However, in some cases, the displayed value for long term fuel trims can be as high as 6 to 8 percent depending on the application and can be either a negative or a positive number. So what does this mean? Let us look at positive numbers first-. Clearing fault codes has the effect of erasing stored fuel trim data as well, and the ECU generally requires at least a few drive cycles to relearn how to adapt fuel delivery strategies.
Note though that since sudden changes in the engine speed can cause short-term fuel trim values to fluctuate wildly, all fuel trim values should be taken at a minimum of at least three steady engine speeds, these being at idle, at about RPM, and at about RPM. Note also that fuel trim values should only be taken when the engine had been running at the above speeds for at least 30 seconds. Note that if the long-term fuel trim values fluctuate in a pattern that resembles the fluctuations of the short-term fuel trim values suspect a defective catalytic converter.
The DTCs directly related to fuel trim problems are:. Here are some common causes of positive long-term fuel trim:. Here are some common causes of negative long-term fuel trim:. Skip to content. What is Fuel Trim? Why do we need Fuel Trim? How do you Get Fuel Trim Data? This, again, causes excess HC in the exhaust stream, yada yada yada….
There are reasons we say the things we do. We honestly want to help. The negative fuel trim is just another symptom, not a cause. You have to understand what that symptom is telling you. When the underlying issue is corrected, the trims will also. That way when you see the trims out of whack you know what direction to take next. Yes, that would have been good to know from the start. The oiling issue could just be from a really sludged engine. The top end gets its oil from the camshaft bearings.
If they are excessively worn, spun around blocking the oiling holes, etc. If the oiling holes are just plugged from sludge, automatic transmission fluid run in place of a quart of oil should help clean it out. The FPR has a vacuum line on it. Remove the vacuum line or pinch it off and the idle should change. If not, the FPR is probably faulty. If you remove the line from the FPR you can check if there is actually vacuum present on the line.
The injectors can be tested by swapping locations and seeing if the codes move with them. Is the old tissue in the spark plug hole still a good test?
If fuel was pouring out, then I would say it was bad. Learning about modern engine management systems can take quite a bit of time and thought, but it will serve you well as you continue working on your car. Sure, there are more expensive analyzers and test equipment that can help identify problems faster or without having to run several different tests for confirmation, but most things can be diagnosed with basic tools, common sense, and the most important tool of all, your brain, equipped with proper understanding.
You may want to pick up a copy. The fuel trim value is read by plugging a scan tool into the OBD II diagnostic connector located under the instrument panel on the drivers side near the steering column. When the key is turned on, the scan tool will initialize and start to communicate with the vehicle's onboard computer. Depending on the tool and the vehicle, it may be necessary to enter the vehicle year, make, model and engine VIN code before the scan tool can read the data.
The engine must be started and running to read the fuel trim information. Depending on the scan tool and how its menu options are set up, you choose the option that allows you to read system live data. On this list will be two fuel trim values for inline four and six cylinder engines, and four fuel trim values for V6 and V8 engines one pair for each cylinder bank.
This value changes rapidly and can bounce around quite a bit depending on engine load, speed, temperature and other operating conditions. Values normally range from negative 10 percent to positive 10 percent, though the readings may jump as much as 25 percent or more in either direction. Long Term Fuel Trim LTFT is a longer term average of what the engine computer has been doing to balance the fuel mixture over a predetermined interval of time.
How do you know which cylinder bank is 1 or 2 on a V6 or V8 engine? Bank 1 will be the cylinder bank that has cylinder number one in the engine firing order.
POSITIVE fuel trim values mean the engine computer is adding fuel increasing the pulse width or on-time of the fuel injectors to add more fuel to the engine.
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