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How to Diagnose Engine Backfires

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Backfires can be hard to diagnose.

Backfires can be hard to diagnose.

Engine backfires can be produced by:

  • a vacuum leak
  • bad timing
  • problems in the ignition system
  • lean or rich air/fuel mixture
  • 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 damage an intake air temperature (IAT) sensor mounted in the intake manifold, a brake booster vacuum check valve, and even cause severe damage, like cracking an exhaust manifold or plastic intake 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.


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.

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.

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

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

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.

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 relies 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.

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 reintroduce 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 the 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. If necessary, check for vacuum leaks, including intake and exhaust gasket leaks, vacuum hoses, and air intake boot.

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.

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 the 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 damages the exhaust manifold.

Consult your vehicle repair manual to check engine timing. On older vehicle models equipped with a distributor, check for a cracked or damaged distributor cap, and check ignition timing. Replace the cap or adjust timing as necessary, using your vehicle repair manual.

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.

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 vehicle 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, especially 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.

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 break, 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 cause 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 types 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.

Engine Backfire Recap

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.

Whenever you are dealing with a backfire, try to fix the cause as soon as possible. A strong backfire can damage gaskets, sensors, valves, and even a plastic intake manifold.

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.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: I have a 1970 Chevy Monte Carlo. I've had the valves, cam, timing chain, distributor, and wires changed. The carb has been adjusted; timing is at 0, and the distributor is facing #1 cylinder. The car seems to be tuned correctly, but whenever I drive, it backfires. While idling, it sounds fine. Backfires only when driving. Do you have any thoughts?

Answer: Double check on the recommended valve lash setting for the new camshaft and check the pushrods. Those are the things that come to mind.

Question: I have a 1998 Chevy K1500.with a regular exhaust backfire. It has new aluminum distributor assembly, plugs, and wires. In trying to isolate which cylinder the issue is involved, the backfire disappears with the plug wire removed from the number 2 or 4 cylinder. Firing order is 1-8-4-3-6-5-7-2. Does this information point to any particular cause for the backfire condition?

Answer: Assuming the new plugs are properly gapped and new wires are good, and backfire wasn't there prior to installing the new parts, I'd be suspicious of the wires' routing.

In your firing sequence, 8 or 7 cylinder wires could be affecting 4 or 2, if they are running parallel to each other. Usually, they should cross. Try pulling these wires apart with the engine idling and see if this makes any difference. Also, try swapping wires and see if this changes the cylinder backfiring.

Question: My 1997 Mazda 626 DX 2.0, automatic, 4 cylinder has been backfiring and won't start. Since November 1st, I have changed the battery, negative cable, starter motor, ignition coil, coolant temperature sensor, spark plugs, wires, and other components. As of February 14th, it was running good. But when I went to a friend's and began to leave, it was backfiring and wouldn't start. Advice?

Answer: Try checking for trouble codes, even if the engine light is not On. Also, try disconnecting the exhaust pipe a little bit so that it can breath and try starting the engine. If it starts, the catalytic converter is clogged. Hope this helps.

Question: I have a 1975 Chevy Camaro with a 350 in it. My distributor was loose as hell which causes the car to catch on fire but I replaced all of the things that caught on fire and timed it more than once(carb, gaskets, distributor cap, and spark plug wires) and it’s still backfiring. I got brand new spark plugs. What would also be the cause of the backfiring?

Answer: Try setting the timing by ear, probably the marks don’t sync, and make sure the distributor doesn’t move from side to side.

Question: I have a 93 Prelude. I recently did the oil change and spark plugs. After I did that, the car backfires and smokes for a couple seconds then stops? What can cause this?

Answer: Make sure the spark plug wires are properly connected, and in the firing order. Check the distributor cap for carbon traces or cracks and test the ignition coil.

Question: I have a 91 Chevy suburban with the 5.7 L. I’m having issues with backfiring after the truck has warmed up. It will start while accelerating and will not let you push the gas without backfiring. I just replaced the fuel filter. And the charcoal can has water in it. What do I do next?

Answer: Check to see if there are any trouble codes in memory, probably pending codes. The canister, make sure it is water in it. Probably you’ll need to replace it. If it is fuel, possibly a problem with the emission system. Check fuel pressure. A lean mixture is possibly causing the backfire. This is usually due to a worn out fuel pump or a bad pressure regulator. Also, check the MAF (bad or dirty).

Question: I have a 1994 GMC k1500 5.7 when It is cold outside and I start it backfires out the exhaust and the RPMs go up when started then started jumping around. Have changed tops,ect, tested map sensor also changed Inc valve and ignition coil vacuum reads fine so no leaks there. Not sure where to look from here do you have any suggestions?

Answer: Seems the engine is sucking too much air -- too little fuel. The mixture is too lean. The cold temperature doesn't help to burn the fuel at startup. And then it ignites in the exhaust. There could be a problem with the air-injection system, a pump or valve perhaps.

Question: 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?

Answer: 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.

Question: I have a 1966 Chevy Suburban with a 327 4 barrel. I replaced the carburetor with brand new one. It is spitting back through the carburetor trying to start. I also changed the rotor and distributor cap and spark plugs. The wires are two years old and look good. Can it be timing? If so, how do you time it if it's not running?

Answer: There could be several possibilities, and yes, the timing could be one of them.

However, if the vehicle was running fine (proper ignition timing) before you installed the new carburetor, it may be a faulty accelerator pump discharge. Also, check choke operation.

Also, the choke valve initial clearance might be too large.

You may need your vehicle repair manual to check or make adjustments.

Another possibility is the fuel mixture is too lean.

Question: I have a 2006 Ford Expedition with the 5.4 Triton. My check engine light came on, and it gave me the P0300 code which is multiple misfire code along with four other cylinders misfiring. When I get on the gas pedal, it backfires from the exhaust and the engine and chugs real bad. I checked spark plugs and wires and replaced the fuel pump and filter, and none of that helped. Any suggestions?

Answer: The problem can be with the ignition or fuel systems. Do a fuel pressure test, and examine the spark plugs. They can give you an indication of what is going on inside the cylinders. Use the chart in your vehicle repair manual to read the plugs. If necessary, do a compression and vacuum tests.

Question: I have a 90 Ram Charger with a 360 engine. I put in a new MSD distributor, new cap, rotor, wires and plugs. Timing is set to service manual specs; changed carb out 3 times; a new fuel pump with correct amount of pressure, and a new filter. The vehicle backfires at 40mph and will not go over that speed, acceleration and deceleration are perfect, lots of power but just stops at 40 mph with backfiring. It gets worse if try to hold at that speed. What could it be?

Answer: Make sure to check for trouble codes, even if the check engine light is not on. Several faults can cause this problem, for example a clogged catalytic converter. But a common problem is a dirty MAF sensor. Other problems include clogged fuel injectors, faulty MAP, or TPS sensors, or vacuum leaks.

Question: 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?

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

Question: I have a '77 Corvette Stingray whose engine misses and backfires. I've had timing reset, changed plugs, and verified there is no exhaust leak. I've also bought a new filter and carburetor. Any suggestions?

Answer: Bad plug wires (jumping spark) and a bad distributor cap are common to both misses and backfires. Other things you might want to check: vacuum leak, bad fuel injectors, and the carburetor accelerator pump.

Question: I have a 1992 Honda that backfires when started. What can I do to fix it?

Answer: Check the fuel pressure, then check the ignition timing and ignition system, in that order.

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

Answer: 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.

Question: I have a 1957 Chevy with a 283. Sometimes she stutters and then backfires when I first step on the gas and sometimes at about 40 mph. I have checked the timing and it seems correct. Suggestions?

Answer: Check the distributor cap for damage or carbon tracking. Also, this can happen sometimes if the engine is running rich and ignition timing is retarded.

Question: 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?

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

Question: 1994 Chevy 350. Backfires when accelerating. Already changed fuel pump, filter, vacuum hoses, TPS, and distributor cap, but still backfiring. Do you know why?

Answer: Check the fuel pressure regulator

and the air injection system.

Question: Will removing the EGR system cause backfiring?

Answer: If EGR passages to the intake are blocked, it can cause backfire with the increase in cylinders temperature. This is usually the case with carbon-blocked or stuck-close EGR valves.

Question: My Toyota Quantum engine backfires when starting it. What could be the problem?

Answer: Check the ignition timing, it may be too retarded. Also, check the ignition system. If you haven't done yet, try downloading trouble codes. A pending code may guide you here.

Question: What are the causes of backfiring that only happens when running on LPG?

Answer: Double-check the timing. Also, the system may be leaning out. It may come from a vacuum leak (gasket, etc), leaking injector.

Question: I've been working on a 1973 Chevy 351 Mercruiser, and we did all the maintenance on it including new spark plugs, cap, rotor, spark plug wires, and new gaskets for the exhaust manifolds. At first, it was starting up every time and all of a sudden it won't start, and if tries it backfires. Any ideas?

Answer: If you didn't have any issues with the timing, check for vacuum leaks. The problem can also be in the carburetor - issues with the choke, accelerator pump discharge -- you may have a lean mixture.

Question: I have a 1996 Chevy Silverado 1500. It spits and sputters. The plug wires, cap, rotor, 02 sensor in rear, and ignition coil was just changed. It did nothing to fix the problem. It says multiple misfires but can't figure out what the problem is can you please help?

Answer: This other post may help:

Question: I have a 327 sbc Edelbrock tunnel ram carb. It backfires when I start, then runs OK. What could be causing my engine to backfire?

Answer: Check timing; check for a possible gasket leak, carburetor, and/or intake; make sure the choke is properly adjusted.

Question: My car is a Nissan Almera. If I try to start it, instead of starting, it backfires and doesn't start. What could be the problem?

Answer: Check the timing belt, ignition timing and fuel pressure.

Question: My 98 Honda d16 backfires and misses at operating temperature. Do you have any suggestions as too why?

Answer: Check the ignition module. If you don't have the specs, probably you can take it to an auto parts store. Some will check it without cost.

Question: 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?

Answer: Check the ignition system, connections and modules.

Question: I have a 2003 Chevy Venture that backfires through the air filter housing on acceleration. What is the problem?

Answer: One of the common problems is a faulty fuel system. First, make sure you have replaced the fuel filter at the proper schedule. You may need to check fuel pressure. This other post may help:

Check the MAF sensor as well. And download possible trouble codes, even if the check engine light is not on.

Question: 1975 BMW 2002 backfires at idle or when coasting. Possible causes?

Answer: An exhaust valve sticking open may be causing the backfire, or possibly an exhaust leak. Also check the carburetor mixture adjustments.

Question: My 2012 Chevy Malibu is making a whining noise coming from the front of the car. It goes away at a higher speed above 40mph and up. Transmission shifts well. I took it to some mechanics. They told me the belt tensioner, and another told me it was the alternator. What could it be?

Answer: It is possible the tensioner, or the alternator is the cause of the noise. But the problem can also be a belt misalignment or worn bearing. You can momentarily disconnect the belt and start the engine and see if the noise goes away. If it does, rotate the different pulleys by hand and see if you can detect a bad or worn bearing from one of the accessories or tensioner the belt runs on.

Question: I have a 1997 Jeep Grand Cherokee. It the truck cuts off while driving, then starts again, it also backfires sometimes. The check engine light came on then went out and back on. The MAP sensor has been changed. What could be wrong?

Answer: It seems fuel volume gets too lean and cuts off the engine. This will make sense, if the engine starts and backfires. You might want to test fuel pressure. However, pay attention to the trouble code(s) and test any part/system indicated by the code.

Question: I have a 2004 Mazda 6 2.3lL that once it gets into normal temperature, the car RPM drops to about 500 and it just idles rough. It also backfires and it’s not drivable. Once it cools down, it works and runs fine. But once I leave, stop in idle, it starts doing it again. What could cause my Mazda to overheat and fail to drive and does it relate to its having a code P0300 misfire?

Answer: There could be a problem with the idle air control (IAC) valve (carbon buildup or faulty device); check also the PCV valve (sticky valve) and a possible bad charcoal canister valve as well. Any of these faults may be causing the misfire. Hope this helps.

Question: I have a 1992 Dodge Caravan. It turns over, but backfires loudly and will not start. What should I do?

Answer: Check for vacuum leaks and good spark. Hoses or gaskets, bad ignition coil, module, distributor. These are the main suspects. But a sensor can also cause trouble. Even if you don’t see the check engine light on, scan for trouble codes. A pending code can give you some clues about the problem.

Question: I have a 1994 Toyota pickup 2.4 22RE, and it won't start, it backfires through the intake. The fuel pump is working, what else could it be?

Answer: Check EGR valve operation; make sure the valves are opening and closing properly.

Question: For my 1997 Grand Cherokee, can a backfire be caused by a clogged catalytic converter?

Answer: A bad catalytic converter that fails to burn fuel that finds its way back there can lead to backfires.

Question: I have a 1996 Dodge Dakota 4 cylinder 2.5. When It first cranks it’s fine, but when it gets hot, the only way it will drive is with the throttle pressed all the way. If you press the throttle halfway like cruising speed it acts like it’s dying till you press the throttle all the way. It will go but backfires. Then it has a rough Idle and red lights. What could be the problem?

Answer: It seems like a faulty sensor's coil. A bad coil won't begin to fail until it reaches operating temperature. Once it cools, it works fine. This kind of problem is caused by a bad ignition coil, fuel pump, crankshaft or camshaft position sensor. Download trouble codes in case the computer has a pending trouble code stored. This may help pinpoint the faulty sensor. Check the throttle position sensor for dead spots as well.

Question: Toyota with 1AZ engine starts nicely but when gear loaded it backfire only when the engine is cold and when it warms its OK. What could be the problem?

Answer: Check the ignition timing and the EGR valve. Also, scan the computer for pending codes, if necessary, even if your check engine light is not on.

Question: I have a 1999 Jeep Grand Cherokee, I replaced the fuel filter but now the backfiring is worse, what should I do?

Answer: The problem could be in the fuel system. Faulty fuel injector, fuel pressure regulator, pump, may cause the mixture to lean. Start by checking the injectors and fuel pressure:

Hope this helps.

Question: I have a 2006 Chevy HHR. I got it for a good deal because someone had ripped some wiring out under the dash, and the key was broken in the ignition. I repaired the wiring and got another key but all it will do is backfire. It won't start. Do you know why it might be doing this?

Answer: If there are no DTCs, check the whole ignition system. Other possibilities are the fuel and exhaust systems.

© 2017 Dan Ferrell