Now, if the above oxygen (O2) sensor test confirms that they are OK, the next thing is to check the engine for vacuum leaks... since vacuum leaks are usually the biggest causes of the P0171 and/or P0174 diagnostic trouble code(s).
Suggested Test 1: With the engine cold, visually and physically check all vacuum hoses. The most common vacuum leak problem is the PCV tube's elbow, where it connects to either the PCV valve or the intake manifold.
Suggested Test 2: Checking the intake manifold and plenum gaskets for leaks with carburetor spray.
For safety's sake, this type of test has to be begun with the engine completely cold. Once you've started the engine, you'll need to spray around the intake manifold's gaskets, plenum gasket, and around the Spider's fuel injector's connector to check for leaks. If the carb spray hits a spot that's leaking, you'll immediately hear a difference in the engine RPMs.
What will happen is that the RPMs will increase or decrease and this will pinpoint the source of the vacuum leak (before undertaking this test, see: Checking for Vacuum Leaks with carb Spray).
Not only will the RPMs increase or decrease, but if the engine is warmed up enough, if you look at the O2S11 and O2S21 voltage numbers on your scan tool, they will max out a .8 to 1 Volts every time the leaking spot on the intake gasket sucks in the carburetor spray (remember, you can't let the engine get hot and continue to spray the intake manifold to cylinder head areas with carb spray).
If after visually and physically checking for vacuum leaks and none are found, the next step is to check fuel Pressure.
A fuel Pump usually ‘kicks the bucket’ from one moment to the next and without any warning... but every now and then, you'll have it die a very slow death. When this happens, it doesn't send enough fuel to the fuel injectors to keep the Fuel Injection Computer happy.
When this happens, the PCM (Powertrain Control Module = Fuel Injection Computer) will think there's a major vacuum leak and this will set the DTC's P0171 and P0174.
The other very big giveaway, that the fuel pump is going out (on the 4.3L, 5.0L, and 5.7L Vortec engines) is that you've got to use Starting fluid to start the engine. Once it starts, it stays running.
The only way to test this is by doing a fuel pressure test with a fuel pressure gauge and then compare the values to the repair manual's specifications.
Spraying the intake manifold gasket with carb spray is one of the most effective ways to find a vacuum leak, but it does have its risks. To minimize any possible risk of a back-fire and/or fire... this test should only be done when you have made sure the PCV tube's rubber elbow (at the intake manifold or at the PCV valve itself) is not torn or that it does not have dry-rot. Also, the engine should not be at its normal operating temperature.
CAUSE 1: PCV Tube's rubber elbow (where it connects to the intake manifold) is torn open and causing a major vacuum leak.
CAUSE 2: Intake manifold gaskets or plenum gaskets are leaking vacuum. GM, in all its infinite wisdom, uses plastic and rubber gaskets for the intake manifold and plenum gaskets.
CAUSE 3: Fuel pump that's going BAD, but hasn't completely fried yet.
CAUSE 4: A dirty (contaminated) MAF sensor.
CAUSE 5: An exhaust leak right before the pre-catalytic converter oxygen sensors. This is usually caused by BAD Exhaust Flange gaskets.
CAUSE 6: BAD pre-catalytic converter oxygen sensor.
So, you have done all of the tests... you have researched the codes to death (online and in repair manuals) and nothing you have done or replaced has solved the problem!
Trying to solve P0171 and P0174 DTC's can convert your GM pickup (van or SUV) into a money pit. To avoid this... you need to remember one very important thing:
Be Patient: Whatever part, whether it's an intake manifold gasket or an O2 sensor, is causing the codes... hasn't completely fried yet. This happens.
Parts don't always fail in an immediate fashion. So, the gaskets or vacuum hoses or the sensor could seal/work/function/ correctly most of the time, and every now and then they stop doing their jobs. This happens a lot! What can you do about it? Not much but wait till the problem is present all of the time.
Patience is a virtue. If you have checked for vacuum leaks and you have none and you have verified that the Fuel Trims are OK (which means the O2 sensors are OK too)... then the best thing to do is to wait a few days and retest again. Whatever is failing... will continue to fail until eventually the problem will be present to verify it with the tests described in this article.
The important thing to remember, is that you can use your scan tool to check the Fuel Trim Values with your scan tool to verify the Lean Condition (that is setting the P0171 and/or P0174 codes) is actually present.
“Remember, this hobby is supposed to be your therapy, not the reason you need therapy.”