How To Fix Cylinder Misfires
If you have a cylinder misfire, there a couple of common steps that you can take to diagnose what the problem is. This guide goes over 6 common problems and how to diagnose and fix them. I am specifically applying these diagnostics to my 2004 F150 with the 5.4 Triton engine, but the tools and procedures are applicable to any vehicle with cylinder misfires.
Part and Tools
Here is a list of parts and tools you may want. Each section has them in the paragraphs, but for convenience I list them all here. The parts are for the Ford Triton 5.4 engine.
Tools
Scantool: https://amzn.to/39mIKSP
Seafoam: https://amzn.to/3LLlIDD
B12 Chemtool: https://amzn.to/3LLlIDD
Multimeter: https://amzn.to/30kbffn
Spark Gap Tester: https://amzn.to/3yjOGGD
Test Light: https://amzn.to/3OY0uod
Smoke Machine: https://amzn.to/383QadH
Compression Gauge: https://amzn.to/38Y2Rqy
Compression Gauge Adaptors: https://amzn.to/3LesAsf
Mechanic's Stethoscope: https://amzn.to/37ncpuJ
Parts (5.4 Triton Engine)
Spark Plugs: https://amzn.to/37mLT4H
Coil: https://amzn.to/3FkiSTL
Coil Wire Connector: https://amzn.to/3M7pKqh (OEM) https://amzn.to/3yhQUq8 (Cheaper)
Fuel Injector: https://amzn.to/3wajhnx
The Offending CYLINDER
The P030# codes will tell you which cylinder is behaving badly for you, where # is the cylinder number. In my example I am having a P0305 code telling me that cylinder 5 is misfiring. You may have multiple of these codes at once, or even P0300 which means multiple or random misfires. P0300 alone with no other codes can be tricky to diagnose vs a P030# but you can still go through the following steps. Additional things that may be wrong for the multiple or random misfires are:
Low fuel pressure
Engine timing has gotten off
Poor fuel quality
Cam or Crank Sensors
I am not going to go into these specifically in this article, since they are more rare, but if none of the other things help it may be one of these.
Manufactures will have different layouts for their cylinder numbering. Often you can get lucky and it will be listed on the spark plug wires if they are OEM wires. You may have to look up your specific engine. I made a picture of the Ford triton v8 since that is what I am working on, but you can google “car type/engine type spark plug layout” and you should find the info you need.
Bad Spark Plugs
In my experience spark plugs are the number one culprit for getting cylinder misfires. Remove the spark plug from your offending cylinder and check it with a spark plug condition chart to see if it may have issues. This chart by aa1car.com is my favorite one and I have been using it for years to help me diagnose problems. Iridium spark plugs last a really long time, and while it depends on your specifics, I find 100k miles is pretty common. If it is around time to change them and they look a bit worn then I recommend changing them all together. I personally like OEM plugs and have found them to last a long time.
Spark plug removal can be a beast sometimes, and Ford F150 engines are notorious for being tricky. If you have high carbon build up, it’s a good idea to run some carbon cleaner though your gas. I like seafoam and B12 Chemtool. Get your tank below 10 gallons, add the cleaner, then run your tank down to as close to empty as you can. This will help the plugs break free without snapping. Also, if you run the vehicle for just a little bit (5 min) before taking out the plugs, the aluminum will warm up and they should release easier. I tried to take out my plugs originally without doing this to save some time and I broke my ratchet universal joint, so I really do recommend this taking these extra steps.
Note: I go over compression testing later on in this list, but it may be more convenient to do it now while the spark plug is out.
Bad Wires
The second most common problem I have seen has to do with bad wires. It is important to tell if the car is pushing enough power to the spark plugs to get them to properly and reliably fire. If you have spark plug wires (you may not, my truck doesn’t) you should visually check them for nicks, melted parts, to see if they are coming apart, etc. If you see any of those issues, you should replace that wire. Typically wires only last 60-70K miles, so if you are replacing your spark plugs, then it is a good idea to do your wires as well.
To check your wires with a multimeter, you are going to first check that there isn’t a short. After that, you should check the resistance and make sure it is under 12,000 ohms-per-foot of wire. This can vary between wires and manufactures so you may need to check if this applies to your vehicle.
If you don’t have spark plug wires, you can still have faulty wiring. A super common issue on Ford engines is that the wiring clip going to the coil is broken. When I changed my spark plugs in step one, I broke 2 of them and already had one broken (which was to my 5th cylinder). These can often cause random misfires as the connector bounces around a little too much, but you can easily replace them with a new connector. Either replace just the head and clip, or you can cut it off and solder it on new. Often there are cheap 8 packs, but the wiring in these is not very good, so I recommend a proper replacement even though it is more expensive. The OEM Motorcraft WPT986 comes with crimps so you just cut it off, crimp it, and heat shrink if you want. The APDTY 756681 is about a third the price and hopefully works just as well (I went with these since I have to replace three of them).
Bad Coils
It is pretty rare for a coil pack to go bad compared to spark plugs and wiring, but checking it next is good because it can help point to wiring problems that may have been missed in the previous testing. You will need a spark gap tester (they can have a bunch of different names). Often people use in line testers, but I like the gap tester to help see if the coil pack is giving sufficient spark, not just constant spark. Plug the tester into the end of your coil if you don’t have spark plug wires and into the end of your spark plug wire if you do. It is best if you unplug your fuel injector to the coil you are testing so you don’t get a bunch of unburnt fuel in there. Attach the clamp on the tester to a ground point (any grounded metal in the car) and crank the engine briefly. If the vehicle starts, turn it off quickly since one of the cylinders is unhooked. I like to set the gap very close at the beginning to just check that something is happening. If you don’t see any sparks at all then the coil is bad or it isn’t getting any power.
If you didn’t get any spark, you’ll need to figure out if it is the coil pack or wiring. If you have a spark plug wire, switch it out for another one and repeat the test. If you get spark this time, your original spark plug wire is bad. If you don’t then most likely the problem is upstream in the coil or car wiring. To test if the car is supplying voltage to the coil, unplug the coil and probe it with a test light. I recommend a simple one, because digital ones can have problems with really fast changing circuits, like the coil signal, and a multimeter is better for reading voltage and current if you need details. With the test light grounded and probing the positive side of the coil signal wire, crank the engine. You should see the test light flash on and off. If you don’t see any signal then you know that there is a wiring problem and no signal is getting to the coil. If you do see the signal and didn’t get any spark after the coil then you know that the coil is bad.
If you do see spark on the spark tester, then increase the gap on the tester to see if you are getting sufficient power out of the coil. This will vary between vehicles and you will probably have to look it up online, but The triton F150 should be getting a good spark at 35kV. Open up your spark tester to around that voltage gap and then with it still grounded crank your engine like before. If you see a good strong blue arc then you know that the coil is pushing sufficient power for your vehicle. If it is intermittent when it wasn’t at the smaller gap, or if the color is faint or almost brown, then the coil is bad and you should replace it. Personally I would just replace the one bad coil, not all of them, but I know people often like to replace all of them together.
Bad Fuel Injector
The easiest way to check your fuel injector is to use a mechanic’s stethoscope. Simply put the stethoscope up against the fuel injector and you should hear a constant clicking sound. If it is intermittent or doesn’t click at all, then you have a bad injector (or it isn’t getting signal). If you don’t have a stethoscope you can use a long screwdriver and put it up against the injector and put your ear on the handle and hear it that way. I think this is the least likely to be the problem, but because it is so easy to check I moved it from last place.
Vacuum Leak
If you have a vacuum leak then the correct air/fuel mixture could be off and this could give you a misfire. Often you will have the P0300 code with this problem, but if the leak is close to a specific cylinder on the intake manifold then this can show up as a single P030#.
The easiest way to find a vacuum leak is with a smoke machine. They can be a little expensive but I have had mine for years and have used it several times, so it might be worth the investment. Run the smoke machine, then plug it into a vacuum line (I like to do it in the brake booster line). If you see smoke coming out somewhere it shouldn’t, particularly at the intake manifold, that can cause a misfire. Any other leaks you find should also be fixed up.
If you don’t have a smoke machine, you can use a spray bottle of water and try to get the car to run rough when you spray in a leak area, but I have never had good luck with this method.
Low Compression
To check if you have low compression you are going to need a cylinder compression gauge which have gotten really cheap these days. I used to have a nice one that I destroyed, and then bought a cheap one from Harbor Freight which has served me well. Also you may have different sizes of spark plugs and need an adaptor, particularly the 16x1.5mm ones in the old Fords.
Take out your spark plug and then disable your fuel injectors and ignition system. The easiest way to do this is to find the fuse to the fuel pump and the ignition system and pull them out. Put the compression tester into the spark plug hole and screw it in hand tight. Don’t use a wrench! My last one died because I way over tightened it and then ripped the hose when trying to get it back out. It was a disaster. Crank until the needle isn’t climbing anymore (often around 7 cranks of the engine). For the Ford 5.4 Triton engine it can be anywhere between like 110 and 185 psi, but can even vary from that and be ok. A more accurate way to find out if compression is good is to test all the cylinders and see if any vary by more than 15% from the rest (10% is better but I have seen up to 15% and the vehicle still ran fine).
If you find that your compression is low it will typically be the cylinder gasket, valve seals, or the head gasket.
If you suspect it is the head gasket and also have any of the following symptoms, then I would replace the head gasket.
Loss of coolant without a leak
Bubbling in the radiator and coolant reservoir
White coloration in the oil
Other Possible Problems
I am not going to go into these specifically, since they are more rare, but if none of the other thing seem to be the problem, it may be one of these:
Low fuel pressure
Engine timing has gotten off
Poor fuel quality
Cam or Crank Sensors
If you only have one cylinder misfiring, then it could be the cam or crank sensors, but that is pretty rare. Normally I have seen that cause the car to not even start, but it is possible that this could cause an individual cylinder to misfire. Low fuel pressure is typically going to cause a P0300 not just one cylinder and you should see other codes that go along with that. Same with poor fuel quality. If you ran the seafoam and the B12 through your system as discussed in the spark plug section, this should have helped that. You could also try a different gas station. If your engine timing has gotten off, I hope you don’t have to replace your timing chain on the 5.4 Triton engine. I had to do that to mine and it was a beast of a job, and very expensive.
Let me know in the comments if I missed any common issues, or I messed something up. Let’s all learn together.
