Long term fuel trim very slow to go down

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Hi all. I am trying to fix a Peugeot 307 1.4 gasoline.
It had LTFT at ~33%.
I replaced the MAP sensor because it was giving wrong readings. Indeed, LTFT+STFT came to ~ 22%.
What troubles me is why it took LTFT about 700 kilometers to come down to 22%.
Has anyone seen this in any car? Should I consider it normal? I mean I have seen LTFTs on Toyotas come down in a few kilometers.


Certainly looking to bring LTFT as close to 0 as possible. Have ordered oxygen sensors. Engine has good compression. Cleaned up the throttle body and the idle circuit.
No intake or exhaust leaks seem to be present. What else could be wrong?
 
...If you remove negative battery overnight it should clear all fuel trims no? Or get tuning software and use a PC to reset.
 
Once you've actually fixed the mixture problem you'll see a large STFT in the opposite direction which will pull the LTFT back toward zero. Then the STFT also goes down.
 
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Use whatever program/app you're using to view this data to reset the ECU and then report back LTFT in 100 miles.
 
LTFT should come down in within 1 KM, not 700. You still have an issue somewhere.

When is the LTFT high? Idle, cruise, under load? It makes a difference in diagnosis.

Does that car use a MAF sensor? If not, the MAP probably helped a lot. My guess is the oxygen sensors may correct the rest. They may have been fouled out by the extreme rich condition caused by the MAP.

The next spot to look would be a weak fuel pump, clogged fuel filter, or clogged injectors. All 3 can cause a lean condition that would push trims rich.
 
Depending on how long things were out of whack, the O2 sensors could be the problem now. Change those out and reset the trims by unhooking the battery for a few minutes, and then see what the trims look like.

If the O2 sensors are fouled, the catalytic converter(s) could also be in line for replacement.
 
Your lean condition is either bad O2 sensors, fuel delivery issue or vacuum leak...we know that, right. Just start eliminating the usual suspects.

I know it can take a bit for the ECU to develop a new fuel map, try resetting it like others have suggested. Are you going into closed loop after a few minutes at startup? If not, that can be a sign of faulty pre heat on the O2's..Dirty O2 sensors can effect fuel trim readings (it's recommended to replace them in pairs). You could pull the O2 sensors (upstream and downstream) and bench test them...or just buy new ones.

Are you certain there are no vacuum leaks - anywhere?..what is the LTFT at WOT? Does it stay high (above 8%) or does it drop (below 8%)? (8% is considered the threshold). A vacuum leak at WOT has minimal effect on air/fuel mixture but at low/no load a vacuum leak can have a more profound impact on the fuel/air mixture. If your LTFT at WOT drops to near normal but goes back up at idle, you more than likely have a vacuum leak somewhere.
 
Originally Posted by Mad_Hatter
Your lean condition is either bad O2 sensors, fuel delivery issue or vacuum leak...we know that, right. Just start eliminating the usual suspects.

I know it can take a bit for the ECU to develop a new fuel map, try resetting it like others have suggested. Are you going into closed loop after a few minutes at startup? If not, that can be a sign of faulty pre heat on the O2's..Dirty O2 sensors can effect fuel trim readings (it's recommended to replace them in pairs). You could pull the O2 sensors (upstream and downstream) and bench test them...or just buy new ones.

Are you certain there are no vacuum leaks - anywhere?..what is the LTFT at WOT? Does it stay high (above 8%) or does it drop (below 8%)? (8% is considered the threshold). A vacuum leak at WOT has minimal effect on air/fuel mixture but at low/no load a vacuum leak can have a more profound impact on the fuel/air mixture. If your LTFT at WOT drops to near normal but goes back up at idle, you more than likely have a vacuum leak somewhere.


That assumes a MAF is main input for deciding injector pulse width doesn't it? What if system is a Speed Density and only has a MAP, do those diag rules still hold?
 
Originally Posted by The_Nuke
Mad_Hatter said:
Your lean condition is either bad O2 sensors, fuel delivery issue or vacuum leak...we know that, right. Just start eliminating the usual suspects.

I know it can take a bit for the ECU to develop a new fuel map, try resetting it like others have suggested. Are you going into closed loop after a few minutes at startup? If not, that can be a sign of faulty pre heat on the O2's..Dirty O2 sensors can effect fuel trim readings (it's recommended to replace them in pairs). You could pull the O2 sensors (upstream and downstream) and bench test them...or just buy new ones.

Are you certain there are no vacuum leaks - anywhere?..what is the LTFT at WOT? Does it stay high (above 8%) or does it drop (below 8%)? (8% is considered the threshold). A vacuum leak at WOT has minimal effect on air/fuel mixture but at low/no load a vacuum leak can have a more profound impact on the fuel/air mixture. If your LTFT at WOT drops to near normal but goes back up at idle, you more than likely have a vacuum leak somewhere.

Yes, vacuum leaks on a speed-density engine are not unmetered air and thus DO NOT cause a shift in fuel trim. The MAP sensor is able senses the change in manifold pressure and the ECM opens the fuel injectors to compensate for the added air passing through the engine. A vacuum leak on a speed-density engine will raise the engine idle speed since the leak is the same thing as having the throttle blade open. A suspected vacuum leak on a MAP system can be confirmed by (carefully) using a starting fluid or propane to find the leak.. you will see a temp bump in rpms when the fluid or propane is burned.

Since the OP is experiencing high (20+%) LT fuel trims, that tells me his ECU meters air by MAF sensor and either a) the metered air is correct and there's a fuel delivery issue or b) the fuel pressure/volume is fine but he has unmetered air entering the system.
 
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Originally Posted by Mad_Hatter
Originally Posted by The_Nuke
Mad_Hatter said:
Your lean condition is either bad O2 sensors, fuel delivery issue or vacuum leak...we know that, right. Just start eliminating the usual suspects.

I know it can take a bit for the ECU to develop a new fuel map, try resetting it like others have suggested. Are you going into closed loop after a few minutes at startup? If not, that can be a sign of faulty pre heat on the O2's..Dirty O2 sensors can effect fuel trim readings (it's recommended to replace them in pairs). You could pull the O2 sensors (upstream and downstream) and bench test them...or just buy new ones.

Are you certain there are no vacuum leaks - anywhere?..what is the LTFT at WOT? Does it stay high (above 8%) or does it drop (below 8%)? (8% is considered the threshold). A vacuum leak at WOT has minimal effect on air/fuel mixture but at low/no load a vacuum leak can have a more profound impact on the fuel/air mixture. If your LTFT at WOT drops to near normal but goes back up at idle, you more than likely have a vacuum leak somewhere.

Yes, vacuum leaks on a speed-density engine are not unmetered air and thus DO NOT cause a shift in fuel trim. The MAP sensor is able senses the change in manifold pressure and the ECM opens the fuel injectors to compensate for the added air passing through the engine. A vacuum leak on a speed-density engine will raise the engine idle speed since the leak is the same thing as having the throttle blade open. A suspected vacuum leak on a MAP system can be confirmed by (carefully) using a starting fluid or propane to find the leak.. you will see a temp bump in rpms when the fluid or propane is burned.

Since the OP is experiencing high (20+%) LT fuel trims, that tells me his ECU meters air by MAF sensor and either a) the metered air is correct and there's a fuel delivery issue or b) the fuel pressure/volume is fine but he has unmetered air entering the system.





Got it, thanks!
 
Originally Posted by Mad_Hatter
Originally Posted by The_Nuke
Mad_Hatter said:
Your lean condition is either bad O2 sensors, fuel delivery issue or vacuum leak...we know that, right. Just start eliminating the usual suspects.

I know it can take a bit for the ECU to develop a new fuel map, try resetting it like others have suggested. Are you going into closed loop after a few minutes at startup? If not, that can be a sign of faulty pre heat on the O2's..Dirty O2 sensors can effect fuel trim readings (it's recommended to replace them in pairs). You could pull the O2 sensors (upstream and downstream) and bench test them...or just buy new ones.

Are you certain there are no vacuum leaks - anywhere?..what is the LTFT at WOT? Does it stay high (above 8%) or does it drop (below 8%)? (8% is considered the threshold). A vacuum leak at WOT has minimal effect on air/fuel mixture but at low/no load a vacuum leak can have a more profound impact on the fuel/air mixture. If your LTFT at WOT drops to near normal but goes back up at idle, you more than likely have a vacuum leak somewhere.

Yes, vacuum leaks on a speed-density engine are not unmetered air and thus DO NOT cause a shift in fuel trim. The MAP sensor is able senses the change in manifold pressure and the ECM opens the fuel injectors to compensate for the added air passing through the engine. A vacuum leak on a speed-density engine will raise the engine idle speed since the leak is the same thing as having the throttle blade open. A suspected vacuum leak on a MAP system can be confirmed by (carefully) using a starting fluid or propane to find the leak.. you will see a temp bump in rpms when the fluid or propane is burned.

Since the OP is experiencing high (20+%) LT fuel trims, that tells me his ECU meters air by MAF sensor and either a) the metered air is correct and there's a fuel delivery issue or b) the fuel pressure/volume is fine but he has unmetered air entering the system.





It's not a MAF sensor. It's a MAP in this engine.
If it was an air leak, I would only have high LTFT at idle. Now, it's throughout the range.
 
Problem solved. The pre-cat o2 sensor had been replaced, but the threads on the cat had been disturbed. The new o2 had sat against the distorted threads, never seating properly, nor crushing its metal ring.
The result was air entering the exhaust and driving the trims haywire.

Both o2 sensors were re-replaced, while repairing the threads with a tap.

Ltft comfortably sitting at 3% now. It became a very nice and economical car to drive.
 
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