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A. Penland
A. Penland, Ford Senior Master Technician, ASE
Category: Ford
Satisfied Customers: 6216
Experience:  ASE Master Certified, ASE Advanced Gasoline Engine Performance
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I have an 02 f150 5.4L. I have a rough idle and some times

Customer Question

I have an 02 f150 5.4L. I have a rough idle and some times will die with the MAF plunged in. If I unplug the MAF it runs great. I tested the MAF and had voltage any where from .6v to 1.5 v at idle. It was all over the place when idleing up and Down. I thought I had found it and replaced the MAF it ran good for a day then back to the same thing. I went and got another new MAF thinking I may have got a bad one but the next new one had the same problem as soon as I put it in. Any Ideas?
Submitted: 11 months ago.
Category: Ford
Expert:  A. Penland replied 11 months ago.

Have you checked for any trouble codes in the engine computer?

Customer: replied 11 months ago.
Lean bank 1and 2 when MAF is plugged in nothing but No response 0.0 voltage when MAS is unplugged . But no lean codes with it unplugged
Expert:  A. Penland replied 11 months ago.

Inspect the PCV hoses from the passenger side valve cover to the back of the engine.

Customer: replied 11 months ago.
PVC is new and the boot on the back of the intake is also new. No leaks there. Why at idle would the voltage be so up and down on the com wire for the MAF with it just idleing ? The test for the MAF says that is that happens then the MAF sensor is bad but I've tried 2 brand new MAF and still the same sporadic voltage on the com wire at idle. I also had the air temp sensor tested and it tested good.
Expert:  A. Penland replied 11 months ago.

And you are getting no trouble codes from the PCM (Engine Computer)?

Customer: replied 11 months ago.
I get lean bank 1 and 2 with MAF plunged in but with out it plugged in I don't get them I just get MAF codes . Also all 4 of the 02 sensors are new . Ford looked at it on the scanner and found only the miss on cylinder # ***** I replaced all coils plugs and boots then that went away no codes any more for the mis. Fuel pressure is good and fule trims looked good they said also. There as confused as me as why it runs so good with out the MAF in and horable when it's plugged in that changing the MAF should have fixed it. But it did not
Customer: replied 11 months ago.
There are no MAF codes when it's plugged in
Expert:  A. Penland replied 11 months ago.

So you only get codes P0171 and P0174?

Customer: replied 11 months ago.
that's what I get with the MAF plunged in. They go away when it's unplugged
Expert:  A. Penland replied 11 months ago.

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.

Customer: replied 11 months ago.
VMV was checked by dealer and intake gasket was changed 2 weeks ago by the dealer. That is what they thought it was but ended up not being that.
Expert:  A. Penland replied 11 months ago.

Did they check the fuel pressure?

Customer: replied 11 months ago.
they said it was good . I think it was around 40 psi. I changed fuel filter also just to make sure that was not it. I have done a bunch of work and had it at the dealer and another auto shop know one seems to have any idea what it is. Just keep putting in new parts hoping they find it. The last shop checked intake for cracks and disconnected the cats thinking they may be plugged but they were fine . It's driving me crazy!!!
Expert:  A. Penland replied 11 months ago.

Is the PCV valve Ford or aftermarket?
Is there oil in the throttle body or air intake tube to the throttle body?

Customer: replied 11 months ago.
The PCV is a ford I think. It was done at the dealer I will check
Expert:  A. Penland replied 11 months ago.

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.

Customer: replied 11 months ago.
When I took it back in last week the guy swore it was a vacuum leak then he did hook it up and check fuel trim. After he did that he said it was ok and he did not think it was a vacuum leak any more .
Expert:  A. Penland replied 11 months ago.

If a vacuum leak can be ruled out, we are lead to the fuel system, fuel injectors and fuel quality.

Customer: replied 11 months ago.
would injectors being bad or fuel only show up with the MAF hooked up but run fine with it unhooked? I would think that bad injectors would run bad no mater what?
Expert:  A. Penland replied 11 months ago.

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.

Customer: replied 11 months ago.
Well it has done a great job of guessing then because this thing runs better than it ever has with it unhooked. It has no stalling, no rough idle, no codes besides the MAF . I just cannot figure out why the MAF would be sending such a erratic voltage to the pcm at idle. I tested it right at the pug in the short wiring harness from the MAS air. the test says that if this happens then it's bad but how can it be bad 2 times in a row. I'm going to go now and look at the PCV and see what they used. What is the problem will only fords work or something
Expert:  A. Penland replied 11 months ago.

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.

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