Other Important 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 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 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, the end result is 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.
To check to see if the fuel pump in your Ford vehicle is not supplying enough pressure, you need to use a fuel pressure test gauge to check it.
Ford has made it pretty easy to check the fuel pump pressure, since they have installed a Schrader valve on the fuel rail how to connect a fuel pressure test gauge to. The following tutorial will help you test the fuel pump: How To Test The Fuel Pump (Ford 4.0L).
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, but it does have its risks.
What risks am I talking about? It's the fact that the engine could backfire while you're spraying the intake carburetor cleaner spray and start a 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, the engine should not be at its normal operating temperature (meaning that you should not perform this test on a hot engine).
Checking the intake manifold gaskets for vacuum leaks using this method may sound scary but I can tell you that this test is performed all the cross country, pretty much on daily basis, without problems or complications -when the above safety precautions are taken.
Most Common Causes Of P0171, P0174 DTCs
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.
I've Done All Of The Tests And I Still Have Codes P0171, P0174
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 DTCs can convert your Ford vehicle 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.
If this info saved the day, buy me a beer!
