What You Need to Know About Engine Misfires

Engine misfires can be a mysterious, frustrating problem—and information around them often makes them sound worse than they are. The symptoms vary by vehicle but are usually described as a stumble or brief hesitation in power delivery. An engine misfire can be temporary or continuous and will sometimes generate a check-engine code. But don’t be alarmed. Though it seems like an expensive fix, it usually doesn’t have to be. Read on as we demystify misfires.

Vehicle engine bay

What is a Misfire?

First, let’s examine what causes a misfire. You already know an engine needs three components to fire the cylinder: fuel to ignite, oxygen to burn that fuel, and a spark to ignite the mix. Take away any of those elements, and the cylinder will not produce the expected bang. That sounds like an easy enough diagnosis, but other cylinder misfire causes are due to incorrect ignition timing, vacuum leaks, or valve spring wear.

If your engine is misfiring, it’s best to find the problem and fix it as soon as possible. Misfires reduce gas mileage and increase emissions, which can cause you to fail an emissions test. More seriously, cylinder misfires can cause damage to other engine parts, like the oxygen sensors or catalytic converter. Let’s look at what to do when diagnosing this issue.

When It’s the Spark

Ignition parts that control spark to an engine are primarily wear parts that are designed to provide maximum performance for their service life, then be replaced as needed. As these parts wear or corrode, they will gradually increase impedance to the point that little or no electricity makes it to the spark plug to ignite. Since this happens over time, you may initially have small intermittent misfires you don’t even notice that gradually get worse over time. This is a big clue that your misfire is caused in the ignition system, so start there. Fortunately, most of these items are affordable and easy to quickly replace.

Spark plugs are cheap and easily swapped in just a few minutes. Ignition wires that are old can show signs of wear and are simple to replace as well. Older vehicles with a traditional distributor might just need a new cap and rotor. The coil packs on modern vehicles are less affordable but are still easily serviced.

When It’s the Fuel

After the ignition system is checked out, move on to the fuel system. Parts here typically last longer but still wear out. Perhaps just the fuel filter is clogged, or the fuel injectors are dirty. If those are good, the fuel pump or the mass airflow sensor may be starting to fail.

The EGR valve might be sticking with age, letting exhaust dump into the intake manifold. Emissions systems are precisely designed, and spent exhaust in the wrong part of the ignition cycle will cause issues. Or maybe you are lucky and just filled up with a tank of bad gas.

Fuel-system misfire symptoms will suddenly appear and are often more noticeable at idle than at highway speed. If your engine is chugging at a stoplight but smooth at speed, take a hard look at the fuel system.

When It’s Mechanical

Engine misfires can also be a little more complicated. Check the vacuum lines connected to the intake manifold. Look for cracks and replace lines if you find any problems. Also check the condition of intake manifold gaskets, especially around the throttle body. Take a timing light under the hood to make sure the timing belt or chain has not slipped or jumped. Finally, pop off a valve cover and have a close look at the valve train for any obvious damage.

Unlike fuel misfire symptoms, mechanical misfire symptoms will not go away with higher engine speeds, and often get worse. The misfire can be serious enough to cause noticeable vibration in the cabin, or even backfires. By this point, your engine’s PCM should show a code.

About That Check Engine Light

If you have a “Check Engine” light, your car’s computer is storing information about what problem was detected. The great thing about diagnostic codes is that they can be very specific, even often narrowing down which cylinder is misfiring. That’s less time spent hunting down the problem, so it makes sense to use a code reader to get to the root of the problem.

Have you ever solved a misfire problem? Tell us your solution and advice in the comments below!

Comments

  1. I had an engine misfire that I diagnosed as a faulty ignition coil on a BMW 540. I individually removed each of the coil leads until I determined the faulty coil and ultimately discovered that I had two bad coils. I replaced the faulty coils and the rest is history.

    • I’ve had two older high mileage cars (F150 and Odyssey) where the EGR paths had bad carbon build up and caused misfires. After removing the upper intake plenum and cleaning the channels the problem was fixed.

  2. Brian Chiarvalloti says:

    I had an engine code telling my my #5 cylinder was the source of the misfire. I took the plugs out, and that one was in rough shape. New plugs- problem gone! For awhile… 3 months later- problem came back. I checked my new plugs. They were junk again! It seems the coil pack was failing, and that was causing the plug tips to break down badly. Another set of plugs, and new coil packs solved the problem for good.

  3. I had an ’88 Buick Reatta that would jump rapidly between gears when I tried to use passing gear, sounded like a machine gun. I took it to a trans franchise and the guy took an analog multimeter and in 10 seconds told me I needed a new tranny, at the time was about $1200. Feeling a little unsatisfied I took it to the Buick dealer who showed me an oscilloscope reading that showed one cylinder not quite peaking like the others…he couldn’t guarantee that was it. stuck an old coil pack he happened to have and problem solved. The only time it would miss was when shifting into passing gear. You could leave it in first and go through the whole rpm range and not duplicate the miss.

  4. Jennifer Ann Cormier says:

    When I did a diagnostic test on my 2000 Ford mustang it says cylinder 3 and 4 are misfiring well I had some one do a compression test and found that those cylinders are not getting hardly any compression,my question is is there anything I can do besides get a whole New motor?

    • Your problem could be in the heads/valves. If that is the problem you could have heads taken off and have a valve job done but if it is not in your valves then , You can have your engine rebuilt. It can be costly but you end up with a new engine.

      • I would suggest doing a cylinder leak down test where air pressure is put into the cylinder through the spark plug hole with the piston at Top dead center. By doing this you will be able to tell where the air is going (oil pan or valves). This will let you know before removing the cylinder head if the compression leak is in the valves or the piston. There is a small chance it could be a failed head gasket as the cylinders missing are side by side.

    • Sean Robertson says:

      As scary as your issue sounds, a new motor may not be called for. That could be a problem with the valves (exhaust probably) on those cylinders. If the valves are burnt, bent or just carbon contaminated they tend not to seat properly and can cause a no-seal condition which reads as low or no compression. I would try some of the fuels system cleaners available at your local parts store. In this case I would go with one of the “name brand” cleaners. They tend to work better than the cheaper ones. Use of a good fuel injection cleaner on a regular basis helps with this as well. Not knowing exactly what is going on with the motor, that is about all I have as far as cheap advice. And adding chemicals is cheaper than having someone tear into your engine and it might actually solve the issue. Hope this helps, keep up apprised,

  5. Neil Schopp says:

    I have a 2003 Chevy S-10 4×4 LS with the 4.3L vortec. I had a random misfire that I chased for almost 10 months. I followed all the online advice with no luck. I finally replaced the head on that side and this fixed my problem. I believe it was a stuck exhaust valve.

  6. When changing the spark plugs, take a good look at the old plugs. White deposits coating the tip could indicate too lean . Look for the air leaks mentioned in the article. Black oily plugs could indicate too rich condition caused by faulty carburetor or caused by oil intrusion into the cylinder. Oil in the cylinder could be caused by faulty head gasket, worn piston rings or, leaky valve oil seals.

  7. Had a random miss in #1 cylinder on a 2006 Nissan Sentra. Drove me near crazy swapping out parts, changing wires, etc…. Figured it was a sticking valve. Ended up using Marvel Mystery Oil in the tank (follow directions on bottle). Also poured 1 tablespoon directly into #1 cylinder, replaced spark plug. Let sit for a few minutes, start car and let run for a few seconds. Let sit for a few minutes and repeat start. Sticking went away after a few miles. I always add a little MMO with every oil change since then. Problem has not returned. Hope this helps someone.

  8. I had a misfire one time. But i drank some pepto bismol and the rest is history. So is my underwear.

  9. My 04 6.0 2500 hd Chevy Silverado with a p0300 code. Coils were delaminating. Plug wires had excessive resistance and were breaking down. Primary wires feeding electricity to coils were shorting to ground. I replaced all coils plug wires with excessive resistance and installed new primary wiring harness. Results: No codes.

    • to me that is from someone using carb cleaner on the upper intake manifold .. very common issue profession techs use NOT GOOD !

      carb cleaner is only for metal any contact on plastic/rubber/gaskets it will eat at it including the wire insulation.

  10. Ron Jacobs says:

    looking for suggestions. I have a 1990 truck with a 460 engine, the engine runs smooth at idle and even at acceleration where what seems to be a misfire occurs when you are riding along and the engine gets a little load on it. If you press the accelerator and the engine shifts gears to create a higher rpm the misfire goes away. Any help is appreciated.

    • Sean Robertson says:

      Check your fuel pump pressure, fuel filter, vacuum leaks, spark plug gap, air filter, the list goes on but check those things first.

  11. Had a Nissan 350z with multiple cylinder misfires. Changed coil packs and plugs which did not fix it. Ended up removing the Variable Valve Timing (VVT) Solenoids, after a good cleaning it ran like new again. The VVT Solenoids get plugged up and don’t send the right signal back to the computer causing it to misfire. Always change your oil to help avoid this problem and remember if you have to put oil in before changes this doesn’t mean that you can extend the oil change frequency it means fix the leak or whatever it may be. The PO of this car should have fixed the valve cover gaskets..

  12. d kim spurgin says:

    Best advice is to take your car to an Advance Auto store and let them plug their machine in. I have a 2003 ford Thunderbird that I bought use. I have had many engine lights come on! Took it to the Ford dealer and they told me the transmission was shot and needed to be replaced-$4500. Took my car to my Advance store where I buy all my car stuff. The tech checked my car and said that I had several bad COP’s (coil on plug). Even told me which one they were. He suggested that I replace all the plugs and coils because the rest of them would probably fail soon. Took his advise. I have had a few other issues with the car. I go straight to the Advance store. They have been dead on every time and given me a lot of good advice. Thank you Newberry, SC Advance Auto store!

  13. David T Wilson says:

    Dave says:
    My wife’s Jaguar 4.0 V8 was misfiring under acceleration, but no code(s). I pulled all 8 ignition coils and inspected them, including ohmmeter testing, but all appeared OK. Then suspecting it was fuel, I replaced the fuel rail pressure sensor and in-tank fuel pump, and cleaned out fuel tank, no improvement. Then I took it to to a local garage with a good reputation for diagnosis. They were convinced it was ignition, took out coils and inspected them, and concluded the coils were OK, but still missing. Baffled, for $150. But they were absolutely convinced it was ignition. I took it home and took out the coils, again, and inspected them again under a very bright light and a magnifying glass. Behold, 6 out of 8 were tracked (high voltage arcing marks/damage). Replaced all 8 coils, it then ran like a top.
    Moral: inspect coils very, VERY carefully.

  14. My 2005 Ford Tarus does not idle smoothly.The idle fluxulates up and down 50 rpms. Replaced coil assembly, new plug wires. idle air control and throttle position senser plus air filter. It didn’t help. Do you have any info that will help?

  15. Vacuum Leak? Check for vacuum leak after mass airflow sensor

  16. If vtec is not working correctly, misfires can occur due to mistiming, a stumble on acceleration is the result. Fouled plugs , injectors or failing coil packs can also cause misfiring.

    This is why a code check should be the first step in diagnosing what is believed to be a misfire.

  17. the p0300 code can be a tough one depends on the vehicle. on my GM vehicles I use a scan tool ,timing light,vacuum gauge, fuel pressure gauge.

    first the fuel pressure is correct, then the engine vacuum . this then becomes a ignition issue.
    using the timing light place on each plug wire see the the light does not miss flashing.if it does its ignition.

    in the dark look for flashes of light .. look closely around the ign coils and wires. any flashing means its bad.
    many times the ignition issues are from lack of silicone dielectric grease on the spark plug boot ends . the coils do get damaged over time due to contamination and people using carb cleaner which corrodes the electrical system .

    get a good sized tube of the silicone grease . many times they give you a tiny spec you need much more on those plug wires.

    high mile GM V8 engines the distributor is a common failure this can be seen with timing light.
    if the fuel trims are close to zero then the fuel system is not the problem..
    cat converters if plugged up/damaged will throw converter codes so that must be taken care of first . also any other codes must be repaired before the P300 is fixed. make sure the exhaust manifold bolts are not broken or any exhaust leaks.

  18. Carl T Lawendowski says:

    My 2000 Grand Cherokee 4.0L with 168000mi was diagnosed by my auto mechanic as a misfire between cylinders 5 & 6 if I remember correctly and he advised me to change the coil pak. He turned the check eng light off. It hasn’t gone back on yet, with about 10 days mileage. Since ignition paks are not cheap, around $65 to $125, should I be doing something cheaper first?? The previous time the light came on it was diagnosed as spark plugs so I replaced them about a month before this one came on.

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