© DenLorsTools.com Summary: P0171 is a lean code which can indicate a vacuum leak especially if we are talking about a Ford. However, GM cars including Chevrolet, Pontiac, Oldsmobile, Saturn and others are not prone to having vacuum leaks. Questions regarding a lean code and poor gas mileage on a Pontiac Sunfire ( which is similar to a Chevrolet Cavalier) prompted this repair article. In this repair article we give advice on what to look for and how to properly check MPG (Miles Per Gallon).

EricG – Reader Comments & Questions

I have a 1997 Pontiac Sunfire with 134k mileage. Since I bought it about three weeks ago, its check engine has been on. Had it checked at Auto Zone and it kicked out P0171 code. Aside for the check engine light. It sometimes hesitates at stop lights, and burns through fuel pretty fast. My parents live maybe 3 miles away and when I drive to their house the fuel needle literally drops almost half a mark. I plan on checking the vacuum hoses and some other things this weekend, but do you have any thoughts other than that? Thanks in advance, you seem very knowledgeable.

DennisB

Eric, no common vacuum leaks come to mind on the type of car you have. It wouldn’t hurt to look for obvious vacuum leaks though. I would also make sure the MAF (Mass Air Flow) and air filter are clean. Look for air leaks between the MAF and the throttle body. This could be allowing un-metered air into the engine causing the code. I would also clean the throttle plates for the poor idle problem to see if that helps. There are other potential causes of lean codes, like exhaust leaks, the fuel pump could be going out or the injectors could be getting clogged. A fuel pressure test would be the next logical step if problems persist.

Additional Tips on Lean Conditions

1. Aftermarket air filters like K&N if over-oiled may cause damage to the MAF. The oil can contaminate the sensing wire altering its reading.

2. Exhaust leaks, even very small ones can cause lean codes. Outside air can get drawn in making the sensors pick up more oxygen content than what the exhaust actually has. I’ve found very small leaks at exhaust flanges before that have caused a false lean code.

3. Rubber or plastic intake boots that are between the MAF and the throttle body on the engine can have small cracks or tears. Worn motor mounts can sometimes accelerate the wear on these boots. Hesitation upon take off from a stop may indicate the motor moving excessively due to a worn motor mount, increasing the gap in a torn air intake boot while accelerating.

4. Listening for vacuum leaks is a good initial step that sometimes takes you right to the source of the leak. Just be aware that power steering pumps usually sound just like a vacuum leak, don’t be fooled.

Gas Mileage

Watching the fuel gauge needle move while driving doesn’t tell you how many miles per gallon you’re getting. The needle may also bounce a little while driving, further altering any perceived notion about how good or bad the gas mileage is. The gas needle on some cars stay at full for an extended period of time when first filled up. Some car’s gas gauge needles travel slower towards the top end of the gauge as compared to when the needle goes below the 1/2 way mark. The only accurate way to check the gas mileage on your car is to do it manually. Fill the tank and write the odometer miles down. On the next fill-up, calculate how many miles you’ve traveled and divide by the gallons.

More Tips Fuel Readings and Mileage

1. Erratic fuel gauge fluctuations are almost always due to a bad fuel sending unit. This also goes for a fuel gauge that doesn’t work at all. The second most common would be the gauge in the cluster.

2. Airing up the tires is the obvious thing to do when looking to increase the MPG’s. Ever try pedaling a bike with low tires?

3. Stay away from aftermarket air filters that claim they will increase your gas mileage. If you want an aftermarket filter because it looks cool or because you can clean the filter rather than replacing it, go for it. But, no matter how the air is filtered, the car’s computer is going to adjust the fuel trim based upon the amount of air that is entering the engine.

4. Hydrogen generators are a huge waste of time and money. You are better off spending your money on a tune up.

Be sure to check out our other articles on lean codes for more information.