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Importance of Reseting Fuel Trim (LTFT).

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This topic is in the MVS Volvo Repair Database » Importance of RESETTING Fuel Trim (LTFT)
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Brucebo
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Volvo Repair Database Importance of Reseting Fuel Trim (LTFT).

Post by Brucebo »

It is extremely important to reset your Long Term Fuel Trim (LTFT) after any work that could possibly affect the engine fuel and ignition system. This incudes, but is not necessarily limited to:

Oxygen Sensor
MAF Sensor
MAP Sensor
IAT Sensor
Coolant Temp Sensor
Throttle
TPS Sensor
IAC Valve
Any Vacuum Line
Fuel Pump
Fuel Injectors
Fuel Pressure Regulator
Purge Valve
Cannister Shutoff Valve
Spark Plugs

This is done by disconnecting the battery, waiting for 10 minutes or so, and/or turning the ignition key to the starter position to discharge the system. Note, the code "Erase" button on scanners does not reset the LTFT, at least not on my S70. Not reseting the fuel trims provides for a small chance that your adaptive fuel map could fail to converge and/or become unstable.

Bare with me, this article somewhat technical.

The fuel map is a two dimensional table of values, sort of like an Excel spreadsheet, for how much fuel to squirt, based on engine RPM and load value. It consists of the "base fuel map" (BFM) which is hard coded into the ECM, and the adaptive LTFT. In addition there is the Short Term Fuel Trim (STFT). This is a dynamic real time variable fed back to the ECM by the Oxygen Sensor(s).

The BFM is unchanging and is based on the automotive engineer's, or aftermarket upgrader's best estimate of what the fuel map should be, given nominal engine design, fuel, and sensor calibration. Added to the BFM is the LTFT, which adjusts for sensor drift, engine wear, fuel, etc. The LTFT is basically a long term running average of the STFT. If the BFM was perfectly adjusted, and unless something is broken it is usually pretty close, the LTFT would be zero everywhere in the table. It SHOULD be fairly small values everywhere in the table. Once the LTFT is fully adapted, the STFT should swing about zero.

The fuel map needs to be a "smooth" or "continuous" function. By this I mean that small changes in engine operating conditions should produce only small or no change in the fuel map value. For example, all else being the same, for a fuel map value at 2000 RPM, you DON"T what the map value at 2001 RPM to be something completely different! This would wreak havoc on your engine.

Here's where the necessity of reseting the LTFT comes in. If you have a bad sensor, or a loose vacuum line, etc. Your Oxygen sensor will send back large adjustment values of STFT. These will slowly get added to the LTFT and it will adapt. Here's the rub. Not all the values in the LTFT will adapt at the same rate. A value will only get adapted when you're driving in that region of the fuel map (combination of RPM and load). Most drivers are creatures of habit and rarely cover their whole fuel map in any reasonable period of time. Your fuel map will start to diverge, become discontinuous and, especially during Wide Open Throttle (WOT) when there is no feedback from the O2S, you will experience surges and hesitations as you swing through the fuel map. Even during non-WOT your O2S feedback takes at lease 1/2 to 1 second to respond so you may still experience surges and hesitations.

Now let's say you go and fix your mechanical problem but don't reset your LTFT. These fuel map discontinuities are still there! They MAY adapt back over time, but this could take awhile, and in the mean time you will still experience surges and hesitations. But worse things may happen. The fuel map adapting ability is really only reliable for "SMALL" corrections. In electronics it's call "small signal analysis". If the adaptive algorithms are presented with LARGE correction imputes, it is entirely possible that the adaption may diverge and/or become unstable through positive feedback. Let me give a somewhat contrived example. Let's say at 2000 RPM the LTFT is +18% and at 2050 RPM the LTFT is -18%. The engine hits 2000 and gets some more fuel and revs to 2050. The O2S reads "rich" and sends a negative correction, but since there is some time delay it sends the negative correction to the entry for 2050 and the LTFT goes to -20%. In the meantime as the engine hits 2050 it now gets less fuel and goes back to 2000, the O2S reads "lean" and the process happens in reverse, and so on ... . And the owner is pulling his hair out looking for "vacuum leaks", or replacing "bad" sensors when all they really needed to do was unhook the battery for a few minutes.

-Bruce

MadeInJapan
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Post by MadeInJapan »

Bruce,
thanks for the great article- I was not aware of this and will do this for now on- I have gotten the code for this after working on such things as the MAF sensor, replacing broken vacuum lines and taking the intake manifold off of the car.

Just a word of caution to everyone w/ a year 2000 or older Volvo- make sure you have your radio code BEFORE disconnecting your battery!
'98 S70 T5 Emrld Grn Met/Beige Tons of Upgrades Mobil-1
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gigidy5
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Post by gigidy5 »

So all you do is disconnect the positive battery terminal, go have a cup of tea, come back and hook the battery back up?
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db130
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Post by db130 »

how about after an installation of a rechipped ECU(having to physically remove the ECU from the ECU box)?
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Post by FCPEURO »

Forgive me if I am missing something important, but if the point is to kill the battery to reset the ECU / system, wouldn't disconnecting the battery then holding the brake pedal for a few seconds so the battery dies be a little easier then waiting, or trying to crank the engine with only what energy is left in the system, possibly getting half a turn out of it?

When I have to reset the CEL's on my car I just do the brake pedal routine, so I figured I'd throw it out there. I'm not trying to discredit your post at all, because I think it was a very good post!

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Brucebo
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Post by Brucebo »

Hitting the brake pedal, turning the key to "start" or whatever. The point is to put a load on the electrical system to discharge any filter capacitors, etc. The starter solenoid is probably the heaviest load you're going to find.

By the way, on some cars (not S70s) disconnecting the battery will NOT reset your fuel trims. I've heard this is true on some BMWs. Their fuel trims are stored in non-volatile memory and must be actively reset by appropriate hardware/software.

Regarding installing a new ECU, since this is where the fuel trims are stored, one presumes they are reset by default. Also, who in their right mind would install a new ECU with the battery connected.

-B

optowalt
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Post by optowalt »

This may fix my problem. Thanks. But will it work on all years? I have a 98 and found some fixes work on 99's but not 98's.

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Post by MadeInJapan »

Welcome to MVS....yes, it will work on '98's...more so than '99 and later models which stores more information in the computer than '98's and older.
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petek153
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Post by petek153 »

Is this also true for older 850's that this should be done? I have a 93,94 and a 97 850 but have never heard of doing this reset.
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MadeInJapan
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Post by MadeInJapan »

Certainly can't hurt! You do some work, your car throws a code and doesn't run very well- eventually the computer compensates and things are back to normal, but in the meantime, unless you read the codes and reset them, you don't know what's going on- this just eliminates that from the get-go.
'98 S70 T5 Emrld Grn Met/Beige Tons of Upgrades Mobil-1
'04 V70 2.5T Red/Taupe Some Upgrades Mobil-1
'07 S40 T5 AWD 6 speed manual! Silver/Black Stage1 Heico & Elevate
'07 S60 2.5T Blue/Taupe- my kid's Volvo

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