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.
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
,the non steady beat of 4 cylinder firing, is no longer perfect
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
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
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)
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
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.)
- 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?
Fueling. (be that too rich or too lean.) (even vacuum leaks in
the induction system or near the front oxy sensor, will stuff the
For sure misfire , is the most complex topic on engines
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
The scan tool you can see the advance hit 40 degree's racing the
motor fast parked. See that
are seeing DTC# P030x
and family. LOOK IN PENDINGS !! too ! not just stored
codes. P0300 is random misfire, P0301 is #1 cylinder misfiring)
must warn you 96+ , you will get false P030x DTCs, for any reason
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
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)
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.
Measurement error's during misfire testing , see Duane M. In
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
do need tools:
still need Tools and gauges, to repair any car, if not willing to
buy tools, you will fail, or damage car or worse yourself.
Misfire is the most complex and hard to find.
Misfire is bad combustion ! on
one or more cylinders!
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.
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)
I'm sure I missed one. (sigh)
fuel , older
than 6 months it can go bad.
- A Bad motor
causes low compression on 1
more cylinders (150 min. to
190 PSI varies by engine type and altitude. (90PSI x4 is bad Timing
belt, No 60k tuneup, again.
- Valves guide stems sticking. ( can be seen with a
vacuum gauge "bouncing vacuum"
- Cam timing accidentally ,
set wrong (or
belt slipped) or the crank snout damage common failure.
wrong by accident or by poor tuneup action or someone guessed at
timing, or left the clamp bolt lose, or crossed over the wires.
- Not doing the 60K full
- Lash set wrong. (60k
service points ignored.) (too
tight? , misfire and burned valves, too loose? , noise)
- Vacuum is bouncing in the intake manifold. (bad valves
sticking .) 800 RPM 19 inches
HG & steady, is normal.
- 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
- 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.
- The CAT
converter is melted closed,
this chokes the engine to death ,so it can not breath, the engine
Air pump. A bad CAT , can in fact render the good engine to BAD ENGINE.
- Over heating or coolant not reaching 180F to 195F and holding, the coolant must
reach normal temperatures or the engine will misfire rich.
- The Frigid
motor problem will misfire.
- Bad or Weak or late ignition spark or cam
timing. ( Do the 60K mile Tune
belt and all major spark parts and filters )
- 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
- 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)
- Constant misfire?, spark wires crossed over or spark plugs
oil soaked or carbon fouled or dripping in pure raw liquid fuel.
Do look at all 4 (to max) spark tips.
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.
- Cold misfire?, on a good motor is usually spark . ( spark
plugs, wires , cap ,rotor) (or the ECT reading way colder that REALITY)
- 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)
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 )
- 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 )
- If only one cylinder misfires, on 16v, swap its injector
with its neighbor.
- 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)
- 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.
- Injectors not balanced. (partial clogs) (blance test failed)
- Bad or
weak MAF or AIR METER sensor. Usually causes
bogging. Light lean misfires, gross
lean, bogs. (OBD2 LTFT way off)
- The 8v MAP sensor never
fails (hose to it top failure) but will cause wrong fuel mixtures if
leaks (hoses/ gaskets anywhere in the Intake
path) causes fast RPM 8v and lean bog misfire in 16v MPI engines) (OBD2 LTFT way off)
- Too lean Air and fuel mixtures. lean misfire. (
bad MAF) Usually bogs. Spark plug tips "white" (OBD2 LTFT way off)
- 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)
- O2 heater dead.
bad, exhaust cracked near it. heater bad causing idle misfire.
Antifreeze contaminated, or using fuel additives) (OBD2 tells you ,its DEAD)
- 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)
- Too Low fuel pressure causing
misfire. 32PSI good. below 29 running is
bad (OBD2 LTFT way off)
- Too high a fuel
rich MISFIRE. 32PSI good , above 43 is bad. (OBD2 LTFT way off)
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)
- Bad Igniter unit, on all
-98 . extreme rare failure.
- Bad ECT low , runs
rich and rich
(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?)
- 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.
- 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.)
- In a 89/90 kick the
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.
- 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
- 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.
- Blown head gasket? warped head. Antifreeze barfing out
tail pipe end.? some only leak enough to misfire. All these are bad
- Cracked head? (allowing water ingress,
coolant) really a bad engine #2.
- On cars with Hydraulic lifter's; a collapsed lifter. (1.8L
2.0L and larger)
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.
- Bent Head valves or springs, sticky valves, burned valves
or seats (bad engine again, and fails compression tests)
- Bad power to ECU, or EFI system electronics modules,
or bad grounds to same.
- Bad Ignitor or coil. or bad Distributor (complex issues
However , there are many things that can cause true misfire. way too
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:
Common problems caused by:
If the CEL is on? running?, then pull the
everything to gain and nothing to lose by correcting these hard
- 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
- Using cheap parts to tune up the car, (Duraxxxx junk) Try Bosch Mag wires. or NGK, run top
brands of parts never NONAME parts.
Things to look for: Called
"SIGNS or Evidence":
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!!!).
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
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.?
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.
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.)
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)
- Good fuel?and not
old and not running summer fuel in the winter. No water in the fuel.
- 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)
- Check compression
4 look low the timing belt
at 60k miles
was skipped. ??)
You can sneak a peek at it fast.
- Check vacuum (no bouncing allowed, 19 inches (about) at
idle . if idle surges, that is not bounce , bad idle is a whole other , fix
- Ok the engine is good, Raise wrench and yell Hooah ! all the
below, is a waste with a bad engine.
- 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...
- Fuel pressure, (8 tests)
the EFI is
useless with out regulated and proper fuel pressure, yes, that takes a
gauge too. for $20
- The engine thermostat runs at
180f to 195F and never
varies HOT, ever... or FAIL!
- 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)
- EFI or Die...! (that means above is good , we now check
EFI fueling issues.
- On new cars this is much more easy due to advanced OBD2 monitors, ( there is one
OBD1 scan tool, mail order)
- The EFI system needs all the above to function correctly,
so if you skip the order you fail. sorry that it physics facts. not
- 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.
- On an 8V TBI motor I check the IAT sensor, OBD2
allows me to do that , sitting in the drivers seat.
- 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
- 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.
- Check that the 8v MAP or
working. using a voltmeter.
- 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)
- 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
- Look at the spark tips?, are they white,cream or black
with carbon and or stinking of raw fuel.. (old school AFR)
- 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.
- EFI fueling can be monitored in a shop or on the
road.. Pro AFR
tools. (as in, Air Fuel Ratio meters)
- The shop
with 4/5 gas test systems $$$$, can use the same smog tester to find
AFR. Then work lean or rich, and high/low 02 or CO.
- 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
- We make sure the 0²
heater is good and gets hot. (draws
see key notes below.
- 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)
- 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.
- A technician knows
how to force lean or rich , manually ,
to see if the 02 sensor(s) wake up?, I will only casually
this test, See ASE, training
- 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.
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.
converter?, use the vacuum gauge 2 ways , to find that.
(mouse nest or squirrel nuts in the air cleaner, just looking now?)
My lists are for the highly motivated, willing
to spend $100 in shop
tools, (simple gauges, strobe light, etc) HF tools are junk
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.
(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.
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
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
The 96 cars and
OBD2 scan tools ,see my full scan
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
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 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
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
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.
coolant is at 180 to 195F and holding!
AFR means Air fuel Ratio,
or fuel mix.
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
If your 96+ car has misfire, read this book.
book is by Tracy Martin, it is very good and on topic.
"How to use Automotive Diagnostic scanners."
- added major crosslinking to related topics. etc. 2-21-2013