E46 Fanatics Forum banner
Status
Not open for further replies.

Negative fuel trims

1 reading
20K views 38 replies 11 participants last post by  doukas007  
#1 ·
Hello, long time I've encountered this problem, so I thought I should ask for help and ideas. My car is a 2001 320Ci with the m54b22 engine that has the EU2 tune (post cat sensors deleted). The problem is I have negative STFTs all the time, except for when really stepping on it. Strange thing is that my LTFTs are perfectly normal. Maintenance to the car is quite thorough.
Both pre-cat O2 sensors were changed with new Bosch, fuel pump with Siemens VDO, fuel filter with Mann, boots, DISA o-ring changed, DISA rebuilt, VCG changed, car runs fine, no codes. I cannot find any vacuum leaks. The only symptom I have is that if I floor it from low RPM in 3rd or 4th gear, it sometimes bogs, like cutting throttle. MAF sensor is original, but car has 100k miles and it does the same even with MAF sensor unplugged. I checked fuel pressure and it starts at 49 PSI, then drops to 30 PSI. I suspect that's a problem, but gauge drops even if I move it so i don’t know. Also, I checked injectors for dripping, I got none.
It had negative STFTs even before the fuel pump and fuel filter change.
I swapped injectors of bank 2 (which seems to be the problematic bank), with some used ones I had, but STFTs remain negative most of the time. Car starts and runs fine, I maybe have a vacuum leak somewhere but I can't find it. I smoke tested it, and nothing, I also pressurized the intake, as much as I could with my mouth though, it holds pressure.
I have attached a link to some OBD fusion logs, per jjoj’s instructions.

 
#2 ·
Negative fuel trims would indicate a rich mixture which would not be due to a vacuum leak.

Here's some info I found online that may be helpful:

"Below are some tips and tricks to diagnose the most common faults /defects / failures /malfunctions that cause rich air/fuel mixtures that result in high negative fuel trim values-

-Excessive fuel pressure; test fuel pressure with a dedicated fuel pressure gauge
-Leaking fuel injectors; test injector operation by performing a spray pattern and volume test
-Defective MAF sensors can under report the volume of air passing over them, thereby causing rich running conditions
-Exhaust leaks; repair all leaks found since exhaust gas contaminates the ambient air that oxygen sensors use for reference purposes
-Loss of some compression on one or more cylinders; loss of compression causes poor or incomplete combustion, which results in unburned hydrocarbons “fooling” oxygen sensors into reporting a rich mixture
-Marginal misfire; not all misfires are bad enough to set a misfire code, so check or replace suspect spark plugs, and check the operation of all ignition coils with an oscilloscope
-Marginally defective oxygen-, or air/fuel ratio sensor(s); sensors may be stuck at reading rich conditions as a result of not reacting to switching signals from the ECU, or may be slow to respond to switching signals"
 
#7 ·
-Defective MAF sensors can under report the volume of air passing over them, thereby causing rich running conditions
Good advice on everything, except one correction: an over reporting MAF sensor causes rich condition.
An under reporting MAF sensor causes lean condition.
 
#3 ·
You sure you used the right ZUSB (assembly number) for your size engine when programming the euro tune update?

Do you know the old ZUSB and new?
I can check they are correct for you.

Sent from my Redmi Note 7 Pro using Tapatalk
 
#4 ·
thanks for the suggestions but I've done nearly all of that ;
fuel pressure residual seems low , but i didn't notice any fuel leak , priming fuel pressure is fine .
didn't notice fuel injectors leaking and swapped some of them in the seemingly affected bank , didn't change a thing .
maf isn't the problem ,same with maf unplugged ,also not that old.
exhaust leaks , car has full aftermarket exhaust and headers , but i don't think there is a leak somewhere ,also if I remember right i had the same behaviour even before . loss of compression ,thats a little far fetched , car runs healthy and has less than 100k miles.
spark plugs new and not even a code ,O2 sensors brand new .also the one bank reads slightly different than the other
 
#12 · (Edited)
Cold 5 min idle
I uploaded the cold start idle log to datazap for viewing/sharing. I think we should use this from now on, such a useful (and free!) graphing tool.

So, looking at the data most looks ok.
But the short term fuel trims steadily go up indicating lean condition.
The pre-cat O2 sensor voltages are a bit odd as they first go up before switching. The typical graphs I have seen show their voltages go down until switching begins.
919980

919979

Image
 
#13 ·
yeah I know that trims look mostly okay on these logs , but when I stab the throttle , stfts go wayyy negative ,like -17% and dropping .Also , rarely but noticably ,car bogs when stepping on it from low rpm and a long gear.Also on some uphills I am noticing some loss of torque at low rpm Weird also Ive noticed the ltfs never change much and sometimes become sligtly positive , like 3% not more.I ll have the cruising log soon enough too .Im suspecting I have two or more issues that I cant find despite how much I try
 
#14 · (Edited)
Rich conditions are caused by either too much fuel or not enough air. As your LTFT's are within +/- 3% the car is running well. The rich condition is not around long enough to push the LTFT's rich. So, the problem must be specific to some conditions.

You see increasing rich conditions on the STFT's when you accelerate hard at lower speeds and have poor/bogged performance when this occurs.

A dirty air filter would have more restrictions at high flows/rev's, so it not likely a dirty air filter.

The Idle control valve is the device that controls the air flow into the engine from idle and stops its influence at about 2,500 to 3,00 rpm. The throttle body starts its influence from around 1,500 rpm upwards. Do you have a dirty Idle control valve that is gummed up at low speeds? Most likely not as the MAF would see the reduction in air flow.

Looking at the hot idle log, your MAF starts at 3.29 g/s which is about right, then drops down to 2.87 g/s, which is a little low. Makes me think that the MAF is the issue. You have unplugged the MAF and still had the same problems. With the MAF unplugged the DME runs on internal fueling tables. The 02's are not used as feedback as the DME is in open loop control. These internal fueling tables are a little rich compared to normal running. So the fact that you are still having the same problems with the MAF unplugged, could the cause be that your MAF is over reporting at low rev's?
 
#15 ·
I appreciate the lengthy response and what you say makes sense .Although there are some details that trouble me .Firstly , the two banks are not showing the same numbers exactly ,bank 2 I think is worse .Secondly ,Mafs usually under report and this maf is original and also not that old , but okay it may have been disintegrated a bit. Also , when I try to let's say , confuse the engine and stall it , by blipping the throttle and when rpms drop blipping it again repeatedly , stfts will go like -26% negative and I don't think any maf would show that low .
I will keep an eye on maf though , but I dont want to throw any more money to the problem if im not positive
I believe I have more than one issues
 
#16 ·
Blipping the throttle is expected to cause rich condition. Repeated blipping will cause negative STFT until the extra fuel clears out. This does not prove anything other than O2 are indeed responding to normal acceleration enrichement.

Full exhaust, but replacing a 20yo original (not old though?) MAF costs too much? Remanufactured units shouldn't cost too much. 80USD in the States.

What is the intake like, factory? Odd flow patterns will mess with MAF readings. Check the airbox?

Swap precat O2 sensors to see if the minor unbalance follows.

There is also a lot of contradiction in what you describe and logs. Post #1 STFT neg all the time. Post #16 STFT neg when stabbing throttle. Logs shows pos and neg STFT, basically normal looking. After WOT again due to fairly rich expected mixture some neg STFT as it returns to normal. Much like you get some positive after coasting on fuel cut. Logging through OBD sucks for transient conditions. It's very slow so temporary issues like bad transient MAF readings are almost impossible to see.

STFT swings of ±12% are rather large, are you sure the O2 are genuine Bosch?
 
#17 ·
You see why I think there is a problem. Yeah I'm sure O2 sensors are genuine, they came in sealed boxes with a seal and a code that I checked to verify it's new. First time I hear about a remanufactured maf and also I don't see the point if I'm not sure. O2 sensors cost me already 120$ and a new VDO pump another $200 .Car yeah seems fine on idle but there is definitely a problem. Also if I blip the throttle I am not getting rich stfts as you say, but progressive negative. Also, one bank has different values from the other, that's not suggesting Maf. I believe I have an exhaust leak and a slight vacuum leak too that I can't find.
 
#18 ·
Negative STFT indicates excessive fueling. A fundamental understanding of what's happening is required.

O2, fuel pump, and MAF at this age and mileage are essentially preventative maintenance. This is not wasted money.

Remanufactured MAF have replaced the heated wheatstone elements. You can buy such elements to try to do this yourself ( a few have). I didn't have a spare MAF to play with so didn't bother for the small difference to a reman MAF. Eventually the heated elements burn up or get deposits and give faulty readings.
 
#22 ·
Strange thing is that my LTFTs are perfectly normal.
That's all you need to worry about. Short terms are going to be all over the place depending on how cold it is, how you hit the gas when you start it up, accelerate up to normal speed, how long the trip is and all that stuff. The STFTs are just a snapshot of the current tip. If all the trips combined average out to 0, or near it, then I woudn't worry about it. That's what the LTFTs are. You have a problem when the LTFTs start to creep ever further away from 0 in either direction + or -. Then you're looking at problems.
 
#23 ·
I know that the Ltfts are the most significant and the ones that throw codes after a threshold. But there are a couple of things. Car has symptoms, not severe but mild okay, but there is certainly a problem. Also yeah, stfts fluctuate, but for example at idle they are becoming slightly positive steadily, as Balidawg noticed, and I have been witnessed them to go really negative, like -20 % , which I don't think is normal really. On some cars I've monitored, stfts stay around zero most of the time . And also, when I stab on the throttle, there is a whoosh sound, strange as I can't find any vacuum leak, neither with smoke nor by blowing into the intake. I will have to pressurize it more I guess. Also, I need to check the exhaust for leaks.On top of that, one bank looks slightly different than the other.
 
#25 ·
I'm having similar issues as well. The main issue for me was high LTFTs that was an over pressurized system. However, I'm also having fun with rich fuel trims. One thing I would point out is that unplugging the maf should cause the LTFTs to go super rich, probably around -10%. That's what I got, and my car hated it, hesitation near vanos activation and other such WOT issues.

However, now that I have fixed my fuel pressure issues the car idles near perfectly with near 0% LTFT. The 49psi is in spec for the running pressure, but the residual pressure actually causes me some concern. It should stay above 42 to 43 psi no matter what, which means you may have a bad check valve somewhere.

Another suggestion I would have is checking the DISA operation, that wooshing sound may be failing of that flap to open properly which would reduce airflow and probably cause a rich condition.
 
#26 ·
Residual pressure is something that bothers me, but I can't find a leak in fuel and also fpr with filter and pump is new. I have a suspicion that the gauge was ultra sensitive, when I moved it slightly pressure dropped and I don't know what to suppose. I'll book the car to a garage to check pressure exactly after pump, after filter and also to check for leaks of gas. I wouldn't say though that the car hesitates to start
 
#31 ·
Okay I see, good investigation you doing here. So, I'll log maf values and rpm although I tried to be quite accurate but okay. I suppose my maf won't be pretty linear cause it's not brand new, but my stfts are slightly different from bank to bank. Also, hope you get a new VDO maf soon to see it plotted, just to be sure
 
#37 ·
I reused the old gaskets because the ones supplied with the kit didn't look very inspiring. And I sense a faint exhaust smell in the engine bay although I thought this was normal . I used a piece of hose to try to locate a possible source but no joy. I'll pressurize the exhaust with a vaccum machine and see.
 
#38 ·
I fought negative fuel trims on my M54 X5 for a few months. Checked fuel pressure first. Replaced pre cat O2 sensors and MAF with new parts with zero results. Still throwing the CEL with codes P0172 and P0175 coming back. Reset all adaptations with INPA and still no love. Went after wiring connections on the MAF and DME connectors. Unplugged the connectors and inspected pins. Tightened them as best I could to close the gap on the terminals and fuel trim errors went away and LTFT went to 0 and STFT when to under 2%. Not sure this will help you but it may be worth a try, costs absolutely nothing.

Had the same issue on my thermostat heating connector. Code P1620 Map Cooling Thermostat Control Circuit Signal High popped up at 190,000 miles. I had replaced the thermostat and housing with connector a little over 2 years ago so I doubted the Genuine BMW unit had failed so fast. Disconnected the connector and the terminals were very spread and loose. Tightened those the same way and problem solved. Check your looms and connections @doukas007 when everything else obvious has been checked and validated.
 
Status
Not open for further replies.
You have insufficient privileges to reply here.