Fuel and air problems are easy to describe, but difficult to diagnose. If the engine is not getting enough of one or both, it will start to run poorly and may vibrate or have reduced power.
LOW OR NO AIR DELIVERY
A modern engine really needs to know exactly how much air is drawn into the engine, how dense it is, and what temperature it is to allow for efficient combustion.
Clogged Air Filter
A clogged air filter means the engine can’t get enough air, which causes it to reduce the amount of fuel delivered and can result in the engine losing power. Check your filter on a regular basis.
Dirty MAF Sensor
The MAF contains a small wire that heats up to measure the density of the air going into the engine. If the wire gets dirty, it will not read properly. The MAF can be cleaned, but you need to buy a special cleaner from the auto parts store that’s made specifically for cleaning the sensor.
Faulty Throttle Position Sensor
The throttle bod is a mechanical plate, so it usually doesn’t have problems, but the sensor that tells the computer how open it can fail. The TPS may have to be calibrated and aligned; if so, leave this replacement to a professional.
Faulty EGR System
The EGR valve can become clogged and fail. The engine may run rough, stall, or not run at all.
Clogged or Sticking PCV Valve
Like the EGR, the PCV valve can become clogged or stuck. If this happens, the engine will start to lose power or not accelerate, or the pressure can build up in the crankcase and cause seals to fail. Checking the PCV is easy and quick, and in most cases relatively inexpensive to replace.
Clogged or Failed IAC
The IAC allows a small amount of air to bypass the throttle plate at idle. If the valve fails or clogs, the engine may come off acceleration slowly, or it may stall and come down too fast. If it is closed, the car may not start and run at idle.
LOW OR NO FUEL PRESSURE
When the engine starts losing fuel pressure, the injectors can’t deliver enough fuel to allow the engine to run smoothly. Many times the engine will start vibrating if it starts to lose power.
Loss of fuel pressure can happen at any point in the fuel system—the screen inside the fuel tank can be clogged, the fuel pump itself can fail, the fuel filter can be clogged, the lines can be blocked, or the fuel injectors can be dirty.
Clogged Fuel Filter
If your car has a replaceable filter, it can be checked and replaced relatively inexpensively. Some newer cars do not have a replaceable fuel filter, as it is part of the fuel pump assembly mounted within the gas tank of the car.
Faulty Fuel Pump
Some fuel pumps use plastic gears, and running the incorrect fuel can destroy the pump by dissolving the plastic parts. This can happen when you try to run alcohol-based fuels in a car not designed for them.
The fuel pump may also stop running because it wears out, or the electricity running the pump may fail. There are usually a fuse and a relay powering the fuel pump, and these go bad more often than the pump does.
Clogged Fuel Injector
The fuel injector has a little needle opening that moves back and forth. Small particles of dirt occasionally get past the filter and may build up on the injector, or poor fuel can gum it up and prevent it from working. Contaminants can also partially block the opening and prevent the fuel from spraying out properly (atomizing), resulting in the fuel not mixing with the air as it should.
One preventative maintenance step you can take is to run a bottle of fuel injection cleaner through your engine every time you have the oil changed. However, if you have a clogged injector, you will have to use a professional-grade cleaner, or have it cleaned by a professional.
Open Gas Cap
One of the most common computer codes comes from leaving the gas cap off of the gas port, which means the vapors from the tank are not being drawn into the engine through the EVAP system.
Inertia Fuel Switch
Some carmakers use a switch to shut off power in a wreck. If your car gets jostled, the switch can trip and close. If the car won’t start, try resetting the switch. Some carmakers use oil pressure or other means to shut off the fuel.
Clogs and Leaks
Inspect your fuel lines for leaks on a regular basis. Fuel lines are made out of solid and flexible materials, and both can fail. Flexible lines wear out and hard lines can be kinked or rusted.
Since your car relies on sensors to know how much fuel is being delivered, the car won’t run properly if a sensor goes bad. When a sensor goes bad, the computer may send an error code that the sensor is out of range.