Skip the auto shop - Our mechanics make house calls

Q: Poor mpg and a rough idle

asked by on

ok so i just bought this truck and that ran perfect other the it was only getting 13 MPG highway when a got it and i decided to do a tune up on it because i had no idea when the last one had been done. so i changed the oil and cleaned the air filter as well as change the spark plugs. originally i had gapped the plugs at .56 because that is what the manual said it should be and boy it sure ran like crap so i since gapped them at .46ish because that is what was told to me by the auto parts store. and since then i have experienced a really rough idle as if the MAF was going out but i didnt know if it was something that i messed up so i havent changed it. i tried changing the pcv valve which was recommended to me by a friend. that gave me an extra 2 MPG but it is still idleing way rough. the error code that it keeps throwing at me is that it is running too lean. what would you recommend me doing first?

My car has 194000 miles.
My car has an automatic transmission.

Hi There, There are a number of things that can cause poor fuel mileage and a rough idle. A faulty camshaft position sensor is one of them. The camshaft position sensor is a magnetic sensor that monitors camshaft speed to regulate ignition timing and fuel injection timing. It gathers and sends information about the car’s camshaft speed (and as a result the position of each piston) to the car’s electronic control module. This information is received by the computer, which then uses this data to further calculate the time of ignition and the timing of fuel injection required by the engine. When this is not working properly, you may experience a drop in power, poor idling, poor fuel mileage and problems starting the vehicle.

A clogged or dirty EGR valve may also cause poor fuel mileage. The EGR valve is a small component designed to allow the flow of exhaust gases into the intake manifold in controlled amounts. As such, it’s a simple valve that closes and opens as needed. The EGR valve has one single job to do, regardless of the system configuration, type of control and number of sensors: that is, to either open and direct exhaust gases into the combustion chamber, or to close and keep them from entering. Whenever you start the engine, the valve comes alive and waits in a closed position, blocking the flow of exhaust gases. Once the engine reaches operating temperature and speed increases, the valve, either through vacuum or electronic control will gradually open, allowing burned exhaust gases to enter and combine with the air-fuel mixture inside the combustion chamber. When the EGR valve is clogged or dirty, as they are prone to, it may not function properly causing the engine idle rough, stall at idle speeds, cause the car to get poor fuel mileage, or emit a strong smell of fuel due to excess hydrocarbons which also results in failing emissions tests. Should you need further assistance with diagnosing your problem, I would recommend having a professional from YourMechanic come to your location to diagnose your truck.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Takes a long time to crank .

I would ensure you are getting strong spark with a spark tester (available for a few dollars). Depending on the version of the 4.3L you have, they are known for having problems with the fuel system. For example, if you...

Q: All my warning indicator's are on and the car won't start 2008 Chevrolet Malibu

Hi there - the security system is intentionally invoked after a battery replacement. Turn the ignition key to the "On/Run" position, and leave it for 10-15 minutes in this position. The security light will go out, then you can start...

Q: Why does my oxygen sensor heater stay in not ready mode on my 2002 Cadillac Escalade

Hello. If you have performed the necessary driver cycles and have not lost battery voltage or reset the computer at any time then you either have a problem with the heater circuit of the sensor or the computer is failing....

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.