1999 Mustang crap idling and gas


New Member
Dec 23, 2018
Hello guys, I have a 99 ford Mustang regular, V6 3.8L Engine. I'm 22 don't know too much about cars, but lately I've been getting really bad gas mileage, around 13 mpg. I'm used to getting about 20 ish. I can also smell a hint of gas when I start it and whilst driving sometimes. I took it to autozone, and some codes read up. It pretty much said that my mass air flow sensor something with my O2 sensors, the system was too lean, and I had some type of emissions or gas leak. I was wondering what would be the reason I'm getting such :poo:ty gas, and such bad idling/rough acceleration. I have never changed the air filter or cleaned the Mass air flow sensor or changed my O2 sensors. I was wondering if either of those would be the main reason for such :poo: gas mileage or is it the emissions leak?
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SN Certified Technician
Aug 14, 2009
Houston Texas
If you really expect those of us out in Internet land to help you then you need to help us by taking the time to post the full DTC codes. There are 1,000's of them and they each have a story to tell. "It pretty much said" doesn't really help.

But if looking for an educated guess, I would say an exhaust leak. That would cause a FALSE lean code with the smell of gas. The air in the exhaust will be detected by the O2 sensors thus making the PCM "think" that the motor is lean when in fact it is not. The PCM will respond by ADDING fuel. This creates a rich condition that will make the car run like crap and give poor gas mileage.

Remember that today's cars can not run right with even a tiny intake or exhaust leak.

However. It won't hurt anything to clean the MAF just to be sure. Consider it basic maintenance. Use only a product designed for the purpose. Be careful as a MAF is easily damaged.

Double check the battery and charging system.

Howto perform charging system voltage drop test

Consider getting yourself an ODB2 scanner. Here's some information on an affordable Windows based unit.

ForScan ODB2 scanner w ELM327 USB

This will help you answer questions such as:
  • Are the O2 sensors "switching"?
  • Is the MAF signal reasonable?
  • What is the fuel pressure?
  • Ware are the long term fuel trim values (LTFT).
good luck.
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