3000GT/Stealth Fuel Trims

By Jeff Lucius

Below is a summary of what we know about Fuel Trims in our 3S cars, as I understand it. If you have comments, corrections, or information, please send an email to jlucius at stealth316 dot com.

The ECU monitors the switching and voltage of the oxygen sensors during closed loop operation and sets values called "fuel trims" that adjust fuel delivery during low, middle, and high (relatively speaking) air flow amounts. These basically do not affect open-loop operation for 1991-1993 models, and do affect somewhat open-loop fuel delivery in 1994-1995 "hybrid" models. For 1996+ models (OBDII) there is a short-term and long-term trim that affects either closed-loop or open-loop operation.

For 1991 to 1993 OBDI models, if our ECUs are programmed similar to the OBDI DSM ECUs, the fuel trims are used only during closed-loop operation (verified in the computer code by the DSM-ECU Yahoo! Group). However, the high trim may be updated during open-loop if Hz is low enough (examination of computer code is not complete at this time). The ranges below are for the 1G DSM MAF. Our 3S MAF (identical to the 2G DSM MAF) may be a bit different (also 4 cyl DSM vs. 6 cyl 3S), but you get the idea: fuel trims are set at low air flow amounts.

For the 1994 to 1995 model "hybrid" ECU, there are still 3 fuel trims. The ranges may be similar to OBDI. And like OBDI, high trim is all flow above a certain value. However, unlike OBDI, high trim is used as a compensating factor in open-loop mode. O2 Trim is still there, too. The O2 Trim is used in closed-loop mode fuel contol to make small adjustments in fuel delivery in response to the O2 sensor signal, that is, add a little fuel (increase injector actuator time) if the O2 sensor signal is below the middle value, or subtract a little fuel (reduce actuator time) if the signal is above the middle value.

For the 1996 to 1999 model OBDII ECU, from OBDII standards there are two trims: short term and long term. Short Term Trim is equivalent to the OBDI/hybrid O2 Trim and is used only during closed loop. Long Term Trim is persistent, covers all MAF flow rates, and is used as compensation for both closed-loop and open-loop modes.

See the web page below for a decent explanation of OBDII fuel trims and other OBDII info.
The Exhaust Gas Oxygen Sensor Signal
   The O2 sensor compares partial pressure of oxygen
in the exhaust to partial pressure of oxygen in the
atmosphere. It really makes no difference if "stoich"
is 14.7 (gasoline) or near 9 or 11 (pure alcohols). If
exhaust O2 content is the same as in the atmosphere
(very lean A/F or no combustion, such as during fuel
cutoff or spark misfires) then the sensor signal is at
or near 0 volts. If the exhaust O2 content is much less
than in the atmosphere (very rich A/F), the sensor
signal is near 1.0 v. At stoich mixture and combustion,
the O2 content is "just right" and the sensor signal is
~0.45 to 0.50 volts.

In closed-loop mode fuel control, the engine control
module is constantly adding a little fuel (O2 sensor
signal less than 0.45-0.50 v) or reducing fuel (O2
sensor signal more than 0.45-0.50 v), resulting in the
"cycling" of the O2 sensor signal around the

The "ideal" trim is 100 for the Low, Middle, and High Trims (1991-1995 pre-OBDII models). When the trim is greater than 100 then the ECU is adding fuel because it perceives that the closed loop A/F was on the lean side of "14.7". Adjusting the trim is supposed to return the A/F to "14.7". If the fuel trim is less than 100 then the ECU is subtracting fuel to compensate for a perceived previous rich condition during closed loop operation.

Note that these trim values are not a percent (1991-1995 ECUs), well not directly. According to the DSM-ECU Yahoo! group gurus, each 4 trim points is the same as a 1% change. For example: trim = 96 means the ECU decreased fuel delivery amount by 1%; trim = 81 (or 80) means fuel delivery is decreased by 5%; trim = 110 means fuel delivery increased by 2.5%; trim = 141 (or 140) means fuel delivery increased by 10%.

Trim ranges from 81 to 141 (-5% to +10%) on 1991-1993 OBDI models (I think; at least these are the ranges for my 1992 Stealth TT). 1994 Stealth TT owner Jim Matthews reports his Fuel Trims go as low as 72 (-7%); I am not sure of the upper limit. You can see that Mitsubishi prefers to limit how much the A/F can be leaned in closed-loop mode. As long as the trim is not pegged at one end or the other, you are pretty much OK. If the trim is pegged, then there is a chance the ECU may not be able to keep the closed-loop A/F near "14.7". However, as long as the O2 sensor signal is cycling about the 0.45 to 0.50 volt "center", then the ECU is able to maintain a good, if not perfect, A/F regardless of the trim value. It is also possible with some setups and programming that there may be a "jump" in A/F, rather than a smooth transition, when switching from closed-loop mode fuel control to open-loop mode fuel control if the fuel trims are pegged at one end or the other.

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Page last updated March 31, 2006.