My engine is misfiring , why?  
There are vast numbers of reasons for misfire.  over 40 ! (on just 1 cylinder?, far less)
Misfire means , INCOMPLETE combustion , on the firing stroke of the engine (4 cycle engines)
Engines that idle too slow , will misfire,  some idle at 400 RPM with EGR stuck open or ISC stuck closed.

The whole purpose of the engine, is to convert fuel to power , and rotating energy, misfire is the antithesis of that. A Total waste.
Misfire can happen, with a cold engine, hot engine or both (coolant at 180f minimum is hot normal and must hold there ! if not, it's a bad Thermostat)
Misfire at idle, the engine will shake , when driving you hear the misfire ,the non steady beat of 4 cylinder firing, is no longer perfect succession.
In fact most folks can FEEL misfire.
The 1996 and newer cars ,warn you with DTC P0300 to 304 or more with V6 (watch out, for false Misfire, on 1996+ OBD2 cars driven off road violently)
Most misfiring issues, are caused by not doing the FULL 60,000  mile interval tuneup.

Don't forget?: Flooding  causes Misfire too and so  is the rotten core of  bogging.(or  engine power loss)

Misfire has no connotation of only, bad spark, misfire means the combustion chamber misfired. ( it means "gross incomplete combustion")
Base fundamental reasons:
In pure physics, misfire is , too low compression, dead spark (at anytime) , too late or too early spark or bad fueling (too lean or too rich)
or
Bad fuel?  running Old summer fuel in the winter?  or worse 2 year old fuel?  Or E10 Fuel, that  is over 6 months old?

Thermostat bad and not holding 180F or more, solid and does not vary! HOT ?
The IDLE MISFIRE?  can be the EGR main valve stuck open. (causes very  low idle speeds, well below 800 RPM hot)
The correct order of diagnosis,  is:  (these 3 , expand to 43)
  1. Bad engine, one or more bad , very weak or dead cylinder's: check compression? (timing belt never changed, so it slipped? , make sure vacuum is not bouncing.)
  2. Bad spark. (never did a full 60k tuneup?) Spark timing, way off? (use a timing light and check it)  Will the advance go off the scale, after 3000 RPM?
  3. Bad Fueling. (be that too rich or too lean.)  (even vacuum leaks in the induction system or near the front oxy sensor, will stuff the fueling.
Let me add on more, problem that cause #2 and #3 at once, bad 12vdc electrical power at any time to the EFI system !  9V DC power, is DC POWER FAILURE.

For sure misfire , is the most complex topic on engines and driving:
Both the below, are misfiring to the extreme.
Keep in mind, violent misfire is many times, just bad spark.  if the misfire is weaker, it might be lean misfire, usually felt as bogging, lost Horse power, lost engine power,

Misfire can be so bad , lean, that we can  call that Bogging. (or lost power),or even hesitation.

Misfire can be so bad , rich, that we can call it Flooding, flooding causes the spark plugs (even brand new ones) to foul and misfire, and you an look at there tips to see that fact.
OBD2 EFI systems: (1996  and newer cars  in the USA ! the ECU can detect misfire and report it.)
The scan tool you can see the advance hit 40 degree's  racing the motor fast parked. See that here.
You are seeing DTC# P030x and family.  LOOK IN PENDINGS !!   too ! not just stored codes. P0300 is random misfire, P0301 is #1 cylinder misfiring)
I must warn you 96+ , you will get  false P030x DTCs, for any reason the Cam to crank are jogging, or jittering in excess. (bad timing belt or the crank pulley never set to 94ft.lbs)
If you have a loose Timing belt/chain or loose distributor or any loose dizzy CMPs sensor, or crank CKP sensor.  The sensors will LIE TO THE ECU. or in fact  cause MISFIRE.
A Loose, cam cog, loose crank cog ,  causing jogging of cam timing, that is bouncing all over the place for ANY reason, will trip P030x DTC.
So can excessive off road, or dumping the clutch too aggressively, can trip a P0300. (any gross rapid crank shaft speed reduction rates, in time)
On 1996+ cars, the CEL may flash every 2 seconds, warning of Misfire damage to CAT. (an over heat warning that is) The operator guide tells you what to do.
"Back off the throttle and drop loads" (drop the trailer off the hitch. it states)  For non DIY folks a new CAT is over $1000 installed, for me its $100, so consider that.

 See Measurement error's  during misfire testing , see Duane M. In action.

See Duane video's here, for many tests..
The older 1995 and older OBD1 cars ECU, are blind to misfire, they have no OBD misfire monitor at all. (no help from the ECU)  Only your seat of your pants, detects this condition.
YOU do need tools:
You still need Tools and gauges, to repair any car,  if not willing to buy tools, you will fail, or damage car or worse yourself.

Gross lean fueling causes Bogging,  See   power loss

Misfire is the most complex and hard to find. 

Misfire is bad combustion ! on one or more cylinders!   

One large  List: 
This list is not a GUESSING List , it's purpose is to demonstrate the futilty of not doing the tests to find which of 3 pathes are the cause.
Misfire can happen when idling , or driving , hot or cold engine. The when and driving conditions are great clues as to the why!
Replacing parts by guessing is going to fail and make you broke fast.
Misfire  means BAD combustion!  That is IT.
 ( Some folks think using statistics on old cars finds bad parts by guessing, no, your car is unique, and many times we see 3 or 4 failures.)
In no order, I can not guess. Only reality works on mother earth. FATE ! and ENTROPY. (or hacking /hackers, in the works)
  1. Bad fuel  , older than 6 months it can go bad.
  2. A Bad motor causes  low  compression on 1 or more cylinders (150 min. to 190 PSI varies by engine type and altitude. (90PSI x4 is bad Timing belt, No 60k tuneup, again.
  3. Valves guide stems sticking.  ( can be seen with a vacuum gauge "bouncing vacuum"
  4. Cam timing accidentally , set wrong  (or belt slipped) or the crank snout damage common failure.
  5. Ignition timing set wrong by accident or by poor tuneup action or someone guessed at timing, or left the clamp bolt lose, or crossed over the wires.
  6. Not doing the 60K full tune up. 
  7. Lash set wrong.  (60k service points ignored.) (too tight? , misfire and burned valves, too loose? , noise)
  8. Vacuum is bouncing in the intake manifold. (bad valves sticking .)   800 RPM 19 inches HG & steady, is normal. at sea level.
  9. The cam lobe's can be flat and still pass compression, so check valve lift carefully ,during the 60k lash tune up steps. (see targets of opportunity)
  10. Valve springs broken, see targets , in line above. and again, will be found at the valve lash checks,  so skipping service, means you missed these faults.
  11. The CAT converter is melted closed, this chokes the engine to death  ,so it can not breath, the engine is an Air pump. A bad CAT , can in fact render the good engine to BAD ENGINE.
  12. Over heating or coolant not reaching 180F to 195F and holding, the coolant must reach normal temperatures or the engine will misfire rich.
  13. The Frigid motor problem will misfire.
  14.  Bad or Weak or late ignition spark or cam timing.  ( Do the 60K mile Tune up  (timing belt and all major spark parts and filters )
  15. Use a new set of Bosch or NGK MAG wires,do no use junky cheap, no name $10 wire sets.!  By a top brand name . Don't buy noname parts off Fleabay (ripbay) Just Don't.
  16. Make sure the Timing advances fully off scale with the strobe light after 3000 RPM.  If not? the ECU is slammed into Limphome mode. (easy to see with OBD2)
  17. Constant misfire?, spark wires crossed over or spark plugs tips oil soaked or carbon fouled or dripping in pure raw liquid fuel.  Do look at all 4 (to max) spark tips.
  18. On a 16v motor the PCV can leak air and  cause  , idle misfire. On a 8v, engines, air leaks cause RACING RPM, never a misfire, this is because the MAP corrects the condition.
  19. Cold misfire?, on a good motor is usually spark . ( spark plugs, wires , cap ,rotor) (or the ECT reading way colder that REALITY)
  20. Injector(s) clogged. send them out for cleaning.    TOP reason for P030x single cylinder failure codes?(open coils in injectors,badwires, or ECU has  a dead injector driver (bad ECU)
  21. Injector(s) leaking. ( do a leak down test on the injectors during the fuel pump pressure tests.) Bad Injector o-rings , on a 8v they can be seen in action. (again, single injector fails )
  22. The injector has a bad base gasket, called the cushion, if  one leaks?, that cyclinder for sure will single cyclinder misfire, P030x (x = 1 to max )
  23. If only one cylinder misfires, on 16v, swap its injector with its neighbor.
  24. The 16v engine, is MAF based, all air leaks from the MAF to the head intake valves will cause a lean condition, if the ECU can not correct it and for sure WOT (wide open throttle)
  25. EGR MAIN leaking. (or gasket below it cracked? or any EGR piping leaking or missing or cracked gaskets. on 16v misfire. The main or its base gasket MUST NOT LEAK.
  26. Injectors not balanced. (partial clogs) (blance test failed)
  27. Bad or weak MAF or AIR METER  sensor.   Usually causes bogging. Light lean misfires, gross lean, bogs.  (OBD2 LTFT way off)
  28. The 8v MAP sensor never fails (hose to it top failure) but will cause wrong fuel mixtures if way off. 
  29. Vacuum leaks   (hoses/ gaskets anywhere in the Intake path)  causes fast RPM 8v and lean bog misfire in 16v MPI engines)  (OBD2 LTFT way off)
  30. Too lean Air and fuel mixtures.  lean misfire.  ( clogged injectors,  bad MAF)  Usually bogs. Spark plug tips "white" (OBD2 LTFT way off)
  31. Too rich Air and fuel mixtures ( spark plugs turn black and foul) Why too high fuel pressures, leaking injectors,  ECT stuck cold. (OBD2 LTFT way off)
  32. O2 heater dead. (plain bad, exhaust cracked near it. heater bad causing idle misfire.  Antifreeze contaminated, or using fuel additives) (OBD2 tells you ,its DEAD)
  33. Exhaust leaks near the 0² sensor,  causing the 0²  to mis-read,  this can cause 15 MPG on early cars. (low MPG on all cars) (OBD2 LTFT way off)
  34. Too Low fuel pressure causing , LEAN misfire.  32PSI  good.   below 29  running is bad (OBD2 LTFT way off)
  35. Too high a fuel pressure causing  , rich MISFIRE.  32PSI  good ,  above 43 is bad.   (OBD2 LTFT way off)
  36. Bad ignition coil(s) pretty rare,  1.8L has 4 coils. called C.O.P.s (swap them around to see if failure moves with new location)
  37. Bad Igniter unit, on all Kicks 89 -98  . extreme rare failure.
  38. Bad ECT  low , runs rich and rich misfire  (the ECT need to rear 180F if water at 180f) If the ECT ever falsely reads below 158F the EFI goes way rich. (OBD2 scan, see value, wrong?)
  39. IAT false low causing rich misfire, IAT falsely too high, causing lean.  The IAT is a major sensor on 8v and does little on the MPI , in fact missing on 1992 16v Sdekicks.
  40. EVAP canister flooded with raw fuel gets sucked into motor and it starts to misfire. ( unplug Canister  and plug engine side of  vac hose and test car like this.)
  41. In a 89/90 kick the Distributor can mess up vast numbers of ways, and is complex, get a rebuilt at rockauto.com and try it. This years, all spark is created by just it, never the ECU.
  42. You pressure washed your engine and messed up the dizzy or filled up the 16v  spark wells with water. ( I bet you won't next time.... LOL)
  43. 16v MPI spark wells filled  full of oil, a bad valve cover gasket ring seal kit.(not replaced at the 60k tuneup, it seems to me) My Felpro kit never leaks.
  44. Blown head gasket? warped head. Antifreeze barfing out tail pipe end.? some only leak enough to misfire. All these are bad engine.#2
  45. Cracked head?   (allowing water ingress, coolant)  really a bad engine #2. 
  46. On cars with Hydraulic lifter's; a collapsed lifter. (1.8L 2.0L and larger)
  47. Cracked cylinder walls. (  lets water ingress at any time or after a hot stop and restart).  See overheats page for simple tools to find this easy.
  48. Bent Head valves or springs, sticky valves, burned valves or seats (bad engine again, and fails compression tests)
  49. Bad power to ECU, or EFI system electronics modules, or  bad grounds to same.
  50. Bad Ignitor or coil. or bad Distributor (complex issues inside.)
I'm sure I missed one.  (sigh)
IN GENERAL:
However , there are many things that can cause true misfire. way too many.
If all cylinders are running grossly lean, then it is smooth power loss, or a bogging issue (see above link) Just feels like weak power and a bog.

( OBD1 cars that is pre-96 years, there are two catastrophic spark codes 41 and 42 , but they are for total loss of all spark or the CMP sender is dead)
These 2 codes can be intermittant ,so drive with the diagnostic jumper planted.
The common list of  Misfire reasons:

The short list.
Common problems  caused by:
  • Never doing the maintenance on the car , the top winner here, is skipping the 60K mile service scheduled Service Points.
  • Never changing the fuel filter, now you have low fuel pressure (lean misfire) and the pump overheats and fails. (or air filter skipped in above list)
  • Ignoring the Check engine lamp stuck on running? 
  • A Bad head gasket, and leaks coolant into the combustion chamber. (tiny leaks too)
  • EGR main sticking open.
  • A Slipped or stripped timing belt (or worse)
  • Low compression  (or bouncing plenum vacuum) The plenum, is that air space below  (behind on 16v) your throttle body.
  • Using cheap parts to tune up the car, (Duraxxxx junk) Try Bosch Mag wires. or NGK, run top brands of parts never NONAME parts.
If the CEL is on? running?, then pull the codes, you have everything to gain and nothing to lose by correcting these hard failures.

Things to look for:  Called "SIGNS or Evidence":
Engine oil:
Milky oil on  the engine dipstick or anything other then the  dirty honey look , as you drain the sump.
Coolant checks: (there are tools to find these faults)
Contaminated Engine coolant, oil backing up to the coolant over flow tank or by looking down Radiator cap (cold engine ONLY!!!).
Antifreeze (AF)
Look for carbon dust, oil or anything not perfectly green ( replacing Antifreeze every 2 years helps diagnosing problems, by having clean antifreeze in the RAD all the times)
Carbon or oil floating in the AF, is good sign of head gasket failure or far worse (a cracked head/block)

Not always true but mostly:  "Shooting from the hip !"
If the misfire is smooth , it's lean burn. (lean fueling errors!) Aka. BOGGING.
If violent, pulsing misfire, it's spark.  (can be heard too) rat-a-tat,tat,tat.  Bida Boda boom? Like a misfiring chain saw, ever hear that?
Too rich misfire?, feels like spark failure, because it is! the rich fuel fouls the spark plugs and they misfire. 
Remove the spark plugs and look at them , they will talk to you.  (was the 16v wells full of water or oil, fix that now)
Spark tips Black and wet with stinking fuel ?, that is rich misfire.
Ar the tips prestine  white at the spark plug tips? and dry then it may be too lean.?
Misfire can cause the engine to shake  hard,   See the shaking engine at  idle, looking at it .? That is gross misfire, usually the EGR stuck open. (push it closed with a finger.)
Old spark plugs and wires, love to fail when damp and cold,  and worse up hills.
If you flat refuse to replace spark wires at the correct service points, the insulation breaks down in time and finding this with diagnosis is very costly and labor intensive.
Not to mention, chasing false demons and replacing good parts.
Testing , and what do I do, to find the cause of HOT misfire.  I will make a list of all checks.  (just DIY , no advanced shop tools, AFR meters. etc.)
  1. Good fuel?and not old and not running summer fuel in the winter. No water in the fuel.
  2. Do the full tune up now, with cap,rotor,spark wire set, spark plugs and air filter and fuel filter. (we call that , a no brain'er..and fail on cold wet days)
  3. Check compression   (if all 4 look low the timing belt maintenance at 60k miles was skipped.  ??)  You can sneak a peek at it fast.
  4. Check vacuum (no bouncing allowed, 19 inches (about) at idle . if idle surges, that is not bounce , bad idle is a whole other , fix last issue.
  5. Ok the engine is good,  Raise wrench and yell Hooah ! all the below, is a waste with a bad engine.
  6. Fat blue , white spark at 4 spark plugs, and timed to perfection with a strobe timing light.  All Pro mech's do this at TUNE UP TIME, it's SOP, standard op procedures...
  7. Fuel pressure, (8 tests) the EFI is useless with out regulated and proper fuel pressure, yes, that takes a gauge too. for $20
  8. The engine thermostat runs at 180f to 195F and never varies HOT, ever... or FAIL!
  9. Defeat the EGR now.  after making sure it don't leak. (pull the hose and put in a golf TEE) It must not leak or idle will misfire. and RPM drops like a rock.(as a test only)
  10. EFI or Die...! (that means above is good , we now check EFI  fueling issues.
  11. On new cars this is much more easy due to advanced OBD2 monitors,  ( there is one OBD1 scan tool, mail order)
  12. The EFI system needs all the above to function correctly, so if you skip the order you fail. sorry that it physics facts. not opinion.
  13. I check the ECT sensor HOT, and will read 300 ohms, about, not 1000 or 0 or 100 .  use an ohmmeter on the 2 pins. Find parts here.
  14. On an 8V TBI motor I check the IAT sensor,  OBD2 allows me to do that , sitting in the drivers seat.
  15. Not having a scan tool makes this hard, but check that the CEL flashes 12s, when the DIAG jumper is planted, you can drive and watch it too, for intermittent DTC's
  16. Make sure the ECU is not stuck in Limphome mode, the advance of spark will be weak and very limited,  that is the best symptom besides 15 mpg.
  17. Check that the 8v MAP or   16v  MAF are working.  using a voltmeter.
  18. Check that the Exhaust manifold has no leaks (cracks?) near the front 0² sensor.  (a new 0² sensors is needed every 100k miles or go broke fuel costs)
  19. No induction air leaks, on a 16v engine or it will lean misfire. 8v races at full power and no misfire.  Very tedious inspections on 16v cars.
  20. Look at the spark tips?, are they white,cream or black with carbon and or stinking of raw fuel.. (old school AFR)
  21. In a shop we check the AFR hot and then work the failing lean or rich running.  At home you have only your eyes, and the above check for spark tips conditions.
  22. EFI fueling can be monitored in a shop or on the road..  Pro AFR tools. (as in, Air Fuel Ratio meters)
  23. The shop with 4/5 gas test systems $$$$, can use the same smog tester to find the AFR. Then work lean or rich, and high/low 02 or CO.
  24. We check that the ECU is holding closed loop at idle and light cruise.  If not ?,we work that first. On OBD1 we use a scope at the 02 cell pins.  OBD2 we do this.
  25. We make  sure the 0² heater is good and gets hot. (draws current)   (1) see key notes below.
  26. If the Fuel system is lean? we make sure all the injectors are working and that they are not clogged. (ECU is not commanding them correctly?) Scope views!.(2)
  27. If rich?, we make sure the injectors do not leak. or the ECU is not commanding them to be full on. (scope view again) and pressure leak down tests. already done above.
  28. A technician knows how to force lean or rich , manually , to see if the 02 sensor(s)  wake  up?, I will only casually mention this test, See ASE, training books.
  29. He adds air? to force system  lean, on a 16v. (he pops off a vacuum line) or adds propane gas, to force rich. On the 8v ?, to get lean, you hand pump the vacuum line on the FPREG (front edge of TB). (this lowers fuel pressure ,un-naturally and results in leaner AFR) we know the pressure drops because we already did that at line 7, the 8 tests.
  30. The key factor with EFI? , is the ECU doing the bad fueling? ,due to bad inputs? or are the outputs bad? (injector) Bad injectors or Injector drive is bad.
  31.  Clogged Catalyst converter?,  use the  vacuum gauge 2 ways , to find that.  (mouse nest or squirrel nuts in the air cleaner, just looking now?)
The real question?, is how far can you get, in this list ,with no tools? or tuneup parts? Step 1. or clean the EGR.  (tools, I'm not talking simple spanners)
My lists are for the highly motivated, willing to spend $100 in shop tools,  (simple gauges, strobe light, etc) HF tools are junk but are cheap and throw away after tools.!
This is just a basics list and is not all inclusive, engines can do funny things, like a partial slow coolant leak to any cylinder. >>  There is a Radiator leak down test for that.
You can have 1,2,3 failures at once, even one failure hiding the other , for sure. (eg: high fuel pressure hiding air leaks?)
I made it to 31 and gave up.  Only Fate determines what fails, only that, and luck. Nothing made by man lasts forever, not even close.

Key Notes:
(1) The 89/90 kick has no 0²  heater and may not run closed loop at idle , in very cold weather, but will at cruise.  0² = means oxygen sensor., or its readings.
(2) The injectors are checked for Pulse Width with a scope then we use tables for the correct valves, hot ,cold, idle,unloaded, or a loaded engine (advanced ASE studies)
Is the exhaust manifold glowing cherry red?, yes?,your running in limphome.  (96+ cars_ flash (every 2 second) the CEL lamp for gross rich running)
Catalyst running real hot? and stinks, again, that can be limphome  mode or just very RICH  AFR.
No idle controls? No full spark advance? ( you lost control of idle speeds? )
The Jargon words are all here.
OBD2: Checks: 1996 and newer cars..
In 1996 Suzuki added a newer and smarter OBD system called OBD2, and has a new added Monitior, called MISFIRE MONITOR. (among many others)
P0300 , all cylinder random misfire and P0301 to P0304 for each cylinder only misfiring.
If you get P0301 then look for things that make #1 cylinder only misfire. 
The 96 cars and newer have OBD2 scan tools ,see my full scan session here.,
  The False Misfire Phenomena
Watch out for false P0300 DTC  because a loose timing belt, on the cam, can cause this failure and is not misfire, but is doom for the belt regardless (you forgot the 60K tuneup)
Loose timing belt, loose idle on same belt, loose crankshaft pulley , loose cam cog pulley.!  If the Belt whips and flops, 300s HIT.
AFR caused Misfire on newer cars:
If you get P0300  ,random misfire. look at the LTFT. (Longterm fuel trim) if the engien is lean it will misfire, and LTFT is the clue.
The Long Term Fuel TRIM, PID.   (PID is parameter Identification)  Each ECU parameter has a name. (and number , under the hood)
Mine runs -3% hot.  if not near center 0 %  that means the ECU is doing a huge correction to the fueling.
If the LTFT is huge + plus value, EFI is running lean, if a huge negative value -, it's running rich.  (air leaks for lean, leaking injectors for rich, or excess fuel pressure.)
Make sure the engine runs in CLOSED LOOP HOT, at idle and racing motor at 2500 or at light cruise, if Closed loop  fails, fix that first.
Some Suzuki 4wd cars have Misfire monitor detuned due to the off road usage.  (I've seen this on 1.8L) So may not report felt misfires.

HOT means coolant is at 180 to 195F and holding!
AFR means Air fuel Ratio, or fuel mix. 
Tricks:
You can use a small rubber hose to your ear, say 3 foot long (1meter) you can now do 2 things.
  • Hear vacuum leaks as you play the hose over all induction paths. (besides using propane raw gas to find a leak , noting idle speed changes as you do.
  • Hear exhaust leaks anywhere near the 02 (oxy) sensor, behind exhaust manifold too.(shields off)  you will hear putt putt putt, (FAILURE)




Further Information.
If your 96+ car has misfire, read this book.

The book is by Tracy Martin,  it is very good and on topic.
"How to use Automotive Diagnostic scanners."
Revision.9  -  added major crosslinking to related topics. etc.  2-21-2013