Fuel filter, throttle body and valve cover gasket problems

What does a fuel filter catch?

AA1Car, an automotive repair library, calls the fuel filter the fuel system's primary line of defense against dirt, debris and small particles of rust that flake off the inside of the fuel tank. AA1Car says a mesh filter sock on the end of the fuel pickup tube inside the tank helps keep out the big pieces of dirt and rust, but does not keep out the smaller particles.

AA1Car says contaminants that the filter does not trap can do damage further along. In fuel-injected engines, it says, debris can clog the injector inlet screens and starve the injector for fuel. It also says that debris inside an injector can wear or jam the pintle valve and seat. For size, AA1Car says many filters for fuel-injected engines trap particles as small as 10 to 40 microns.

Where the filter sits depends on the vehicle. AA1Car says that on most fuel-injected vehicles it is found under the vehicle in the fuel line from the tank to the engine, or on the firewall in the engine compartment.

It adds that some returnless systems place it on top of the fuel tank as part of the fuel pressure regulator assembly, and it tells readers to refer to the owner's manual for the exact location. One Kia owner's manual describes a petrol model with a lifetime fuel filter integrated with the fuel tank. Kia says regular maintenance or replacement is not needed on that filter, but that this depends on fuel quality.

What does a clogged fuel filter do?

AA1Car says the filter media eventually becomes clogged with debris and creates a restriction. According to AA1Car, a filter that is not replaced before it plugs can cause major driveability problems, such as hard starting, lack of high speed power and stalling.

One Kia owner's manual, for the petrol model with the lifetime in-tank filter described above, gives fuel flow restriction, surging, loss of power and hard starting as examples of matters that call for the filter to be checked or replaced.

Maintenance advice varies by maker. AA1Car says some vehicle manufacturers call for replacing the fuel filter every 50,000 miles, others say to replace it only as needed, and some make no recommendation at all. AA1Car also says that on most fuel-injected engines, extra fuel pressure from the fuel rail is routed back to the tank, so if the fuel is dirty, abrasive particles can recirculate through the fuel pump again and again.

AA1Car says this accelerates pump wear. Separately, AA1Car's hesitation article lists a restricted fuel filter next to a weak fuel pump, dirty fuel injectors and a leaky fuel pressure regulator as fuel delivery problems. The same article lists vacuum leaks and ignition misfires among other causes of a stumble, and it says a scan tool that shows sensor data and fuel trim values can help diagnose a hesitation problem.

What does the throttle body do, and what builds up inside it?

Wikipedia describes the throttle body, in fuel-injected engines, as the part of the air intake system that controls the amount of air flowing into the engine in response to the driver's accelerator pedal input. It says the throttle body is usually located between the air filter box and the intake manifold, and that the largest piece inside it is the throttle plate, a butterfly valve that regulates airflow. On cars with electronic throttle control, Wikipedia says, an electric actuator controls the throttle linkages and the accelerator pedal connects to a sensor that sends a signal to the engine control unit.

Some throttle bodies also contain a valve for idle airflow. Wikipedia says throttle bodies may contain valves and adjustments to control minimum airflow at idle, and that there is often a small solenoid-driven Idle Air Control Valve that the engine computer uses to control the air that bypasses the main throttle opening so the engine can idle when the throttle is closed.

AA1Car describes the same part. It says the idle air control valve opens a small bypass circuit around the throttle, that more bypass air raises idle speed and less lowers it, and that the engine's computer commands the valve up or down when idle speed is outside its preset range.

On what fouls these parts, MechanicBase, an automotive repair site, says that even with an air filter on the car, dirt and soot from the turbocharger, the EGR valve and the crankcase ventilation get stuck on the throttle body valve. It says that after a while they cover the valve enough to affect engine speed.

In its throttle body service article, MechanicBase says the service involves cleaning the throttle blade and bore, and that carbon deposits on the throttle body can keep the right amount of air from being added. AA1Car says a buildup of carbon or fuel varnish deposits in the idle speed control valve can cause an erratic idle that surges.

What can a driver notice when a throttle body or idle air valve is dirty?

MechanicBase lists eight signs of a bad throttle body: a check engine light, rough idling, rough acceleration, misfires, a high idle speed, stalling at idle, poor engine performance and changed fuel consumption. It says the engine is most sensitive at idle, so symptoms of a dirty or bad throttle body show up there first. MechanicBase also says that a lot of soot and dirt on the throttle flap can reduce the air entering the engine and give a low idle speed, and that the idle can be low enough for the engine to die.

AA1Car says that if a vehicle idles erratically, with idle speed increasing and decreasing, the problem may be carbon or fuel varnish deposits in the idle air control valve. AA1Car also warns that if an engine idles too fast, too slow or stalls, the cause may not be the idle control system at all but a vacuum leak. It says an open or short in the idle air control circuit, or an idle speed out of range, will usually set one or more fault codes and turn on the check engine light, and that a scan tool plugged into the diagnostic connector is needed to read those codes.

On timing, MechanicBase says most manufacturers do not set a recommended interval for throttle body service, and that some people suggest every 75,000 miles. It says to consider the service when symptoms appear, and that if the car is running perfectly the service is probably unnecessary.

Why does a valve cover gasket stop sealing?

MechanicBase says the valve cover sits on top of the engine and that the gasket creates a tight seal between the engine and the cover. Apex Tech Nation, a technician training site, says the rubber gasket between the valve cover and the cylinder head compresses slightly under torque to create the seal, and that with heat cycles over time the rubber hardens and loses its ability to maintain that compressed seal. MechanicBase says the gasket becomes brittle and cracks, and that this can be caused by old age or by excessive heat in the engine.

AA1Car says engine oil leaks occur most often at the valve cover and oil pan gaskets, the timing chain cover and the front and rear crankshaft seals. It says that as an engine ages, heat can cause cork gaskets to harden and shrink, and can cause rubber gaskets and seals to harden and lose elasticity. According to AA1Car, after six or seven years of service an engine may start to leak oil, and the older the engine, the more likely it is to leak oil from aging gaskets and seals.

What can a driver notice when a valve cover gasket leaks?

Apex Tech Nation says a valve cover leak typically starts as a seep along the gasket rail and gets worse as the gasket continues to harden. AA1Car, writing about engine oil leaks in general, says the first sign of trouble is usually drips or stains under the vehicle after it has been parked overnight, and that dark brown or yellow liquid that feels slippery or greasy is probably motor oil. It also says that more often than not the oil just slowly seeps out, building up grease near the leak, and that because oil attracts dirt, greasy stains around or below gasket seams are a clue.

Sources also describe a burning smell. Apex Tech Nation says oil from a valve cover leak runs down the side of the engine and often contacts the exhaust manifold, where it burns and produces a burning oil smell.

AA1Car says oil can produce blue smoke and unpleasant odors if it drips onto a hot exhaust manifold or exhaust pipe. MechanicBase lists a burning smell and low engine oil as signs of a bad valve cover gasket, and it says running the engine without the proper amount of lubrication can cause the engine to cease and fail.

Sources also warn that where oil ends up is not where it starts. Apex Tech Nation says the oil stain under the car is not where the leak is, that airflow through the engine compartment blows oil backward and downward, and that oil may travel 12 to 18 inches from the actual source.

MechanicBase says oil leaks have many causes, so a leak is not necessarily a sign of a bad valve cover gasket alone. AA1Car says that if a driver suspects an oil leak, the oil level on the dipstick should be checked with the engine off, and that a low level suggests a leak. AA1Car adds that no additive will stop a major leak or repair a broken gasket.