How to Diagnose Engine Backfires

Updated on September 23, 2017
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Backfires can be hard to diagnose.
Backfires can be hard to diagnose. | Source

Engine backfires can be produced by a vacuum leak, bad timing, problems in the ignition system, a faulty sensor, an exhaust leak, or some other system fault. Sometimes, locating the source won't take you much time, other times it can prove difficult.

The backfire is produced when unburned fuel ignites inside the intake or exhaust manifold instead of a cylinder. You can hear the combustion as a mild, cough-like ignition or a loud bang. A strong explosion, though, can cause severe damage, like cracking an exhaust manifold.

Although a backfire can be the fault of a system malfunction, this malfunction may come from maintenance neglect. Have you forgotten to service one or more engine systems lately? Start with those if you see them mentioned here. This strategy will make your diagnostic easier and your repair faster.

Yes, many backfires can be prevented with a little maintenance as suggested in your car owner's manual or the vehicle repair manual. If you don't have your repair manual yet, you should get one. Get an inexpensive Haynes aftermarket manual at Amazon for your particular vehicle make and model. These manuals include many maintenance tasks, troubleshooting strategies and repairs you can do at home.

Also, remember that some faults will trigger the Check Engine Light (CEL). But, even if you're dealing with an intermittent backfire issue and you haven't seen the CEL coming on, scan your car's computer memory with an OBD-2 scanner (like this Ancel scanner) to get diagnostic trouble codes (DTCs) anyway. The computer may have one or more pending codes that could give you a clue about a potential system involved in the problem. DTCs are very helpful in locating fault sources.

This article will describe the causes of backfires, starting with the most common and progressing towards the more rare causes, to help you speed up your diagnostic.

Index
I. Ignition System Problems
II. Injection System Problems
III. Engine Sensor Problems
IV. Problems with the EGR System
V. Ignition Timing Problems
VI. Exhaust System Leaks
VII. Valve Train Problems
A bad ignition coils and other ignition system components can cause backfires.
A bad ignition coils and other ignition system components can cause backfires. | Source

I. Ignition System Problems

Any problem that upsets an ignition system spark can cause backfires and other engine performance problems.

An engine needs a few thousand volts of potential for the spark to jump the gap between the center and side electrode at the tip of a spark plug.

Lack of proper ignition system maintenance can cause problems that ultimately lead to backfires.

For example, a spark plug gap may widen after the plug has been in operation for months and make it difficult for the spark to jump. Also, carbon buildup may isolate the electrodes, preventing proper combustion. Unburned fuel then is allowed to pass into the exhaust system where it can backfire.

The same type of problem can be caused by a worn out or damaged spark plug wire. A bad wire will make the spark's travel difficult, weaken the spark, or simply push it into an adjacent wire or to ground, resulting in a backfire.

The same result can come from a faulty ignition coil, distributor or rotor and cause a more repetitive backfire.

Car owners usually forget to check the ignition system at the recommended manufacturer schedule.

If necessary, consult your car owner's manual or repair manual.

Get Help From Your Car Computer

Even if your Check Engine Light is not lit on your instrument panel, scan for possible pending diagnostic trouble codes that can guide you in your diagnostic.

A clogging fuel injector will lean the mixture and lead to backfire.
A clogging fuel injector will lean the mixture and lead to backfire. | Source

II. Injection System Problems

Backfiring problems can also originate in the fuel system.

Usually, when an injector clogs or wears out, causing the air-fuel mixture to lean, the combustion process weakens and fails to properly burn the fuel. Too much unburned fuel then enters the exhaust system where the fuel ignites with a loud bang.

Check the fuel injectors for proper operation. You can use a mechanic's stethoscope and a digital multimeter to check the operation of the injectors in your vehicle.

Also, a lean fuel mixture can be caused by a bad fuel pressure regulator, fuel pump or even a clogging fuel filter. Consult your vehicle repair manual, if necessary, to check the fuel system and maintenance items in your particular model.

Check your MAF sensor for dirt and operation.
Check your MAF sensor for dirt and operation. | Source

III. Engine Sensor Problems

An engine sensor malfunction can also lead to backfires.

Take for example a bad mass air flow (MAF) sensor. The engine computer uses this and other sensors to compute the amount of fuel to inject into the engine according to operating conditions. If a sensor fails to send the correct signal, the computer may deliver too little fuel, creating a lean fuel condition.

Usually, a bad MAF sensor will trigger the Check Engine Light (CEL). And many other emission related sensors the computer rely on can fail as well, causing a CEL to come on.

Just remember that even if your computer points to a possible bad sensor, make sure to test the sensor before replacing it. Many times a fault in a different component or part can make a sensor look bad. For example, a duct in the air cleaner assembly with a tear or not properly connected may cause unmetered air to enter the engine and make the computer 'think' that the MAF sensor has gone mad, causing all kinds of problems.

Check and replace the EGR valve as necessary.
Check and replace the EGR valve as necessary. | Source

IV. Problems with the EGR System

The exhaust gas recirculation (EGR) system, usually the valve, can cause all kinds of trouble when not functioning properly. Backfire is one of them.

The EGR system is designed to reintroduced a measured amount of exhaust gases back into the cylinders for reburn. This reduces combustion temperatures and dangerous emissions like NOx (oxides of nitrogen).

The EGR valve opens when engine speed increases and closes when engine is resting at idle. However, a stuck-close valve won't allow exhaust gases to recirculate back into the combustion chamber. Sometimes this happens when carbon buildup blocks valve passages, a vacuum leak fails to operate the valve (on vacuum operated valves), an electronic control sensor fails, or the valve itself is damaged.

This condition will increase cylinder temperature during engine operation, causing a flame to expand rapidly through an open intake valve and burn the fuel coming in, resulting in a backfire. The fault is similar to the one shown by a carburetor with a faulty accelerator pump. Watch the video at the bottom to see a backfire taking place through the intake manifold, possibly from a vacuum leak.

If you haven't checked the EGR valve in the last two or three years, do so now. Make sure the valve still opens and that passages are not blocked. Removing the EGR valve for inspection may be easy or difficult, depending on your particular vehicle model. But in most cases you can do this maintenance task at home. Check for carbon buildup along the valve passages and the intake manifold where the valve mounts.

A worn out timing belt can upset the combustion process and lead to backfires.
A worn out timing belt can upset the combustion process and lead to backfires. | Source

V. Ignition Timing Problems

Gasoline engines need to fire spark plugs on time to properly ignite the air-fuel mixture inside a cylinder. On time means that sometimes the spark needs to be fired ahead in advance or retarded depending on engine speed and load conditions.

But firing a spark too much in advance before a cylinder compression stroke can cause engine knocks or pings.

On the other hand, retarding the spark too much after the cylinder compression stroke will not only cause engine to lose power and waste fuel, but can also lead to exhaust backfire (aka afterfire). This is because flames from the combustion can jump through an open exhaust valve and cause unburned fuel to explode in the exhaust system.

Usually, this condition causes the engine to overheat and, with high enough temperature, seriously damage the exhaust manifold.

Consult your vehicle repair manual to check engine timing. On older vehicle models equipped with a distributor you can also adjust timing as needed as part of a tune-up procedure.

On newer vehicle models, though, the engine control module (ECM-car computer) usually controls ignition timing. You still can check the timing but you can't adjust it yourself. If timing is not correct, consult your vehicle repair manual or have your engine checked at a car shop to correct the problem.

A backfire can originate in a cracked exhaust manifold and or leaking gasket.
A backfire can originate in a cracked exhaust manifold and or leaking gasket. | Source

VI. Exhaust System Leaks

Exhaust backfires can also be the result of air leaks in the system.

As oxygen content increases, it causes partially burn or unburned fuel entering the system to ignite loudly. The extra oxygen may come through a leak in the exhaust manifold gasket, an exhaust pipe sealing ring, or a damaged pipe.

Also, some vehicles models are equipped with an air injection system. If the system is injecting air when it shouldn't or has stopped working, it may cause unburned or partially burned fuel in the exhaust gases to backfire. Check the system for proper operation, specially the check valve. Consult the repair manual for your vehicle make and model.

A sticking-open valve can cause a backfire.
A sticking-open valve can cause a backfire. | Source

VII. Valve Train Problems

To let air-fuel into the combustion chamber and remove exhaust gases, intake and exhaust valves need to open and close at their proper time

But valve train components can also fail and cause a number of problems.

For example, a valve spring may weaken or brake, causing a valve to fail to properly seal the combustion chamber as needed or cause it to remain partially open; carbon build up and other substances may caused a valve to stick open as well. Any of these conditions can allow flames to burn fuel back in the intake manifold, throttle body, or exhaust manifold, leading to a backfire and other problems.

Valve issues may also involve poor fuel economy, rough idle, stalling, high oil consumption, hard starting, and exhaust smoke.

Valve problems of these type tend to create a more consistent backfire condition.

You may diagnose valve problems at home with the use of a compression gauge or a vacuum gauge. These tools can help reveal the mechanical condition of your engine. If necessary, consult your vehicle repair manual.

Diagnosing engine backfires can be difficult at times. But, as you've seen, the root cause of a backfire can be something as simple as a clogging fuel filter or something more complicated as a sticking valve. So paying attention to regular maintenance items in key systems like fuel, ignition and emissions can go a long way in keeping your vehicle running in top shape. And whenever you encounter a backfire, this guide will help you trace the culprit.

Questions & Answers

  • I have a 2001 Cherokee Sport Straight 6. We put in a used jasper motor and now after a few minutes when the engine starts to warm up, the engine starts to rev up to 2000 RPMs. About 30 defends after that you hear the popping from the gas exploding. Too many backyard mechanics with too many choices. I've heard computer or maybe IAC VALVE. Do you have any ideas?

    Check the ignition system, connections and modules.

  • I have a 1988 GMC Candida 2500 with a 4.3 l motor. I just did a tune up on it, and afterward, it ran like it's supposed to. At least it ran, but now it won't start. All it does when I try to start it is backfire, and with force. I'm scared to try to start it because I don't want to damage anything. What should I do?

    If you didn't change the distributor cap, check it for cracks, also check or change the spark plug wires. If the ignition system parts are fine, check the timing, it may be too retarded.

  • I have a 95 Wrangler that had a bad ignition coil, causing it to hesitate at high RPM. After changing the coil, plugs, wires, cap, and rotor. I now have an intermittent backfire at high RPM. What do you think I should do?

    You might want to check the fuel system, possibly a weak pump, clogging fuel injector or filter. Other possibilities, a dirty MAF.

  • I have a 2006 Yamaha 1100 that is backfiring in the carburetor. Do you have any idea why?

    I don't know much about bikes, but have you played with the air injection system? Try to disable it and see if it stops the backfire. You may need to have it serviced.

  • I have a 1979, gas powered, Datsun pickup. Just completely rewired, carburetor jets, etc. cleaned six months ago. Fuel filter & fuel pump ok. Timing chain, chain tensioner, rings & valves replaced three years ago. Distributor cap looks OK & coil is only a few weeks old. The truck ran fine, until I hit a 2' deep pothole, and it simply died. Won't start, & backfires during the starting procedure. What is the cause of the problem?

    It's possible the starter or part of the circuit got damaged. Possibly the carburetor will need adjustment as well.

© 2017 Dan Ferrell

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    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      2 months ago

      Hi Kyle,

      Replacing the plugs, boots and seals was probably a good idea; still there might be other problems. You might need to do some tests. Check this post and see if it helps:

      https://axleaddict.com/auto-repair/Engine-Rough-Id...

    • profile image

      Kyle 

      2 months ago

      I have a 04 chevy trailblazer ls before i changed the plugs and rubber boots and seals on the coil pack it had lack of power where i would accelerate and it wouldnt go anywhere for a couple seconds and at a dead stop the car would sit and vibrate after i changed all that stuff the car runs rough at an idle it will sit and shake the last time i heard and felt that noise on my 92 ranger i replaced the coil packs and everythin worked fine but ive been hearing a lot of different things it could be i just need more advice all the rubber boots and seals were cracked light brown it looked like rust almost can anyone help me

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