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AMEDEE
AMEDEE, FORD TECHNICIAN
Category: Ford
Satisfied Customers: 24435
Experience:  ASE CERTIFIED TECHNICIAN ADVANCED LEVEL SPECIALIST. Wisconsin certified emissions state inspector
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2005 Taurus: OHV..a very rough idle but runs smoothly above 1000 rpm

Resolved Question:

2005 Taurus with OHV U engine with 87k miles. Just started a very rough idle but runs smoothly above 1000 rpm. No driveabliltiy problems except rough idle at stop. Codes are 171, 174 and 300. After starting cold, engine runs smoothly until warm up when rpm drops to 700-800 and rough idle starts. Due to misfire code back in July, changed plugs, wires and coil pack. Have checked everywhere for vacuum leaks, cleaned IAC, TB and MAF, installed new fuel and air filters, checked impedance on all fuel injectors which are within spec. Fuel pump was replaced two years ago. What should be checked out next?
Submitted: 5 years ago.
Category: Ford
Expert:  AMEDEE replied 5 years ago.
Hello!

Do you have access to a scanner that can read live data?
Customer: replied 5 years ago.
No, I am trying to avoid having to pay a shop for a full engine scan.
Expert:  AMEDEE replied 5 years ago.
Ok.. I will be honest with you. This is either a bad mass air flow sensor or a an air or a vacuum leak. The PCV system elbow/hose right below the IAV valve tends to leak vacuum causing this codes. The intake plenum and lower intake gaskets are also very common and are hard to spot. If everything checks out, chances are the mass air flow sensor is at fault.
Customer: replied 5 years ago.
Why would bad MAF sensor allow smooth operation at higher rpms? I have air and vacuum locations you mentioned and have found no leaks. Can the MAF sensor itself be replaced or the total unit must be replaced?
Expert:  AMEDEE replied 5 years ago.
It is possible for the mass air flow sensor to read incorrectly just at idle and be fine off idle causing your condition. However, my experience has been that this is just a vacuum leak. The mass air flow sensor can be replaced separately.

Does that help?
Customer: replied 5 years ago.

If it is a vacuum leak, why does engine run smoothly above 1000 rpm? Does a scanner indicate a vacuum leak is present?

Expert:  AMEDEE replied 5 years ago.
That is a good question..

Engine vacuum is at its highest point at idle. SO if you are leaking vacuum, it is going to be more noticeable at idle and will smooth out when you pick up the engine rpms.

A scanner that can read live data will show you the fuel trims and the barometric reading. If the baro reading is off, chances are the mass air flow sensor is bad. If the fuel trims are high at idle, then this would indicate a vacuum leak.
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