Other Important P0171 And P0174 Tests
Now, if the above oxygen (O2) sensor test confirms that they are OK, now you can confidently conclude that the engine has a vacuum leak and that this vacuum leak is the one causing 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 the intake manifold.
Suggested Test 2: With the engine completely cold, start her up and spray around the intake manifold's gaskets to check for leaks. If the carb spray hits a spot that is leaking, the engine RPM's 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 0.8 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.
Low Fuel Pressure Causing A Lean Condition
A fuel pump usually goes to the 'big gig in the sky' from one moment to the next and without any warning but sometimes, you'll have it die a very slow death. When this happens, it sends enough fuel to keep the engine running, but not enough for the PCM to effectively control the Air/Fuel Mixture.
When this happens, this will result in a lean condition that will set the DTCs P0171 and P0174. In most cases, you'll also see the engine idle rough and/or misfire upon vehicle acceleration.
The only way to test this is by doing a fuel pressure test with a fuel pressure gauge.
Checking For Vacuum Leaks With Carb Spray
Spraying the intake manifold gasket with carb spray is one of the most effective ways to find a vacuum leak (it's the method most consistently used in auto repair shops) but it does have its risks -mainly the risk of fire.
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) is not torn or that it does not have dry-rot.
Also, and probably the most important, the engine should not be at its normal operating temperature. This is because the hotter the engine, the greater the risk of fire as you spray the areas around the intake manifold with carb spray.
Most Common Causes Of P0171, P0174 DTCs
Although this is not the most complete list of causes, here are the ones that I've encountered on a regular basis:
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 that are leaking vacuum.
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.
CAUSE 6: Bad pre-catalytic converter oxygen sensor.
Remember, the most important thing to consider (or keep in mind) when you're troubleshooting a P0171 or a P0174 trouble code is that whatever is causing it, is making the PCM think that too much unmetered air is entering the engine.
