Test Fuel Pump Pressure VideoThere are two kinds of backfires, one is produced from the engine air intake and the other is produced from the tail pipe of the exhaust system. The cause for a backfire condition can vary depending on the malfunction of a particular system. Some backfires can be extreme enough to cause damage to related components. Anytime a backfire is observed immediately diagnose and repair as needed. If the engine is running rough it can produce a backfire. A backfire occurs when there is an imbalance in the air to fuel ratio required for your vehicle to operate properly. If the fuel mixture is too lean (not enough fuel) you may have a backfire in the intake manifold gasket, or too rich (too much fuel) you may get a backfire out of the exhaust system. It is important to observe the location of the backfire. The information below is separated into two sections; the first section contains possible causes for backfires in the engine intake. The second section contains possible causes for backfires in the exhaust system. (Note: if the engine is misfiring and the engine backfires while the engine is under load please visit, engine misfires. Car Repair Information Guide - Backfire In The Engine Intake System
Step 1 - The ignition system in your car is designed to operate at a specific voltage and amperage output. If this voltage/amperage is forced to "spike" due to faulty or inferior tune up parts it could cause the system to "crossfire". A crossfire condition can allow the electrical discharge for a particular spark plug circuit to transfer to a alternative cylinder. Depending on were the cylinder is in the combustion process it can produce a backfire in either the intake or exhaust systems. Always replace ignition components on a regular mileage schedule and use high quality replacement parts. Step 2 - Your engine's fuel system is designed to run on a constant fuel pressure. If this fuel pressure drops it can cause a lean mixture which in turn causes the engine to backfire. There are several reasons this can take place such as a faulty fuel pump, clogged fuel filter or a malfunctioning pump or a fuel pressure regulator. The best way to check for this condition is to perform a fuel pressure test. This is done by locating the fuel pressure test port and attaching a fuel pressure gauge and reading the fuel pressure while the engine is running. Step 3 - Engine ignition timing is to be set at a specific degree in relation to the crankshaft of the engine. If this timing becomes mis-aligned it can cause low power, poor gas mileage, engine detonation (pinging) and backfiring. To check the engine timing locate the number one cylinder of the engine. This number one cylinder will be the forward most cylinder of the engine. On straight 4 and 6 cylinders engines it is pretty obvious, but on "V" style of engines like the V8 it is a little more tricky. The cylinders heads on every "V" style of engine are offset from one to the other. One cylinder head is more forward than the other. The most forward cylinder head is the side of the engine where the number one cylinder is located. The forward most cylinder is the number one cylinder. Next, locate the crankshaft timing marks, most engines timing marks are at the front of the engine on the harmonic balancer. Near the balancer there is a small pointer or scale to align the balancer mark to. When the engine is running this gauge or pointer should align with the mark on the balancer. When the engine is running the timing light will strobe and illuminate the balancer mark as it spins. To adjust the timing loosen the distributor and turn slightly one way or the other to achieve the desired setting. Once an adjustment has been made re-tighten the distributor hold down bolt or bolts. Some engine's timing marks are located at the rear of the engine on the flywheel. Before you begin start and run the engine until warmed to operating temperature. Connect the positive and negative leads of a timing light to the battery. Observe the timing tag either on the engine or engine compartment for the timing specification and procedure. If the specification and instruction cannot be found consult a car repair manual. Step 4 - Distributor style ignition systems are designed to operate at high voltage. If moisture is present inside of components like the distributor cap it can cause internal ignition crossfire. This crossfire condition can allow the electrical discharge for a particular spark plug circuit to transfer to a alternative cylinder. Depending on were the cylinder is in the combustion process it can produce a backfire in either the intake or exhaust systems. To check for this condition remove the distributor cap and inspect for moisture. If moisture is present dry components thoroughly and reassemble. (some cars are not equipped with a distributor)
Step 5 - The engine in your car is designed to run on a specific ratio of air to fuel (14 to 1). If this ratio increases by withholding fuel it can cause the engine to back fire through the intake system. The number one reason for this condition is a plugged fuel filter. If your fuel filter has not been changed for over 30,000 miles it probably needs to be changed. Remove the fuel filter and inspect if dirt is observed replace the filter. (note: some cars fuel filter is located in the gas tank and is not serviceable, these cars are quipped with a lifetime filter). Next the ratio can be incorrect because of an air intake boot that has failed or other vacuum leak. If a failure is detected replace part with new and recheck.
Failed Air Intake Boot