Have Ford Questions? Ask a Mechanic for Answers ASAP
Have you checked for any trouble codes in the engine computer?
Inspect the PCV hoses from the passenger side valve cover to the back of the engine.
And you are getting no trouble codes from the PCM (Engine Computer)?
So you only get codes P0171 and P0174?
With the MAF unplugged, the PCM cannot monitor the amount of air entering the engine. So you will not get lean codes. You may want to have the VMV (Vapor Management Valve) inspected for sticking open allowing fuel vapors to return to the engine. Also, check the intake manifold gaskets for vacuum leaks. The fuel pressure should be around 35 to 45 psi.
Did they check the fuel pressure?
Is the PCV valve Ford or aftermarket?Is there oil in the throttle body or air intake tube to the throttle body?
OK. Also, you will need to have them monitor the long and short fuel trims while they bring the RPMs up to 2500 rpms. If the fuel trims change less than 15 %, it is not a vacuum leak.
If a vacuum leak can be ruled out, we are lead to the fuel system, fuel injectors and fuel quality.
The only way the PCM can monitor the amount of air entering the engine is through the MAF. If you unplug it, the PCM has to use a stored value which is basically a default value and that is why the lean codes go away and the engine runs somewhat ok.
The PCM basically calculates fuel trims by reading the temperature and volume of air entering the engine by the MAF. Then it applies the fuel through the fuel injectors and then it monitors the 02 sensors to read what is coming out of the cylinders in the exhaust and then it can adjust the fuel to help the exhaust to be clean for emissions. It takes away or adds fuel to allow for clean emissions. If it cannot monitor the amount of air coming into the engine, it cannot properly calculate and it guesses.
I do not think it is the MAF. Below is the actual pin point tests we use to diagnose the P0171 and P0174 lean codes.
The Ford PCV valves are designed specifically for that vehicle or engine. The aftermarket ones are basically generic for about 20 or more vehicles.