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Discussion Starter · #1 · (Edited)
Nissan defines Long term fuel trim as "overall fuel compensation carried out long-term to compensate for continual deviation of the short term fuel trim from the central value. Such deviation will occur due to individual engine differences , wear over time and changes in the usage environment." It is important to realize that the central value that Nissan talks about is based on the theoretical stoichiometric air fuel ratio of 14.7 to 1 for perfect combustion of non ethanol gasoline. Because the car's computer doesn't know what % of ethanol is added to the gasoline that you put in the tank, it has to have a base reference from which to decide how much fuel to spray into the cylinder to then mix with the air for combustion. If you put high octane gasoline ( which usually has no ethanol added ) into your tank, then the car's computer will not add or subtract any fuel for the short term fuel trim ( which is the immediate correction if something isnt right about the combustion). In that case your long term fuel trim will stay at zero because there were no short term corrections. Of course we are assuming that everything about combustion is working perfectly in the car and there are no diagnostic trouble codes (DTC). If that wasn't the case, then the short term fuel trim( which can be + or -) would show a number ie +1 +2 +3 etc if the car's engine is running too lean(too little fuel) and -1 -2 -3 etc if the car's engine was running too rich(too much fuel). The computer always rounds the fuel trim number to the nearest integer %. There is about 10% ethanol on the average for regular gas (mandated by the way by the US government). I wonder if Ted Cruz would do away with that regulation? Actually, the US government makes each seller of gasoline submit and adhere to a national yearly average which amounts to close to 90.7% gasoline and 9.3% of ethanol in the total gasoline sales of non diesel and non racing vehicles. The calculations for this is that total sales of high test is 7% and the rest is ethanol blends. I didnt calculate the blend of E85 as it isnt large enough yet to matter in the totals. The bottom line is that the result of this ridiculous regulation is that in the future if an increasing number of engines require high octane, then the gasoline companies will have to start adding ethanol into the high octane gas. Because it has been shown in scientific studies on actual vehicles that ethanol damages catalytic converters even at ratios as low as 5%, then because the government forces us to have catalytic converters on the cars, then they will be looked upon as a car wear and tear item. Fortunately the minimum warranty on converters is 8 years. In California it is 15 years.

So back to the fuel trim situation. Because non ethanol gasoline E0 has a stoichiometric air fuel ratio of 14.7 to 1 and ethanol blended E10 (regular gasoline) has a stoichiometric air fuel ratio of 14.04 to 1 , if you put the E10 gas into your tank, the car's computer will take time to learn to use it so that it eventually will have a long term fuel trim of +4 or +5. That means that the computer is constantly relearning the new stoichiometric air fuel ratio of the E10 blend, so that it keeps adding fuel while constantly adjusting the fuel trim. Once this process has been completed then things settle down after about 20 driving trips( each car manufacturer will have a different moving average adjustment for the long term fuel trim). Of course the LTFT is stored as a number for each RPM, so to get it to ZERO for each RPM, you will have to drive your car over a whole range of RPMs. Thus this will be the new baseline with which the cars computer will work from and the short term fuel swings will disappear and everything is hunkydory or in other words perfect again. So to those car owners that like to switch back and forth between regular and high test( high octane), I would strongly advise against it. The reason is that you will force the car's computer to constantly relearn the stoichiometric air fuel ratio and your car's engine will be running too rich whenever you will have the high test gas in the tank and will be running too lean whenever you have the E10 fuel in the tank. Stick with one or the other. I recommend the high test because it has no ethanol ( damages the catalytic converters) but the difference can be as much as 14 cents a litre or 53 cents a US gallon. This can add up to $150 -$200 extra per year but if your catalytic converters warranty is only 8 years it may be worth it. Ethanol probably does some damage to other engine parts but unless someone can point to scientific studies we wont discuss it here. So if you are running high test with no DTC codes and all your sensors are working properly with no vacuum leaks and brand new catalytic converters and brand new 02 sensors and a brand new redesigned camshaft position sensor then your long term fuel trim should be ZERO. I confirmed this with my local garage who ran his $18000 generic scanner on my 2006 Altima 2.5 litre QR25DE engine with 78000 km (48000 miles) on it. The test came in at 100% which means ZERO long term fuel trim in the way that his scanner presents the data. To doublecheck this I then took it to the Nissan dealer who ran his $25000 Consult diagnostic tool on my car. The results confirmed that my car was running perfectly but the Consult tool does not give the Long term fuel trim number unless there is a DTC !!!!!!!!!!!!!!!!!!! I guess that since car owners who used regular gas with 10 % ethanol would end up with long term fuel trims of +4, Nissan decided that they didnt want to use Long term fuel trim as a diagnostic tool for gasoline engines. This is extremely unfortunate because I believe that if you use high test E0 in your car, and you then have Long term fuel trim = Zero, then whenever the Long term fuel trim changes to some integer other than Zero, Houston WE HAVE A PROBLEM. The Long term fuel trim is the most important number in today's car engines as long as you run the car without an ethanol blend. In that case it is the only number you need to know about your car's combustion and that number should be on every gasoline car's dashboard.
 
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