Fuel Trim

Kenya

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May 12, 2021
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Dear friends ,
I checked a fuel trim by oBD and the result is as follows. Can you help me to analyze the results?

Firstly, I checked O2 sensor voltage and it looks ok.

When a cold engine, STFT is over +10% but after some time, STFT will be around 0% at the idle but it will go up to +15% more if over 2500 rpm.

LTFT is a negative 24%. This is the result after I drive 500km on the above condition.

air flow is 2.7-3g per second

Recently, I feel less power over 3-4000 Rpm. Mass Air Flow Sensor is replaced recently


I appreciate any advice.

Regards
Kenny
 

colb

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British Zeds
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Location
Newport, South Wales,UK
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Z3 M43 1.8 (1999) and Z4 E85 2.5 (2003)
Firstly was the replacement Maf an oem part such as Bosch or Seimens, cheap pattern parts are a gamble, they seldom work correctly or not at all, been there done that paid twice, lesson learnt. Lack of power is a symptom of a bad Maf in my experience, feels like the car is being held back.
Sounds like you have a scanner capable of looking at live data so take another look at the O2 sensors in graph form if your scanner supports that. You should see the pre cat sensor(s) producing a wave form as the voltage ranges up and down the scale, this proves they are working. Wait until the car is warmed up to view these graphs. The post cat sensors should be showing a pretty static voltage unless they are picking up unburnt fuel in the gas.
Turning to fuel trims you should be seeing as near to 0% on the scale at idle as the engine management should be controlling the mixture the engine is getting taking into consideration what the Maf is telling it as to the amount of air that has passed through it and comparing it to what the exhaust sensor is seeing. If it sees more air it will try and compensate for what it sees as a lean mixture and command the injectors to put more fuel in. It will continue to do this until it reaches around 20% more fuel being added then give up and set the EML on the dash to draw attention to the problem.
The most common cause of high fuel trims is usually air leaks from holes or splits in the cars vaccum system pipes and their joints, a smoke test on the system is the best way of checking the system if you cant find any obvious split or holed rubbers.
 

Kenya

Regular Member
Joined
May 12, 2021
Points
21
 
Firstly was the replacement Maf an oem part such as Bosch or Seimens, cheap pattern parts are a gamble, they seldom work correctly or not at all, been there done that paid twice, lesson learnt. Lack of power is a symptom of a bad Maf in my experience, feels like the car is being held back.
Sounds like you have a scanner capable of looking at live data so take another look at the O2 sensors in graph form if your scanner supports that. You should see the pre cat sensor(s) producing a wave form as the voltage ranges up and down the scale, this proves they are working. Wait until the car is warmed up to view these graphs. The post cat sensors should be showing a pretty static voltage unless they are picking up unburnt fuel in the gas.
Turning to fuel trims you should be seeing as near to 0% on the scale at idle as the engine management should be controlling the mixture the engine is getting taking into consideration what the Maf is telling it as to the amount of air that has passed through it and comparing it to what the exhaust sensor is seeing. If it sees more air it will try and compensate for what it sees as a lean mixture and command the injectors to put more fuel in. It will continue to do this until it reaches around 20% more fuel being added then give up and set the EML on the dash to draw attention to the problem.
The most common cause of high fuel trims is usually air leaks from holes or splits in the cars vaccum system pipes and their joints, a smoke test on the system is the best way of checking the system if you cant find any obvious split or holed rubbers.

Thanks for your advice! I will check the MAF maker first. Can I ask 2 questions? STFT should be always 0%? In my case, over 2500RPM, I see a higher STFT % and that is why you suggest a smoke test?

secondly, what indicate bt a negative 24% LTFT? Any action is required on this?

regards
Kenny
 

colb

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British Zeds
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Points
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Location
Newport, South Wales,UK
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Z3 M43 1.8 (1999) and Z4 E85 2.5 (2003)
Yes smoke test to check for air leaks should be your first move.
Negative Long Term will be the Engine Management decreasing the fuel at the injectors to lean the mixture.
A good read for an explanation on fuel trims here, worth reading to get your head around it all
 

colb

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British Zeds
Joined
Nov 25, 2012
Points
178
Location
Newport, South Wales,UK
Model of Z
Z3 M43 1.8 (1999) and Z4 E85 2.5 (2003)
When it comes to smoke testing for air leaks its not just the obvious places to check out for escaping smoke, close attention to some of the lesser known parts of the engine also need a good look for leaks. Often overlooked are the dipstick tube which has O rings in the top, they are known to degrade and allow air in. Close attention should also be made to the plastic cam cover, these go hard with age and go brittle and start breaking up, had a 2.2 that suffered from this around No4 plug well, only smoke test showed that up. Solved that with a good used cover being fitted. At the back side of the air intake manifold there are a number of rubber blanking plugs used to seal stubs that are not used in the system, these again degrade and split eventually falling off and allowing unmetered air to get in. Check if yours are intact and sealing ok.
 

NZ00Z3

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Which engine do you have in your Z3? You are only giving us one set of fuel trims (bank), so do you have a 1.9L?

Your Z3 is running seriously rich (-24% LTFT). There is a problem (or 2) to be found.

Total fuel trim = STFT + LTFT.
Cold Idle = 10% -24% = -14%
Hot Idle = 0% - 24% = -24%
>2,500 rpm = 15% - 24% = -9%

Rich conditions are caused by:
  • Too much fuel. High fuel pressure, leaking injectors etc.
  • Not enough Air. Blocked air filter or air ducts. Poor/dirty idle control valve/throttle body etc
  • A sensor fooling the DME into thinking that one of the above is happening. Pre-Cat O2 sensors, MAF etc
  • Blocked Cats. Remove pre-cat O2 sensor from the exhaust and go for a drive. Engine will be very loud. If performance returns, then you have a blocked cat.

If you get totally stuck, Set up the OBD Fusion smart phone app as per this thread. Collect the 3 standard logs and post links to them here. I'll look at them and let you know what I can see. I do this over the internet engine diagnostics all the time for the E46 crowd. https://www.e46fanatics.com/threads/obd-fusion.1240733/#post-18380161
 

Kenya

Regular Member
Joined
May 12, 2021
Points
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Which engine do you have in your Z3? You are only giving us one set of fuel trims (bank), so do you have a 1.9L?

Your Z3 is running seriously rich (-24% LTFT). There is a problem (or 2) to be found.

Total fuel trim = STFT + LTFT.
Cold Idle = 10% -24% = -14%
Hot Idle = 0% - 24% = -24%
>2,500 rpm = 15% - 24% = -9%

Rich conditions are caused by:
  • Too much fuel. High fuel pressure, leaking injectors etc.
  • Not enough Air. Blocked air filter or air ducts. Poor/dirty idle control valve/throttle body etc
  • A sensor fooling the DME into thinking that one of the above is happening. Pre-Cat O2 sensors, MAF etc
  • Blocked Cats. Remove pre-cat O2 sensor from the exhaust and go for a drive. Engine will be very loud. If performance returns, then you have a blocked cat.

If you get totally stuck, Set up the OBD Fusion smart phone app as per this thread. Collect the 3 standard logs and post links to them here. I'll look at them and let you know what I can see. I do this over the internet engine diagnostics all the time for the E46 crowd. https://www.e46fanatics.com/threads/obd-fusion.1240733/#post-18380161
Thanks for your advice! Yes, mine is 1.9L. I am going to do a smoke test first to check the air leak. I also plan an injector replacement later. I see the total fuel trim %. It should be always 0% ideally?
While the ST is 0% or positive , how the long term fuel trim can be a high negative?

regards,
Kenny
 

Kenya

Regular Member
Joined
May 12, 2021
Points
21
When it comes to smoke testing for air leaks its not just the obvious places to check out for escaping smoke, close attention to some of the lesser known parts of the engine also need a good look for leaks. Often overlooked are the dipstick tube which has O rings in the top, they are known to degrade and allow air in. Close attention should also be made to the plastic cam cover, these go hard with age and go brittle and start breaking up, had a 2.2 that suffered from this around No4 plug well, only smoke test showed that up. Solved that with a good used cover being fitted. At the back side of the air intake manifold there are a number of rubber blanking plugs used to seal stubs that are not used in the system, these again degrade and split eventually falling off and allowing unmetered air to get in. Check if yours are intact and sealing ok.
Thanks for your detailed advice. I will try to do a smoke test.
Regards
Kenny
 

NZ00Z3

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In the BMW world, LTFT of +/-4% are great, +/- 6% OK, +/- 8% and you have problems to find before they leave stranded on the side of the road.
 

Kenya

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Points
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In the BMW world, LTFT of +/-4% are great, +/- 6% OK, +/- 8% and you have problems to find before they leave stranded on the side of the road.
In the BMW world, LTFT of +/-4% are great, +/- 6% OK, +/- 8% and you have problems to find before they leave stranded on the side of the road.
Thanks for your advice. Then, can I assume the LTFT will be a better ratio after I drive more? The problem I have to fix is the STFT over Idle rpm? The other problem why I checked the Fuel trim is my engine slightly shake at a lower idle. My manual sift started to shake a bit after warming up.
Regards
Kenny
 

NZ00Z3

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Your engine is getting too much fuel over all rev ranges.

Start with Fuel:
  • What is your fuel pressure at the Schrader vale on the fuel rail
    • With the key in position 2 and the engine not running? The prime test.
    • With the engine running at idle? The running test.
    • After 20 minutes since you turned the engine off? The let-down test that looks at either injector leaks or loss of pressure through the pump/regulator.
 

Kenya

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Thanks for your detailed advice. I will try to do a smoke test.
Regards
Kenny
I am able to do a simple smoke test and found a leak from the end of the intake boot. The boot seems not so old and tightly fastened. Is there any way to stop leaking before replacing to new boot?

Regards
Kenya
 

colb

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The intake boot(s) are in two parts with a plastic joining ring between them, the lower one has a flap on its end that should fit in the corresponding cut out on the throttle body so it is fitted in the correct angle to join up with the plastic ring and upper boot. All ends of the boots are secured with jubeleee clips to provide the airtight seal they need. Make sure they are all in the correct position if you are getting a leak at the joints then they are not seated correctly or not clamped up tight enough. If there are any cracks or splits then the only option is to replace them with new ones.
 

Kenya

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The intake boot(s) are in two parts with a plastic joining ring between them, the lower one has a flap on its end that should fit in the corresponding cut out on the throttle body so it is fitted in the correct angle to join up with the plastic ring and upper boot. All ends of the boots are secured with jubeleee clips to provide the airtight seal they need. Make sure they are all in the correct position if you are getting a leak at the joints then they are not seated correctly or not clamped up tight enough. If there are any cracks or splits then the only option is to replace them with new ones.
Thanks. You are right. After I fit the intake boot, the leak was stopped. However, I also found a leak around PCV valve. Is it a serious that I should fix?
Regards
Kenya
 

colb

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Yes any leaks should be fixed, these engines require a totaly sealed vaccum system otherwise it will affect the correct fueling and cause running problems. The age our cars are the rubber pipes and plastic pipes componants are all showing age and heat cycle degredation, these parts need to be in good condition change any you find holed or split.
 
Last edited:

Kenya

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Your engine is getting too much fuel over all rev ranges.

Start with Fuel:
  • What is your fuel pressure at the Schrader vale on the fuel rail
    • With the key in position 2 and the engine not running? The prime test.
    • With the engine running at idle? The running test.
    • After 20 minutes since you turned the engine off? The let-down test that looks at either injector leaks or loss of pressure through the pump/regulator.
Thanks for your advice. I bought the pressure gauge but it is to ckeck the pressure of combustion chamber and did not hit to the Schrader valve. I tried to search the adopter but can not find it. Is it a special adopter that I have to order to BMW?

regards,
Kenya
 

colb

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Newport, South Wales,UK
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Z3 M43 1.8 (1999) and Z4 E85 2.5 (2003)
You bought a compression tester for the engine cylinders, you need a fuel pressure test kit that connects to the shrader valve on the fuel rail. Kits on ebay at various prices, most are from China so delivery can be long.
 

Kenya

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You bought a compression tester for the engine cylinders, you need a fuel pressure test kit that connects to the shrader valve on the fuel rail. Kits on ebay at various prices, most are from China so delivery can be long.
Thanks very much for your advice. I could order the correct tester.
Regards
Kenya
 

NZ00Z3

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Make sure that the tester has a Schrader valve adapter. Many do not have this adapter. You will need it to connect to the fuel rail.
 

Kenya

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Make sure that the tester has a Schrader valve adapter. Many do not have this adapter. You will need it to connect to the fuel rail.
While I am waiting for the pressure gauge, I proceeded the fuel injector replacement but the result is not a stable idle and after OBD check, I noticed the o2 sensor did not work as it show the last voltage and no movement. The short term trim remains 0%. Did the injector replacement cause a o2 sensor issue?

I would appreciate any advice.

Regards,
 
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