Long Term Fuel Trim: A Nod in the Right Direction
longer control fuel delivery adequately it’s given a new range to operate under. Switching to this new range is a func- tion of Long Term Fuel Trim. What that means is that a prob-
lem in the computer system, such as a faulty MAP, MAF, or TP sensor, will usually show up as a shift in the Long Term Fuel Trim reading. According to Robert Moreau, technical market- ing support coordinator for Sonnax, a normal Long Term Fuel Trim reading on most vehicles will range between about 0 to +7 if the system is operating properly. Anything outside of this range
should be a red flag; it indicates that something’s out of specs in the com- puter system. What’s more, if the Long Term Fuel Trim is within the accepted range, you can be pretty sure that everything outside the transmission is working the way it should.
Examining Long Term Fuel Trim Let’s look at how Robert recom-
mends checking the Long Term Fuel Trim to help identify a problem in the computer system. First, it’s important to understand
that this test procedure only works before anyone’s played with the com- puter system. Once someone clears the computer’s memory, all of the readings — including Long Term Fuel Trim — go back to baseline. The Long Term Fuel Trim won’t be useful again until
Figure 2b: Here, we’ve added a vacuum leak and let it idle for about 15 minutes, to illustrate
its effect on Long Term Fuel Trim. Long Term Fuel Trim went up 7 points on bank one and 7.8 on bank two. We’re a long way from setting any codes but we’re getting near that area where drivability problems begin to occur.
Figure 3: At 1,500 rpm the readings are in range. At a higher rpm that fuel leak has no affect on the system.
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www.transmission-specialties.com 30 GEARS January/February 2012
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