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    Thread: Fuel Trim Information

    1. Forum Sponsor Andy@Ross-Tech.com's Avatar
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      01-20-2004 06:17 PM #1
      Some of the most common fault codes (DTC's) pertain to fuel trim. Here is an explanation of fuel trim and what it does for us.
      The ECU controls Air/Fuel mixture in order to maintain power, efficiency, and emissions. A/F is expressed as either a ratio (14.7:1 for example) or as a Lambda value. With iso-octane ("ideal" gasoline), Lambda of 1.0 is equal to 14.7:1 A/F. This is known as "Stoichiometric", a condition where there is a perfect balance between oxygen molecules and the various hydrogen and carbon based molecules in petroleum. With the oxygenated gasoline that most of us use, actual A/F ratio of 15:1 is closer to stoichiometric.
      If Lambda is greater than 1.0, then there is a surplus of air and the engine is running lean. If Lambda is less than 1.0, then there is a surplus of fuel and the engine is running rich. It should be noted that the ratios are mass-based, not volume-based.
      So, why don't we always run at 1.0 all the time? Well, we do MOST of the time. At cruise and idle, mixture is held tightly to 1.0 to keep the catalytic convertor at optimal efficiency, so the emissions are minimized. However, when we need acceleration, the mixture gets richer. Why? Maximum power is made between 0.85 to 0.95 Lambda (12.5 to 14.0 A/F with iso-octane). So, under acceleration, mixtures get richer. Sometimes you want to get even richer under acceleration to keep detonation (pre-ignition of the mixture from excess cylinder temperatures) away. The 1.8T, for example, has a relatively high compression ratio for a turbocharged engine, which especially under lots of boost, is very succeptible to detonation).
      So, now that we know that the ECU wants to be able to control the A/F ratio. It has a prescribed set of values (maps) for a given RPM, Load, etc. So, the ECU tells the injectors to pulse for exactly XX.X milliseconds and that SHOULD get us the proper A/F ratio that we want. Well, if you tell an employee to go do something, you want to make sure they actually did it, right? The ECU has some snitches (the front O2 sensor and the MAF, for the most part) that will report back whether or not the desired mixture has been attained. The rear O2 sensor is used mostly to monitor the condition of the catalytic convertor, although in some applications it also contributes to trim information.
      Based on feedback from the snitches, the ECU learns to apply a correction factor to its commands to the fuel injectors. If you know that your employees take longer than the standard allotted time to do a specified job, you will need to adjust for that in your planning (injectors are in a union, so it is tough to fire them ). The learned values go between the maps in the ECU's Flash ROM (the "chip") and the signal to the fuel injectors. These learned compensations are known as "trim". So, when you see "trim", it means "compensation".
      "Add" means additive trim, which is addressing an imbalance at idle. When the ECU is using additive trim, it is telling the injectors to stay open a fixed amount longer or shorter. The malfunction (e.g. vacuum leak) becomes less significant as RPM increase. For additive adaptation values, the injection timing is changed by a fixed amount. This value is not dependent on the basic injection timing.
      "Mult" mean multiplicative trim, which is addressing an imbalance at all engine speeds. The malfunction (e.g. clogged injector) becomes more severe at increased RPM. For multiplicative adaptation values, there is a percentage change in injection timing. This change is dependent on the basic injection timing.
      You can check your current state of trim by using VAG-COM or equivalent to look in Group 032 in your engine measuring blocks. The first two fields will have percentages. The first field tells the fuel trim at idle (Additive). The second field tells the fuel trim at elevated engine speeds (Multiplicative). Negative values indicate that the engine is running too rich and oxygen sensor control is therefore making it more lean by reducing the amount of time that the injectors are open. Positive values indicate that the engine is running too lean and oxygen sensor control is therefore making it richer by increasing the amount of time that the injectors are open.

      It is totally normal for both the first and second fields to be something other than zero. In fact, zeros indicate either you just cleared codes (which will reset fuel trim values) or something isn't working properly. If values get too far away from zero, it will cause a DTC (fault code) and can set off the MIL (commonly referred to as the Check Engine Light, or CEL).
      Specifications for normal operation are usually somewhere near +/- 10%.
      In general, an out-of-spec value in the first field (Additive) indicates a vacuum leak since it is mostly present at idle, when vacuum is highest. An out-of-spec value in the second field (Multiplicative) indicates a fault at higher RPM, and may point to a faulty MAF.
      Here's a good sanity check for the status of your MAF. Do a full-throttle run all the way to redline in a single gear (second works fine). Group 002 usually shows air mass in g/s. Your peak airflow should be roughly 0.80 times your horsepower. So, if you have a stock 150 hp 1.8T, expect around 120 g/s. If you have a 201 hp 24V VR6, expect around 160 g/s. If you see significantly less than that, you MAF may be on the way out. This still works if you are chipped, but "race" programs may make more power through timing, rather than airflow. Therefore, take all readings with a grain of salt.

    2. 01-20-2004 07:16 PM #2
      WOW, a forum with good information...
      and yes, I do (HEX/)VAG-COM, thank you very much, why do you ask?
      u said trim.
      (obviously from the MkIV forum)

    3. Member Junk T.I.'s Avatar
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      01-20-2004 10:58 PM #3
      awesome write up andy.........
      explained well.
      01 GTI 1.8T big turbo ghost...
      02 Jetta GLX
      VW/AUDI MASTER GUILD WRENCH

    4. Banned Taddd's Avatar
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      01-20-2004 11:41 PM #4
      Appreciate it! [IMG]http://**********************/smile/emthup.gif[/IMG]

    5. 01-21-2004 08:46 AM #5
      That is some useful info.

    6. Forum Sponsor Andy@Ross-Tech.com's Avatar
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      01-21-2004 09:52 AM #6
      Glad you guys like it. You do realize this same text has been in the 1.8T forum FAQ for over a year.

    7. 01-22-2004 10:42 AM #7
      Yup, saw it in the 1.8T faq.. but making it a sticky here would be good.

    8. 01-26-2004 10:34 AM #8
      Wow... Nice! Good info to have considering I have a car at the Stealer right now for the O2 recall, and it threw a multiplicative this weekend.
      Holy timing, Batman - all this info, and I just stumbled upon the Vortex last week...


    9. Member Junk T.I.'s Avatar
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      05-12-2004 09:48 PM #9
      also another way to reassure whether a maf is on the way out or is bad is to check block 006.....
      usually I see this reading anywhere from plus or minus 3.1%
      anything higher than this within reason could confirm a bad MAF.
      best thing to do is to check this block when you know your maf is good and then use that as a base line reading.....
      this is basically an atmospheric or barometer reading..... it can vary, but as I said before, within reason
      different altitudes will of course give different readings, as well as different humidity measures
      Pat
      01 GTI 1.8T big turbo ghost...
      02 Jetta GLX
      VW/AUDI MASTER GUILD WRENCH

    10. 05-13-2004 12:16 PM #10
      Very interesting [IMG]http://**********************/smile/emthup.gif[/IMG] , will you please elaborate on block 006? What is it's main parameters?
      What does that figure (atmospheric barometer) you gave +,- 3% relate to?
      Don't flame, I'm still a nooooobie but learning hella lot in this and the technical forum.
      TIA

    11. 05-16-2004 11:55 PM #11
      Andy
      Is it possible to read fuel trim on AGU engines?
      I suppose ECUs as mine also have this adaptive feature, please let me know.... [IMG]http://**********************/smile/emthup.gif[/IMG]

    12. 06-03-2004 09:13 PM #12
      Andy!
      [IMG]http://**********************/smile/emthup.gif[/IMG] [IMG]http://**********************/smile/emthup.gif[/IMG]
      Two thumbs up! You should write a book.
      This was some great information and extremely relevant on a recent problem I had.

    13. 08-16-2004 10:39 AM #13
      Andy thank I just got my new Vag-com from you so I will check it out
      thanks
      larry mc gregor

    14. 09-22-2004 08:31 PM #14
      Hello,
      I just ran the vag com beacuase i keep getting the "1 DTC's Found:
      17536 - Fuel Trim: Bank 1 (Mult): System too Lean
      35-00 - -" code

      the first Lambda value was -8.6

      the second Lambda value was 24.2

      I read this thread but i dont understand it fully. can anyone help me. I can send you the logs i took.

    15. 09-22-2004 08:50 PM #15
      Increase secondary and decrease primary fuel tweek with lemmiwinks until you get closer to zero values, reset fuel trims before made any changes.

    16. 09-22-2004 08:58 PM #16
      Quote, originally posted by 27psiBoom »
      Increase secondary and decrease primary fuel tweek with lemmiwinks until you get closer to zero values, reset fuel trims before made any changes.

      huh ? is there a how2 for this?
      thanks [IMG]http://**********************/smile/emthup.gif[/IMG]

    17. Member o2bad455's Avatar
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      11-04-2004 10:46 PM #17
      Great info, but I might have spotted one error.
      Andy@Ross-Tech.com said:
      "With the oxygenated gasoline that most of us use, actual A/F ratio of 15:1 is closer to stoichiometric."
      That doesn't sound right to me. If the fuel is "oxygenated", then it carries some of its own oxygen. Thus, while stoich for the standard fuel is an A/F mass ratio of 14.7:1, stoich for the oxygenated fuel should be slightly less (such as 14.5:1) since LESS external oxygen is required. Make sense?

    18. 11-05-2004 07:44 AM #18
      Quote, originally posted by Lotust »
      Hello,
      I just ran the vag com beacuase i keep getting the "1 DTC's Found:
      17536 - Fuel Trim: Bank 1 (Mult): System too Lean
      35-00 - -" code

      the first Lambda value was -8.6

      the second Lambda value was 24.2

      I read this thread but i dont understand it fully. can anyone help me. I can send you the logs i took.


      Sounds like a bad MAF.

    19. 11-05-2004 10:44 AM #19
      first channel is idle fuel trim (additive), second one is run fuel trim (multiplicative), you have problems with both of them....
      they mean yur running too rich on idle and too lean on part throttle.
      do you have a stock turbo?, what software are you running?, did you change any values with lemmiwinks?
      You may have a faulty maf but I'd check the first O2 sensor first

    20. 01-09-2005 12:28 AM #20
      I read this in the nick of time...I just calculated the worst mileage i've ever seen in my 1.8t (20mpg). Now where is that VAG-COM cable?
      Thanks Andy!

    21. 02-09-2005 06:32 PM #21
      andy, i sent you an IM
      max

    22. 02-11-2005 08:10 PM #22
      Quote, originally posted by digitalhippie »

      Sounds like a bad MAF.


      I agree. A multiplicative fuel trim close to the max value of 25% usually means the MAF is dead on a MKIV.
      Gary

    23. 02-12-2005 12:15 PM #23
      Fuel trims of 1.7 and 2.3 are pretty normal, so they shouldn't need much adjustment correct?

    24. 02-12-2005 05:01 PM #24
      Quote, originally posted by Mosquito »
      Fuel trims of 1.7 and 2.3 are pretty normal, so they shouldn't need much adjustment correct?

      Those values are completely normal. What do you mean by "shouldn't need much adjustment"? The trim IS the adjustment.
      Gary

    25. 02-12-2005 05:24 PM #25
      I meant I have no need to mess around with it.
      BTW. would you happen to know if the breather hose connected to the side of the air box goes to the distributor cap? I have no idea why it would, but that's what it looks like.
      It just so happens that I have that hose connected to the CAI directly and I got some water in there a little while back. behavior has been better since I changed the oil recently, but it just crossed my mind that water could have gotten in that hose too. I'm looking to clean it if possible.


      Modified by Mosquito at 5:25 PM 2-12-2005

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