11/11/2011 10:37 AM
by Marie Ray
LTFT and STFT values are off and don't make sense. Ask Question 22. RPMs go all the way up to 1800 rpm to warm up the cats and when they drop, there is some up/down movement around 700-900 rpm and it goes on for maybe 1-2 seconds and stops. So the fuel trim would have to be positive to compensate for a lean condition, not a rich one.
By Ralph Kalal
“Fuel trim” is the amount of fuel added or subtracted by the PCM. The PCM starts out with a baseline fuel/air mixture that it believes will achieve a stoichiometric ratio. When the feedback from the HO2 sensors shows a deviation, the PCM adjusts the amount of fuel the injectors deliver, in an effort to hit the stoichiometric goal. It either adds or subtracts fuel from the baseline.
The amount it adds or subtracts is called the “short term fuel trim” (STFT) and is expressed either as a plus or minus. Because the PCM’s definition of “short-term” is milliseconds, it acts incrementally, adding or subtracting fuel in small increments in an effort to hit the stoichiometric target.
All this time, the PCM is watching what’s happening in response to the changes in STFT. If STFT adds, for example, 3 percent fuel to the baseline and this makes the oxygen sensors happy, the PCM concludes that it ought to adjust the baseline. So it changes the “long term fuel trim” (LTFT). “Long term” is between 30 seconds and a minute. This creates a new baseline.
The baseline is “injector duration,” which is the length of time the fuel injectors are open, supplying fuel to the cylinders. Both STFT and LTFT are adjustments from the baseline that make the fuel delivery leaner or richer, as needed to best achieve the stoichiometric ratio that represents the most efficient combustion.
As a general rule, STFT can deviate as much as +/- 20%, but normally should be within +/- 10%. If it deviates by 10% or more for very long, LTFT will adjust the baseline to bring the STFT deviation back within the 10% range. This will then be reflected by a change (+/-) in the LTFT.
To illustrate: suppose the vehicle were operating at 0% correction in LTFT and +10% in STFT when an air leak occurred. STFT initially adjusts for this increase in oxygen by increasing fuel +20%. It does that in 0.6 millisecond. After 30 seconds, LTFT increases +10%. This brings the STFT down to +15%, which is still too much. So, after another 30 seconds the PCM moves LTFT +20%, and STFT drops to less than 10%.
Long term fuel trim enables the PCM to compensate for variations within the engine itself resulting from production variations, engine and component wear, and minor malfunctions. If you install an aftermarket air filter that is less restrictive than original equipment, it will affect intake airflow. Increased air flow will prompt the PCM to add fuel. It will quickly establish a new LTFT baseline that includes that addition. Similarly, as the air filter becomes more restrictive from the accumulation of dirt and debris, the PCM changes LTFT to adjust for the decreased air flow.
LTFT also enables the PCM to make the best decisions about the fuel/air mix when it is in “open loop” operation, or is not giving priority at that moment to achieving the stoichiometric ratio. For example, when you suddenly floor the accelerator, the PCM is programmed to recognize that wide open throttle requires a richer fuel/air mixture. But how much richer should it be? LTFT is the data upon which the PCM relies to answer that question.
Adjustments to LTFT by the PCM are normal. However, fuel trim data may point at a problem elsewhere, so fuel trim data can be very useful in diagnosing engine problems. When LTFT is adding to injector duration, it is compensating for a lean condition. That condition could be caused by an air leak, weak fuel pressure, an exhaust leak in the manifold upstream of the HO2S, clogged fuel injectors, wear in the throttle body, or water in the fuel. When LTFT is negative, there might be a leaking fuel injector, excessive EGR flow, high fuel pressure, or a malfunction in the evaporative emissions system. As with other components of the electronic engine management system, the key to using STFT and LTFT data is in understanding how they affect, and are affected by, other parts of the system.
- AxleAddict»
- Auto Repair»
If you learn how to test a fuel pressure regulator (FPR), you may save some money on repairs and get your vehicle going sooner. A fuel pressure regulator commonly fails in one of two ways: When it fails to hold pressure, it provides too little fuel to the engine, causing a lean mixture (low pressure) condition; when the FPR gets stuck and builds up more pressure then it should, it'll cause the injectors to deliver too much fuel, causing a rich mixture (high pressure) condition. A worn out spring or valve, a leaking diaphragm or lack of vacuum to the pressure regulator may cause these conditions.
Common bad FPR symptoms include hard starting, misfiring, stalling and hesitation.
However, other worn out or failed components—like the fuel filter, fuel pump, and automatic transmission issues—can also cause similar symptoms to those of a failed pressure regulator. So you need a way to troubleshoot the regulator in your vehicle whenever you suspect a malfunction.
Here, you'll find a couple of tests you can do at home with the use of a fuel pressure gauge, a fairly inexpensive tool. If you don't have this gauge, you may buy one from your local store or online.
I. Checking for FPR Fuel Leaks |
REMINDER - Fuel Filter Replacement |
WARNING - Fuel Pressure |
I. Checking for FPR Fuel Leaks
The easiest way to test an FPR is with the use of a fuel pressure gauge. But first, you'll start your tests with a quick preliminary check:
1. Open the hood and locate the fuel pressure regulator on one end of the fuel rail. This rail holds the fuel injectors in place. A standard regulator is a small metallic cylinder with a thin vacuum hose connected on top. Also, you may see an incoming fuel line and a return fuel line connected to it, depending on your particular configuration.
Note: the tendency on newer vehicle models is to place the fuel pressure regulator inside the fuel tank as part of the fuel pump assembly. So you won't find the regulator or a return fuel line on these models. The computer helps maintain pressure along with the pressure regulator.
2. Check the vacuum hose for a tight connection. A loose hose will prevent the regulator from working properly. Also, check the hose for damage and wear. Then disconnect the vacuum line from the regulator.
3. If you see any signs of fuel in the vacuum line, the diaphragm inside the pressure regulator has a leak and you need to replace the regulator. Otherwise, continue with the rest of this test.
Note: try to do this test right after you've taken you car for a ride on the highway which helps reveal small fuel leaks.
II. How to Check a Fuel Pressure Regulator
1. Locate the Schrader valve. Most modern fuel injection systems come equipped with a Schrader or test valve located on the fuel rail. The valve is similar to the air valve on your tires.
2. Connect your fuel pressure gauge to the Schrader valve or test port. If your system doesn't have this valve or has an older fuel system, check the repair manual for your particular vehicle model for the best way to connect the gauge to the system.
Note: if there's no test port or you don't have the correct adapter, remove the test port valve and connect the gauge directly; or disconnect the fuel inlet line and connect a T-adapter. Then reconnect the fuel line and connect the gauge to the T-adapter.
3. Then find the fuel system pressure specification for your particular model with the engine running and off. Look up this number in your car repair manual.
4. Start the engine and let it idle (or have an assistant crank the engine for a few seconds, if the engine doesn't start). Then turn off the engine.
5. Observe the fuel pressure gauge while the engine runs and after shutting off the engine. Fuel pressure should hold to specs, while the engine is running, and after turning off the engine.
Note: While conducting this test, also remove the vacuum line while the engine is running. It should cause the pressure to rise. Otherwise, there's a problem with the FPR.
6. Make a note of the pressure gauge reading.
7. Compare your readings to specification in your repair manual.
- If your gauge reading is lower than the specification while the engine is running (pressure goes down quickly or after a few minutes, or doesn't build up), you'll need to check the fuel pump (failing to pump at the adequate rate), fuel filter (restricted or clogged), or the anti-drain valve (failed—draining fuel back into the fuel tank) on the fuel pump assembly. Continue reading.
- If fuel pressure begins to drop soon after shutting off the engine, you probably have a leaking injector, a leaking anti-drain valve in the fuel pump assembly or problems with the FPR itself.
- When you detect low pressure, and you have not changed the fuel filter in more than a year (or at the interval suggested in your car owner's manual), it is a good idea to replace the filter, and repeat the test again. A clogged or partially clogged filter may be your problem. After installing a new fuel filter, if pressure is still too low or system loses pressures after shutting off the engine, head over to the next section Testing for Maximum Pressure.
- If your gauge reading goes above specification and you know the vacuum line is in good condition, properly connected and not clogged, most likely your FPR is to blame.
REMINDER - Fuel Filter Replacement
When you fail to replace a fuel filter at the recommended car manufacturer intervals, trapped particles will begin to clog the filter, which causes fuel pressure to drop and damage to the fuel pump as well. So it's more cost effective to service the fuel filter at regular intervals.
III. Testing for Maximum Pressure
You want to do this test if your previous test showed low fuel system pressure after shutting off the engine. The test may help locate the point of failure, provided your fuel filter is not clogged or due for replacement.
This particular test is similar to the previous. You'll use your fuel pressure gauge. But this time, you'll try to locate the likely source for low fuel system pressure.
1. Connect the fuel pressure gauge to the fuel system as you did in the previous section.
2. Now, place a rag over the fuel incoming line (hose) and another rag over the fuel return line (hose). If you need more help to locate these lines, consult the repair manual for your particular vehicle model.
3. Have an assistant start the engine, let it idle for a minute, and then turn it off.
4. As the engine turns off, use a pair of slip joint or vise grip pliers to squeeze and block the incoming and return lines. The rag should be between the pliers and fuel line to prevent damage to the hose.
5. Note the fuel pressure gauge reading for a few minutes.
- If pressure remains steady, your fault if probably in the fuel pump anti-drain valve.
- If pressure still drops, you have either a leaking fuel injector or bad FPR.
Check the video at the start of this section so that you have an idea how to do a maximum pressure test.
WARNING - Fuel Pressure
If you need to replace the fuel pressure regulator—or whenever you need to disconnect any part of the fuel system—relieve fuel system pressure before beginning to work on the system. Fuel pressure on some models can reach up to 60 psi (414 kPa). This means fuel can suddenly discharge with force, even with the engine off, and injure your eyes. Consult the service manual for your particular vehicle make and model for the proper way to bleed off system pressure on your car.
As you can see, testing a fuel pressure regulator can be a simple process. With the help of a pressure gauge, a fairly inexpensive tool, you can find out if you need to replace the regulator or concentrate on other components or systems. If you need to replace the regulator head over to How to Replace a Fuel Pressure Regulator, where you'll find the steps to install a new one. Most of the time, though, you'll learn that the root cause is a device in need of maintenance. And often, you can replace a failed or worn out component yourself and save some money over paying a repair shop.
Test Your Knowledge of FPRs
view quiz statisticsThis article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Is there any way you could check the vacuum on the regulator to see if the incoming and outgoing towards open, or how does that work?
You need to have the car at idle. You can unplug the vacuum hose and put your finger at the hose opening. If there's vacuum you'll feel a slight pull on the tip of your finger. If you need to know how much vacuum there is, you need to connect a vacuum gauge to it. Check the specification on your vehicle repair manual.
14For how long does fuel line pressure remain?
Fuel pressure decreases slightly after shutting off the engine. Then the pressure will hold for about five minutes then decrease slightly. But some pressure will remain steady usually after about 20 minutes.
12What if the car just doesn’t start and I’ve never had it running? How can the fuel pressure system be tested?
Locate the fuel pump relay; you may be able to connect battery power to it. Have a fuel pressure gauge connected to the test port. Check the specification for initial pressure on your vehicle repair manual. The manual will help you locate the relay as well.
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how to clean and ceck the injacktors of a geely 1.5 ck 1and the rail and pressure regulator of rail
Changed fuel injector Check ingine light came on for egr valve..... Since then car stoped on me while i was driving i changed fuel pump thinking it was the cause yet car will not start it started up once in 4 days and died about 20 secs later.... Would egr valve cause this problem?
I would like to know why it's so hard for some one to tell the fuel pressure psi on 1992 vw cabriolet it would really be nice if the simple question could be answerd i just hope every one is having a better day then i pease out