Reader Question: Hi Austin,
We have a 1999 Toyota Corolla with 92k miles on it. It used to have very good gas mileage in the past 3 years, but during the last couple of months, it declined from 35 MPG to 26 MPG gradually. While my driving habit, route, places traveled, everything are still the same. We wonder what is wrong with it.
I have replaced the air filter last month. I checked the gas cap was still tight, holding the pressure.
Spark plugs were replaced at 82k miles. Last year, I replaced all the brake shoes, pads, drained and refilled radiator (replaced radiator cap), and brake fluid. The PCV valve was replaced 6 months ago.
I got all these done in a shop myself, when doing an auto maintenance evening class. They have a retired mechanic there, to help us if we are stuck in changing some parts.
However, they will not help us to do any diagnosis. Therefore, I am hoping to get some clues about which part could be faulty to look for.
Would you please suggest what might be causing the problems? Would oxygen sensors, spark plug wires or canister failed to cause this low gas mileage?
Thank you so much for your help. Your website is great, really helped a lot of us, as a non-mechanic!
Are you sure about the initial 35 MPG figure for this vehicle? I would expect the normal MPG to be about 26, which is really great!
If you feel confident that the mileage has substantially decreased lately then I would start from scratch and recheck the below mentioned items that can cause a change in MPG.
1. Change in driving habits: You probably do not drive exactly the same way each day, and MPG will vary daily (you seemed to have ruled this out, but I just want to mention it anyway).
2. Weather conditions: Strong winds, drastic changes in outside temperature can affect MPG
3. Tire pressure: Could you have one or more tires with a slow leak – a huge factor in reduced MPG is under inflated tire pressure
4. Proper engine oil weight: Too thick of engine oil will reduce MPG, if you had the oil changed recently you might want to make sure the proper weight was used (probably 10W30 or 5W 30) if in doubt you might want to drain and refill the oil with the proper weight just to rule this out.
5. Air filter: Dirty air filter will reduce MPG and engine performance.
6. Different grades and sometimes just using different brands of fuel can change MPG figures and overall engine performance. I recommend getting fuel from the same station or at least the same brand name whenever possible.
You also might experiment with different grades of fuel from your station as well i.e. premium versus regular unleaded.
7. An engine misfire problem due to a worn out spark plug, spark plug wires, distributor cap and ignition rotor. Since you installed your own spark plugs, I would remove the plugs and double check the spark gap with a spark plug gap tool. Sometimes the gap can close during installation and this close in the gap can cause decreases in engine performance and reduce fuel MPG.
8. Oxygen sensors that are worn out or defective can cause poor engine performance and reduce MPG and will usually cause the check engine light to come on. The sensor is cheap and easy to replace and some mechanics replace them regardless of condition as part of regular routine maintenance.
9. An engine that is running at a colder operating temperature than it is expected to run can affect MPG. 210 degrees Fahrenheit is the normal operating temperature for a gasoline fuel injected engine. If the wrong thermostat is installed the engine will not reach 210 degrees, and a decrease in engine performance and MPG will result.