Car runs rough, Check Engine Light on, and bad fuel mileage

My car has developed a few problems. First, it has been running with a moderate level of roughness; not extremely bad, but annoying. Also, the CEL light started flashing intermittently. At times it is a solid light and other times it flashes. Yet another problem is that I am getting bad fuel mileage. I am thinking that my car might be misfiring, but I have not pulled the plugs yet, so I’m not sure about that. Could you please give me any advise as to what I need to do to repair my car?
Experienced mechanics share their insights in answering this question :
Hi there. I would start first by checking for codes that are causing the Check Engine light to come on. This will direct diagnosis and let you know if it related to fuel, spark, compression, or another system. Have a professional mechanic, like YourMechanic, diagnose the Check Engine Light and correct the issue. Proper addressing of the Check Engine Light will likely solve the rough running issues as well.

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Possible causes

If you had filled up the tank without turning the engine off, the sensors would`ve still been operational then, and would`ve detected a loss of pressure. Other factors that can cause pressure loss and trigger the check engine light are a loose fuel cap, a bad fuel-cap gasket, or a damaged vacuum hose.

Bottom line: Yes, if you see a pattern in which the engine light comes on during periods that your tank is low, then turns off right after you fill it (and tighten the gas cap!), one of the above issues is probably going on.
The check engine light should go off after you drive for several minutes if the light was caused by a loose gas cap. Pay attention to the dashboard after the check engine light experience. If you find that the light keeps coming on, and goes off again once you tighten the gas cap, then your gas cap is too loose.
It can cause your engine to misfire

If you run low on gas, your fuel pump may start to suck in air. Consequently, your engine won`t get the fuel it requires and will begin sputter and misfire. This is a sign that your vehicle will soon stop working and that you should look for a safe place to pull over.

The reasons for a check engine light turning on can range from something minor like a loose gas cap to something more serious like a faulty catalytic converter. It could even be a sign of internal engine failure. Ignoring the problem can turn a potentially quick fix into a costly and time-consuming repair.
Bad Fuel Pump, Fuel Pressure Sensor Can Trigger `Check Engine` Light, P0087 Code. Your car`s “Check Engine” or “Service Engine Soon” light can flash on for more than 1,400 different reasons.
To make sure the check engine light does not reappear, it`s recommended that you drive your car 30 to 100 miles. This enables the vehicle`s “Drive Cycle” to reset, as the various sensors need time to recalibrate.
Many manufacturers recommend tightening the fuel cap until it clicks three times, which is just a random number to make sure the cap is tight. At this point — even though the engine will run fine — it would be best to bring the car into have it looked at.
A clogged or failed exhaust gas recirculation or crankcase ventilation valve or faulty oxygen sensor can send the wrong signals to the computer and cause misfires.
Dirty fuel injectors may cause your vehicle`s engine to misfire. This problem makes the motor feel as though it is sputtering — sending vibrations through the car. Such misfires can happen when a fuel injector problem mixes up the delicate balance between fuel and air entering the engine.
Here are some the main causes for a sudden drop in gas mileage: Bad oxygen sensor and air filters-in all forms will affect the fuel mixture and your fuel efficiency. Bad or misfiring spark plugs-will result in poor performance and subsequent lack of power will cost you fuel efficiency.
A leaky, loose, or missing gas cap may also cause the Check Engine light to come on. Whatever the cause, if your garage has started to smell strongly of gasoline, call your NAPA AutoCare Center before you attempt to start the vehicle.
Here are some the main causes for a sudden drop in gas mileage: Bad oxygen sensor and air filters-in all forms will affect the fuel mixture and your fuel efficiency. Bad or misfiring spark plugs-will result in poor performance and subsequent lack of power will cost you fuel efficiency.
Clogged or Damaged Fuel Injectors

One of the most common culprits for a drop in fuel efficiency is dirty fuel injectors. Fuel injectors are the nozzles that spray fuel into each engine cylinder. A fuel injector`s spray pattern must be very precise to properly mix with air and combust inside the engine.

Many of the main causes for poor gas mileage are mechanical and include things like improper air flow, faulty injectors or bad oxygen sensors. “Your vehicle needs both fuel and air to give the best fuel mileage,” says Nicole Miskelley, manager at PMR Auto & Diesel Repair in Marion, Illinois.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

Car runs rough, Check Engine Light on, and bad fuel mileage – 2005 Suzuki Verona
ANSWER : Hi there. I would start first by checking for codes that are causing the Check Engine light to come on. This will direct diagnosis and let you know if it related to fuel, spark, compression, or another system. Have a professional mechanic, like YourMechanic, diagnose the Check Engine Light and correct the issue. Proper addressing of the Check Engine Light will likely solve the rough running issues as well.

Check engine light refuses to go off after engine replaced
ANSWER : Hello there, you have a long series of issues going on with your 2013 Hyundai Sonata. If you keep getting a repeating oil light on than there is likely a fault in that system.

Since this fault has lasted after the engine was replaced it could be a oil pressure sensor, wiring harness fault, ECU fault, oil pump, oil strainer clogged, or low oil pressure.

To diagnose this issue properly you need a complete vehicle inspection to start from square one and identify the root cause of all this.

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ANSWER : Hi there. It is possible that the battery cable came loose or the wires to the oil pressure sensor has came loose or failed. Check the battery cables and make sure that the terminals are tight and clean. Check the wires to the oil pressure sensor and make sure that they are connected and not broken. If everything looks good, then the oil pressure sensor may have failed. Plus, check the computer for codes and see what is stored. This will lead you to the part that could have failed. I recommend replacing the oil pressure sensor then checking to see if the lights will go out. If you need further assistance troubleshooting your vehicle, then seek out a technician, such as one from Your Mechanic, that can assist you with the warning lights that came on.

replaced bad coil pack and new coil not sparking car starts but runs rough engine light on blinks sometimes
ANSWER : Hello and thank you for your questions. If your engine was rebuilt about 10 months ago, the question is to determine if the management system has been installed as well. With the symptoms that you are having, it sounds like the coil packs are old and in need of replace. You replaced numbers 1 and 4 and now number 6 is not firing. I recommend replacing number 6, 2, 3, and 5. This will ensure that you have all 6 coil packs that are new.

Just because the coil packs are okay during a test doesn’t mean that they are functioning correctly. Coil packs on Buick vehicles from 1998 – 2005 last between 150,000 to 200,000 miles. I also recommend having your fuel injection system flushed. Unless you have new injectors, the old injectors could cause a cylinder to fail, even though it is working correctly. These symptoms can cause the engine to have a rough idle and rough running during conditions. If you need assistance troubleshooting the rough running condition, I recommend having a professional, like one from YourMechanic, inspect the car and move forward with repairs.

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ANSWER : Hi and thanks for contacting Your Mechanic. If sometimes your vehicle will start with the new alternator and sometimes it won’t, then there is a loose connection in your charging system. Check all of the wires to the alternator and to the battery. Make sure that the battery cables are clean and tight. Make sure that the wires to the starter are clean and tight. A loose ground to the ignition system will cause the vehicle to not crank over. Check all of the ground wires from the battery to the starter. Make sure that they are clean and tight. If you need further assistance troubleshooting your no start situation, then seek out a technician, such as one from Your Mechanic, that can assist you.

Can an open gas cap which causes the Check Engine Light to come on cause the car to not start and the ECM to appear bad?
ANSWER : Yes, there are a lot of things a bad gas cap or open gas cap can affect. If you don’t have a gas cap then you may be experiencing issues with debris getting into your tank or fuel filter and affecting the drive performance. Since the ECM is a costly repair, you may want to have another independent technician diagnose the vehicle’s loss of power. They will be able to confirm the cause of the issue or advise you on another fix.

2006 Ford Mustang has check engine light on
ANSWER : There are countless potential causes for your illuminated check engine light. The mechanic manually turned off the light, but it came back on because the problem has not been fixed. The first place to start is retrieval of the stored diagnostic trouble codes (DTCs). These codes serve as a starting point for any further diagnostics a mechanic may perform. Codes will be stored anytime the check engine light is on. Since the first mechanic was unable to fix your concern, I recommend giving the professionals at YourMechanic a call. We offer expert check engine light diagnosis and repair.

Car goes into limp mode when the check engine light and VSA light comes on
ANSWER : The lights that you have on and the vehicle going into fail safe mode is an indication you have a major system failure that will require computer to be tested for codes first. There are many possible codes it can be so the additional testing will be needed to isolate the cause of the problem.