank more significant than the other you may have a bad injector seal or dirty injector, something that effects one bank more than others.

I would do the VANOs then do an engine air test. In the early days I came up with a way to idle the car and block the incoming air to determine how big the leak is but that does not help find the leak, just tells you how urgently you need to find it. I came up with a much better way to test and have posted it many times but always in threads like these so I have no way to tell you where it is.

So here we go again, I should do a thread for the DIY and might, so I will give you the short version. You need 4 caps and clamps to seal the exhaust pipes. I have a Tubi but I got 4 plastic test caps that fit inside drain plumbing, that slid over my tailpipes perfect and clamped them tight. If the caps are to big then weather stripping might make up the difference. You will have to be creative.

Next you need to block off the intake air. This is simple if you have the tools. Cut two pieces of plywood/MDF or other stiff sheet stock the same size as the air filters. Add some foam weather stripping and drop those in the place of your air filters.

Now you need to add air pressure, you will start with one or two psi as you proceed you might go to 6 psi but not above. Really the engine needs to be tested for vacuum but you don't have a vacuum pump so pressure will have to do. To add the pressure it depends on your set up. I have done it through the dipstick, cut a vac line and added a "t", but on my M5 I drilled one of the exhaust caps and added the fittings to that. How is unimportant, but it must not have any leaks of its own.

If the leak turns out to be under the plenum that will take a few paragraphs so by then maybe I will written a full DIY so I can point you there. Normally what happens is I explain how to do it and the person doing it takes the time to do the DIY.
 
#26 ·
I need to explain the crankcase air leak a bit better. Not only is it un-metered air, but it adds air flow into the crank which grabs a ton of oil and overwhelms the separators even if they are in perfect working order. So replacing the hoses is last on the list not first. It could be while you are in there because all cars need that anyway.
 
#27 ·
I took the plenum off and took a video. I think that it shows the oil better. After reading Sailor’s post I think that it’s significant.

*Note, I apologize ahead of time for filming this in portrait. I was putting the plenum back on when I realized it. I also wasn't able to get video to embed so here is a link.

https://youtu.be/zL1Ap4BYt0w

So I am going to do the test as Sailor suggests. I think I understand how it works but I have a few questions:

1) Rather than plug up the airboxes can I just take off the MAFs and plug up the intake tubing? That seems like it would be easier.

2) Is there a reason you stopped using the dipstick tube to pressurize the engine? I think this would be the easiest method for me.

3) What am I going to be looking for? A sound? It doesn’t intuitively seem like 5 psi is going to be enough to cause a sound.

I also took this video of the Analog 2 output as xrviz outlined. The AAs are continuing to increase. I have about 60 miles with the VW MAFs now.

https://youtu.be/yO8HhkFCIfw
 
#28 ·
Oil in the plenum looks slightly more than mine, still seems minor to me.
Fuel trims look good other than the AAs. Not sure what that means, never seen that before. But if you had a significant vacuum leak your STFTs/LIs should be high at idle then decrease with higher rpms. Dirty fuel injectors should be affecting your MAs not AAs.
The only thing I can come up with is the VW MAFS and old style CPSs as being the potential problem.
It might be worth clearing adaptations to reset the AAs back to 0 and then recheck analog 2 after a few drive cycles.
 
#29 ·
I stopped using the dipstick likely because I started using a vacuum pump and was pulling oil up the tube into my pump. Don't really remember though. The dipstick tube will work well. If you can plug the MAF hole that will work except having the MAFs in place is better so they get tested but it is unlikely they will leak.

You will hear a leak at 1 psi you may need to go to 5 only to hear it really leak. Depends how quiet things are, and how big a leak. The bigger the leak the less likely you are to hear it so watch the pressure gauge for fall. The faster it falls the more pressure you can add, but if the leak is big you will still only have 1 psi and a ton of air. This creates the problem of the compressor turning on and preventing you hearing anything.
 
#31 · (Edited)
Ok I will try to rig something up this weekend. I actually haven't pulled the plenum in the time that I've owned the car. So I will do the test and then maybe pull it to see if anything jumps out at me. I've seen a lot of threads where the injector seals are brought up and it seems like no one really looks into that. Some of these threads don't have a resolution either. Whatever combination of small leaks I have, I can tell that it's slightly worse on the left side (bank 1, correct?), so I will make sure I inspect them and likely just replace them.

Without the test I can see that at the bare minimum it looks like I need new grommets for the valve cover.
 
#32 ·
You likely have a couple of leaks. Injector seals would be a good guess. AAs are affected most when the ICV is controlling the air so less than 2000 rpm.

If you need new grommets then you need new valve cover gaskets. The grommets start leaking when the valve cover gaskets get old shrink/become hard and apply no pressure to the grommets. The studs are a fixed length and without pressure pushing up from the valve cover gasket they leak. I learned that 15 years ago on E34s. Installed a new grommet only to have it leak too.
 
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