How do you fix a cylinder misfire?

Answer 1

Cylinder misfires come in many different flavors. Sometimes only 1 cylinder will fail, sometimes more, and sometimes it will jump around. The lesser amount of cylinders you have, the more noticeable the misfire will be. In all cases it depends on the condition of the electronics, and hardware (i.e.; spark plugs, spark plug wires, coil/coil pack(s), etc.) In my experience, the lesser amount of electronics you have, the easier solving the problem will be. This answer applies only to gas powered internal combustion engines. Not diesel powered engines.

First you want to make sure that ALL hoses and electrical connectors/wires are located in their correct place, and connected properly. Next to identify the problem. When does it miss? At idle or throughout the driving range (while at idle and in motion). Is it intermittent (does the miss occur randomly)? Does the vehicle seem to have 1 miss? Or more than one miss (This is more easily noted on vehicles equipped with motors that contain a smaller amount of cylinders)?

Now to explore each question, I will use a hypothetical situation. I have a small car with a 4 cylinder motor. I drove it home last night, and when I started it this morning, it had a definite miss.

To identify which of the cylinder(s) are misfiring, no actual tools are required. To find which cylinder(s) are having the problem, simply open the hood with the key "off," and check to see if anything is loose (i.e. a spark plug wire). If everything is snug, try removing one of the plug wires (NOTE: Only remove 1 plug wire at a time!). Then reach inside the car and try to start it. If you have a 4 cylinder motor any difference will become immediately apparent. In fact, most times the car will fail to start and stay running. If the plug wire you removed made no difference in the way the car runs, that means that cylinder is not firing. Replace the wire to its original location, and make sure that it is snug. Try another wire. Continue this process until you find the cylinder(s) that are causing the problem. (NOTE: Keep notes on which cylinder(s) are having the problem, and how the car behaves.)

Once you have identified which cylinders are having the problem, it is now time to determine the source of the problem. On newer cars (approx. 1985-present, depending on make and model) the problem can be more complex to try and solve. In this scenario, I am using an older car. Now it is time to see if the spark plug is getting any spark. To test this, remove the spark plug wire from the bad cylinder's spark plug, and set it against something metal on the motor, for best results, place the metal tip of the wire slighty away from the metal object (like the valve cover), but close enough for the spark to arc across (approx. 1/4 inch is best). Start the car. If you look at the tip of the wire and there is a spark arcing across the wire to the motor, that wire is transmitting spark. If it does not, skip down to the next paragraph. Since the wire is transmitting spark, the next thing to check would be the spark plug. (At this point you will need tools) Remove the sparkplug from the misfiring cylinder, and check for detonation of the tip of the plug (plug tip is destroyed), corrosion(white chalky residue), and foulage (if the plug is wet, and it smells like gas, that means that there is fuel getting to the plug, but the plug is not igniting it. If the residue is oily, then there is oil seeping in through to the combustion chamber. If it is not oily or wet, but is black, that means that the plug is carbon fouled, either from too much fuel (a rich fuel mixture), or that the spark plugs have not been cleaned/ replaced in a long time. To test the spark plug, plug it back into its wire, and set it again against something metal (NOTE: Make sure the location your choose, has the least chance of falling over and into other moving parts). Start the car, and check for spark at the end of the plug, if there is no spark, the plug is dead, and needs to be replaced. If it does spark, that means that there is either a fuel delivery problem, sensor malfunction, or vacuum/vacuum hose leak, and not a spark issue.

Its always a good idea to start off with an Ohm meter. This device measures the resistance through a given medium. This device will show you if there is a problem with a wire, plug, etc., and takes out alot of guess work. If you know how to operate one, it will be very useful, if not, find someone who does. When you've found someone that knows how to use one (or yourself), take the spark plug wire off that was not transmiting any spark. Set the ohm meter to check for continuity. Place one ohm meter wire at one end of the spark plug wire, and do the same with the other ohm meter wire on the opposing side of the spark plug wire. (NOTE: Most newer ohm meters are digital, so any continuity will make the digital readout change, on some older models, or special edition new ones, there is a needle, continuity detected on this model will cause the needle to move) When you measure the amount of resistance the wire has/does not have, compare you reading with what the resistance should be (On most wires, there is a printed readout stating what the resistance should be, if not, either look it up on the web, or contact the manufacturer or an auto parts store). Since all ohm meters are different, there may be more than one setting for checking for continuity. When you have found what the reading should be, compare it to what you got as a reading. Then (If applicable) switch to the other resistance measure setting, and take another reading. If both of the readings are a ridiculously large amount higher/lower then what the specifications read for that wire, then the wire is most likely bad. In which case, head to your auto parts store and/or call the dealership and check on a price for a new set. You will want to replace them all at the same time, and when you buy the wires in a box designed for your motor, it will come with the correct amount of wires. On most newer cars (and even some older ones) the wires are a certain length as to make it easier to determine which wire goes where (assuming that you do not know where they were previously).

When you have the cables, replace 1 at a time. In most cases, the cables will be marked as to which cylinder they attatch to. On most V6 and V8 models, the even numbered cylinders are on the right side of the motor when the motor is facing forward, and also to the right of the motor when it is mounted transversely. The quickest way to tell which cylinder is #1, is to locate the end of the motor that has all of the belts on it, and it will be the fist cylinder on the left. Then #2 would be the first on the right, then #3 would be the 2nd on the left, and so on. Take all of the cables out of the box, and lay them on the ground. Arrange them in order from short to long. Depending on where your coil/distributor is, the arrangement of the cables may be different on the motor than on the ground. Assuming the the coil/distributor is in the middle of the motor (NOTE: On 4 cylinder models, the coil/distributor may be located on top of the motor, or on the side/front where the motor is mounted parallel/transverse respectively)

Once you have located the wire for the #1 spark plug, take the end off of the coil/distributor cap, and replace it with the new one. Make sure that the wire you choose is long enough, but also that it is not too long. You should have enough slop in it to bend the wire up at least an inch. Then proceed with the next wires 1 at a time. Once they have all been replaced, and are on snug (you should hear a "snap" when the wire is connected properly to the plug), try starting the car. It should start up and run normally. If it does, the problem is solved.

Fuel delivery problems are fairly easily found. The only problems the fuel delivery could face, are: Dead and/or malfunctioning fuel pump (may be intermittent, or only when hot/cold, under a load, etc.), partially/fully plugged fuel filter(s), very rarely a fuel line will break, however you would see/smell gas if it was. After that you have (if applicable) malfunctioning/dead fuel injectors, or the carburetor/throttle body has a mis adjusted air/fuel ratio. For these kinds of things and the even more complex things like sensors, if you don't have much finess with vehicles, and/or arent very good with troubleshooting, it would be wise at this point to consult a professional. I consider myself a professional at what I do, although there are tmes when I need to consult others for ideas. Back to the problem.

Fuel pumps come in as many different locations and forms as peoples DNA comes in different sequences. So depending on the make model, and mostly the year, you may have to take it to a professional to have it diagnosed. For my hypothetical situation, the fuel pump is manual (Meaning it is not electrically operated) and is mounted on the side of the motor. 2 nuts, and the fuel lines are keeping it on the motor. CAUTION: Always relieve the fuel pressure in the lines before servicing the fuel system, and keep heat sources/burning materials (i.e. cigarettes) away from the gas and fumes. This can usually be done by removing the gas cap on older vehicles, and on newer vehicles, there is a small pin button on a gas line under the hood that you can depress to relieve the pressure. Once the pressure has been released determine where your fuel pump is. My vehicle as stated above, is on the motor, though I think you will find on most newer vehicles where the pump is operated electrically, the pump is either in the gas tank itself, or is of an "inline" type, meaning that there is a hose from the tank, to a filter, and then to the fuel pump, then from the fuel pump, another hose leads all the way up to the carburetor/throttle body. If the pump is located in the fuel tank, your best bet again if you are not very "good" with vehicles, is to take it to a professional, but if you are confident that you can work with the pump in the tank, and the removal of the pump from the tank, then continue on.

Find the fuel pump, and turn the key to the "on" position. If you have an electric fuel pump, you should hear it running quietly for a few seconds. If it is making a lot of noise (Meaning you can hear it well over the motor while it is running), the pump either has something stuck in it, or is on its way out. In both cases, replacing the pump will become necessity soon enough. If the pump is running well enough to start the engine, that means that to test for the remaining part of the fuel pump will have to be done on the road whilst driving. To do this, get in the car, and drive out onto the road.(NOTE: If the car is missing badly, do not drive into traffic areas. Drive only in quiet areas, or at night when there is little traffic.) Determine if it is missing while driving it all the time, or just when you press on the accelerator. It is easiest to determine this with the driver window open all the way, so you can listen to the exhaust. If it not missing under a normal load, go out to a place that you can safely put your foot into it and place the fuel pump under a load. If it was missing at normal speed, there are still quite a few possibilities, however, if it is only missing when you have your foot in it, that eliminates a lot of things.

Lets start with the idea that it only misses when you have your foot in it. Does it miss badly when you have your foot in it, or only intermittently? Does it feel/sound like it is missing at a steady pace, or randomly? If it is missing all the time you have your foot in it, and is steady, that means that chances are only 1 cylinder is having a problem. Which makes thing easier yet. Assuming that there is only 1 cylinder with a problem, if your vehicle has fuel injectors, grab the ye ol' trusty ohm meter, and set it up to test resistance/continuity. Take the wires off of one of the fuel injectors. Put the 2 prongs from the ohm tester on each prong of the injector. You should get a reading. Now, put those wiers back on just as you had them before, and take the wires off of another injector. Test it with the ohm meter. All of the injectors should have a similar reading. If there is one that is way off of the others, that means that injector you are testing is most likely bad.

If it is missing randomly, or intermittently, its now time to check the fuel pump, and filter(s) out. On older vehicles, there is usually a easy to locate fuel pump under the hood. Mounted on the side, or front of the engine block. If it is located on the front, chances are, its sits close to the bottom of the motor.

We will first test the filter(s). On my vehicle, the filter is small and white, out in the open, under the hood. Fuel filters come in about as many different forms if not more than the fuel pumps themselves. Determine whether your vehicle has 1 or 2 filters. To do this, start at the fuel tank, and work your way up the vehicle along the fuel line. There will only be a filter in areas of the line that are visible. No they don't try to hide them from you (unless you count ones in the tank in the tank). Most vehicles with a manual pump on the engine block have 1 filter. That filter is located mostly under the hood, and on rare occasions, underneath the vehicle along the fuel line. Locate the fuel filter and again, relieve the fuel system pressure. Then grab a container that you can hold or set underneath the fuel filter area so that any fuel that drips down can be caught. Remove the line after the fuel pump first (NOTE: If you want to be safer around the area, find a screwdriver that you can use to plug the fuel line to prevent any fuel from leaking). After you have done that, do the same with the line leading to the fuel filter from the tank. Plug this one for sure, otherwise all of the fuel in the tank can drain out. Once plugged, remove anything holding the fuel filter in place, and remove the filter. Drain any gas out if it that might be inside. To test the fuel filter for blockage, you can either: A; Blow through one end with your mouth, or B; use an air hose to blow through it. If you choose A, then take a clean rag, or shop towel, and wipe the end of the filter off. Place your mouth around the end of the filter, so that there is no way the air can escape, except through the other end of the filter. Blow into it. If you find that it is hard to blow through, blow harder but do not make your head spin by doing this. If you choose B, place an air nozzle on the end of the hose you wish to use, and insert it into the end of the fuel filter inlet. Very slowly, allow the pressure from the air nozzle escape into the fuel pump, if it seems to be flowing through well, then the filter is most likely OK. If it does not, apply more pressure to it, and see if you can coax it to blow something out if it. If applying more pressure does not change the amount of restriction, the filter is most likely paritally plugged in which case, a replacement will be needed. If blowing through option A, or B do not work, the filter is most likely 90%+ plugged, and it is amazing that the car ran at all. In which case a replacement is guaranteed to be needed. If a replacement is in order, replace the filter with an identical copy (found usually at an auto parts store, or dealership). After replacement, on electrically driven fuel pumps, turn the key "on" and "off" a few times, allowing the fuel pump to pump enough fuel into the lines to fill the filter back up. Then try to start the car. If that does not solve the problem, continue on.

If the filter blows through fine, then place it back exactly as you found it. For the testing of a fuel pump, unless you feel like replacing it to see if that was it, take it to a professional, or if you have the right tools, you can check it yourself. The best idea here, is to take it to a profeesional and have them test it, in general, it doesnt take long to figure out if a fuel pump is bad. If you have the correct tools (Fuel pressure gauge), connect them via the manufacturers instructions. If the pump passes the test (and to totally test this, you will have to drive the vehicle with the tester/gauge hooked up), that means the fuel pump is in good enough condition to run the vehicle.

Although the possibility of a mis adjusted air/fuel ratio is not negligable, chances are, that this is not causing the problem. If nothing above has solved the problem, your problem lies in these things including but not limited to: Brain Box (If applicable), various sensors (i.e.; O2 sensor, MAP sensor, cam sensor, crank sensor, etc.), and vacuum hoses/valves (i.e.; PCV Valve, EGR valve). Make sure that all of the vacuum hoses are connected properly and are not deteriorated and leaking. A good way to tell if a vacuum hose is leaking, is to listen for a hissing noise while the engine is running. On older cars with an air cleaner unit on top, there may be a mess of vacuum hoses underneath of the cleaner unit. Check to make sure that all vacuum hoses underneath are connected properly in the correct location. For most sensors, you cannot test them. Other than noted in a "Chilton" manual, testing of sensors is basically impossible. Other than to replace them, you will never know. Once again check over ALL electrical connectors, and wires, vacuum hoses, and that there are no leaks anywhere. Worse come to worse, you may have an internal problem, and problems such as those are noticable by means of a knocking noise, or other deep internal noise. Unless the motor is worn out, or abused, the chances of an internal problem are not likely.

If none of the above has solved the problem, and you are fresh out of ideas, take it to a professional and have them check it out. Just make sure you take it to a mechanic that you trust, and not the cheapest way out.