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Disclaimer: I am a novice, so I possibly have made many errors during the process.

I have a Mercedes-Benz A Class w168 (2001).

It has a problem that sometimes, when running low RPMs it shuts down (often while stationary - but it did happen low speed corners).

I have brought it to two different mechanics but they said it's all fine and they can't find the issue. So I have decided to learn about modern engines myself, and bought a OBDII device to read ECU values.

I have read engine errors and it turns out I have one P0100 (Mass Air Flow Sensor).

So I took it apart and cleaned it, cleared the error, and put it back.

Unfortunately I did not measure LTFT and STFT before that.

The problem still persists, so I am trying to understand better what's going on. (although the error did not come up again yet).

All the data below has been collected with engine in closed loop.

I have noticed that the car has LTFT and STFT at ~0% when idle, but when I open the throttle (even moderately), LTFT goes straight to -20%, however STFT goes up, proportionally with RPMs (the more the throttle is open, the more STFT goes up), let's say between 10% and 25%.

Both idle condition and open throttle condition, O2 sensor reports normal conditions.

When returning to idle from an open throttle condition, LTFT stays -20% and STFT get temporarely "stuck" at 10%, and in those condition 02 sensor reports lean condition, which it kind of makes sense.

The thing that I don't understand is, how can LTFT be negative and STFT be positive so consistently? What am I missing?

Summary:

Idle: LTFT 0%, STFT 0% Throttle: LTFT -20%, STFT 10-25%

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2 Answers 2

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Long Term Fuel Trim (LTFT) should respond over time; in a process called 'Block Learn' by many manufacturers; to systemic changes in the operating conditions of the engine. For example, a change in fuel vendors, a change in regular operating altitude. Short term fuel trim will respond to rapid, transient changes in operation, such as gas pedal, hill grade and more.

The short story is that you need to multiply LTFT by 10 (or some manufacturer specific factor) and then add the STFT to get the final trim condition.

In my educated judgement, a change of 20 percentage points for Long Term just throttling up from idle is a symptom something is seriously awry.

Most of the other reported observations don't really support a the theory of a major problem so maybe there's just something wrong with the scanner interpretation of the percentage point readout.

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  • Thanks for your answer. As far as I understand, low LTFT but positive STFT means I can discard MAF under reading as a potential issue, right? Could I discard any other issue, or give more weight to other hypothesis, as leaky fuel injectors for example? My objective would be to narrow down the possible causes
    – milo5b
    Commented Oct 12, 2020 at 8:21
  • Review my post again. In particular reinforce the concept of total trim condition.
    – Andyz Smith
    Commented Oct 12, 2020 at 13:18
  • Many variations in total trim are acceptable but you should see a consistent systemic trend associated loosely with changing external factors. So for example total trim may be 3*10 (ltft) + 15 (stft) => gives 45 basis points. Which should happen repeatedly consistently under full throttle and 10% hill grade. But total trim may be -2 * 10 + 25 => gives 5 basis points. Which is ok too, with closed throttle and AC off.
    – Andyz Smith
    Commented Oct 12, 2020 at 13:25
  • As it turns out, probably this is not true on my car. LTFT and STFT have been the exact opposite, and slowly converging to 0 (it took a couple of days). I take it the total trim was just the sum of the two, otherwise we could not explain convergence to 0. (For example: (-20,+20),(-15,+15)
    – milo5b
    Commented Oct 16, 2020 at 7:33
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LTFT and STFT were converging to a new value: conditions have changed, so STFT was not aligned with LTFT direction, and eventually all settled to approx 0%.

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  • Any changes in the other symptoms, smooth idling, smooth throttle response and power up, smooth power at mid-range RPM. I would hedge a bet you have some as yet undiagnosed intermittent failure. MAF or MAP electrical connectors, IDLE control connector, intermittent 'physical' vacuum leak.
    – Andyz Smith
    Commented Oct 16, 2020 at 13:14
  • You'd definitely win the bet: I do have an undiagnosed intermittent failure indeed. Sometimes STFT starts going down to -40% and consequent loss of power, and as far as I understand it seems caused by O2 sensor which is reporting rich condition. What causes that is still a mistery to me, but if I turn off and on the car it goes away (temporarely). MAF readings look normal to me, but engine light turns on with P0100 error (MAF Circuit). Could still be malfunctioning MAF? I would discard vacuum leak because AFAIU fuel trims would go positive, maybe something in the exhaust system?
    – milo5b
    Commented Oct 17, 2020 at 14:32
  • Not to be snark, but this is why they have that sign at the repair facility. $80/hr. You want to help and suggest logical plausible ideas that we just know IS NOT IT. $150/hr. You worked on it first...
    – Andyz Smith
    Commented Oct 17, 2020 at 16:30
  • If I was you, I would put a laptop computer in there and strap it to the baby seat. And when this weird performance issue happens, you get on that computer and MARK THAT SPOT on the data record. THEN you can start replacing parts.
    – Andyz Smith
    Commented Oct 17, 2020 at 16:33
  • @AndyzSmith In the post I've mentioned I've brought it already to repair facility, twice, replaced parts and the problem if anything got worst. That is why I want to learn as much as possible before bringing it in again. I am already logging values with OBDII, and "marked" when this is happening. Now the difficult part is understanding what might be causing such values and then I can replace the part (or have it replaced)
    – milo5b
    Commented Oct 18, 2020 at 18:23

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