My cars engine dies(Stalls) at idle , coasting or under power !                       
Related links first:

Pure Physics: (makes the best short list, it does)  Lots of tests using meters and cheap gauges, find all this easy !
  1. Lost 12vdc POWER (11-15vdc) to spark or ECU. It will stall like a falling brick,  check fuses yet ?  (cracked, loose, corroded fuses?)
  2. Engine bad?, up to 50 causes, lost compression, timing belt slipped (cam drive!), burn head valves ,blown head gasket, and many more.? (valve spring broken? keeper missing?)
  3. Weak or lost spark.?  never ever, tuned up, is like a disease here. "folks drive until it stalls ,with zero service" or was tuned-up and spark wires now crossed, oops?
  4. Bad fueling ,Weak or over fueled.?
  5. A lack of fresh air to the engine (air cleaner packed up, IAC valve dead  "cold engine", ISC stuck closed "hot engine"?)
  6. Combustion failure, is really just the above 5, but add EGR main stuck open , or coolant leaks ( to cylinders (green spark tips?) or melted CAT guts)
Top causes (popular):
  • Never had a full 60,000 mile tuneup this decade. The Disease I call this.
  • A Sticking EGR main.
  • Cam Timing belt never changed. (cam drive failure happens on any engine made, be it gears,belt or chain, as does chain tensioner failures big time)

Most times, it's very  weak combution for a variaty of reasons and resulting in gross misfire.
MISFIRING, for any reasons (lots) This one is most complex and is also combustion failure. (takes more tools and skills)
If car will not restart ? but cranks ok, then go here; the nostart page.
Top get help questions, by your mechanic will be , "can you save the stall, with first right foot action's" and will it restart easy:?"

 THE ECU MAY  NOT TELL YOU WHY it stalled (but sure; do look,  SCAN IT.)
(you can put the DTC Diagnostic jumper in place (pre96) and catch an on the fly DTC error  driving  "41 or 42 DTC"?)
{or using a full scan tool on 96'+ and newer cars and logging all the PIDs and DTC's.
 (PID'S are live sensor data  "parameter ID's) DTC P03xx errors? SCAN live RPM drops to zero like a stone? drops faster than Engine spins down?}

EFI magic:
The electronic fuel injection system (ECU/PCM) works overtime to prevent all stalls, so if it did stall, it's a very bad and telling thing.
The ECU opens the ISC adding up to 400 RPM';s worth of air more on a hot engine.
The only question is why?
The engine has 4 cylinders, pretend it's 4 engines, and all this gets more easy,  (check for spark at all 4 spark plugs and check all 4 injectors (MPI).

No Restarts, or it Restarts and then dies:
One of the most difficult to diagnose , unless it stays broken.
The number 1 test for stalls, is can you, prevent the stall with fast right foot action, yes /no?
Yes, easy to save it from stalling,it's starving for fuel. Spark is ok.
Yes, but it's harder, the EGR leaks, and have full power,  on demand, if it do this act.
No,  catastrophic failuers (lost DC power, lost spark , lost all fueling)

TOP FAILS:  (assumes a good Engine and can pump air well )   A bad engine is low compression and top cause is slipped cam timing belt.
  1. The battery is going dead, low or discharged, it needs to read 13.3v to 15vdc running.
  2. The Bad need of the 60k mile tune up. (spark parts and filters and cam drive belt !)
  3. Loss of spark  (check for spark x4 ) The Spark plugs, cap rotor or wires old and no longer serviceable. How to test for spark.
  4. Spark timing way off,  (bingo , cam belt/chain slipped !!!) Compression below 150 PSI (W.O.T)?
  5. EGR main sticking  OPEN at  as you come to a stop.  {a bad EGR main} or is hard to start and EGR stuck open full time. (clean it)
  6. ECU/PCM stuck in failsafe or backup mode. (low power, black smoke, red hot Cat)  Misfire cause the RED CAT/manifold, easy.
  7. Stuck ISC/IAC valve  , (very common) sticking for any reasons (use right foot to see if tiny added air helps. yes?, then suspect ISC) Loss of air.
  8. The MPI engine will not like (MAFbased engines) any air leaks in the main air path from MAF to intake valves. period,  and 20 year old hoses here fail big time.
  9. Low fuel pressure.  (use gauges)
  10. Leaking, stuck or dead injectors. (clean them and do the NOID lamp test) do the balance tests and leakdown tests?
  11. Loss of fueling "injections" ( many reason, even a bad fuel pump ground behind left rear tail light 2-DOOR only) chcck fuel pressure!.(use  a Gauge)
  12. Flooding, will stall any spark engine. (check the symptoms quickly ,while failed) a.k.a.  TOO MUCH FUEL.  SPARK TIPS ALL SOAKING WET?
  13. TPS idle switch stuck open , signaling  not at idle,  this switch is critical every time you stop. This switch must close. Test and calibrate it.
  14. Loss of main elecrical power ( corroded /cracked ,loose fuses? "FI or IG-coil" , can cause stalls.   (generic: EFI fuses)
  15. Bad or weak sensors "ECT, MAP, MAF"  Intermittant CMP or  CKP Sensor (the ECU WILL REPORT THAT, TO YOU, JUST SCAN IT.)
  16. Bad fuel. (or full of water or Diesel)
  17. A bad ECU.  (put in new 2 o 3 large Capacitors (Cap.'s) and be happy,  all fail once up to year 1995. 96+ are ok !
  18. Engine never reaches 180F or more and holds, can run very rich and stall. The ECT sees the cold and ECU adds to much fuel.
  19. The Engine is overheating or the ECT is fibbing saying it is and is not? Use an IR pyro temperature gun to see TRUTH.
  20. The idle up input to the ECU can fail,  A/C or Power-steering overload switches.  (A/C kicks on, and engine stalls?)
The Basic reasons for  a stall are,  loss of  fuel, flooding, or loss of spark. (or EGR is sticking open)  
Under full power stalls is , loss of spark or fuel.  
If at any time, running your ECU thinks (spark monitor input into ECU)  the Spark is bad, for any reason, the ECU will immediataly cut fuel. and disable  the fuel pump .
It will throw DTC' errors , if and only if, the diagnostic jumper is in place at the time of this failure ( code 42/42 many not be stored, only seen live !)
On USA 1996+ cars it throws, Pxxxx DTC errors or you see RPM go to Zero while engine is still spinning (CKP fails)
Learn to use a scan tool and do it parked ,idling ,running, moving, driving  or stalling.  Let the PCM talk to you, EVEN CRANKING for 5 long seconds !!!
The PCM NEVER STOPS LOOKING FOR ERRORS (well, unless in backup mode, Dead PCM/ECU).  (Proven with bench testing, by me)

Fast Testing way's?
If the engine likes to stall at stop signs and the like, then try to save the stall.  (called feathering the gas or throttle, or happy right foot?)
If it can't be saved, at all is usally bad spark (or total loss of fuel pressure)
If it is very hard to save it from stalling but you can with aggressive feathering, you have a leaking EGR
If it is easy to save it , then you have air starvation, suspect TPS failure or ISC sticking closed, as you stop car.
Does the CEL lamp come on , key on, then go out and stay out driving.  If CEL fails to come on Key on, the ECU is dead or the lamp is dead.
Yes, the CEL will come on when any motor stalls and RPM drops below about 300 RPM. The CEL glowing in this instance, is to say motor stalled.
If the ECU is throwing DTC failures you need to check sensors:  See here , to test and calibrate sensors.
Reason for stalling motor , in no order, are listed below:  (bad motor,spark, fuel problems, and electric power)
Stalling is always gross misfire, if the engine misfires at idle, try killing spark one by one (not additive) and see if one cylinder is dead. (no RPM drop is dead cylinder)
The injector can be balance tested with any fuel pressure gauge too.
I will now elaborate on all ways to fail, one by one.
1: A Bad engine:
The engine is an Air pump, if the engine is sick ,  check compression and if  EGR is stuck and Cat melted.
See here for a  BAD motor , tests and solutions.

If motor runs great most the the time, then we can discount a bad motor.

2: Loss of spark ( lack of a tune up for over 60k miles, or bad Distributor CAS/CMP/CKP sender.)  Always check spark first. (on all 4 spark plugs and from coil)
If spark dies intermittanly , Special tools !
A shop lab scope or DSO can see spark misfire easy, most DIY folks have no such tool.
Strobe testing: "automotive timing light ,xenon strobe) sold for like $20 everywhere. Check each spark wire for spark loss or drop outs.
If a stobe  light goes dead, at the motor stall (before 1 RPM is hit) then you are loosing spark. (common on 89/90s with the funky (VR-ignitor) Distributor it uses)
I tape the timing strobe ON, and tape it too the hood at 90 degrees so it shines to my passenger side. Then drive and look !
I can monitor spark in the cab driving with my DSO attached to a mag pickup on the spark wires or the coil minus terminal or the spark read bad pin of the ECU.
I use a mag probe on wires for my DSO scope or wrap hookup wire around the HV spark wires, 5 to 10 times the scopes probe to my DIY coil. I then see spark.
You can attach a test spark plug to each wire 1 by 1 and check spark, like done for 100 years now. Use a real plug, gapped to 0.028" "28 Thou"
The 1996 and newer cars, all , in USA, all can report bad spark (actually misfire) P0302 is #2 cylinder misfiring. P0300 is random all misfiring.

3: Loss of fuel:  ( are  spark plugs are dry? )  (check fuel supply)
Bad fuel,  rust in tank clogged up the filter, and injectors?
Listen for fuel pump buzz at the gas filler cap removed. ( 1991-98' kicks it buzzes , for 3seconds at just key on and stops)
Is the 2 Door pump ground, behind left rear tail lamp rusted out? killing pump dead, (pumps ok,just this)
The PUMP dead.  (its top connectors can fail too, with corrosion only)
The Fuel Pump Regulator FPreg, on fuel rail,  can be bad, stuck at too high or low pressure or leaking from the wet side to the dry vacuum side causing 2 failures!
The 16valve FPreg, must raise fuel pressure 6 to 8 PSI, at wide open throttle, or its bad, (or pump fails shunt test)
The ECU , can stop injection.  ( sees wide open throttle cranking this is a feature "unflood mode) or injector drivers can fail.
The injector can be bad or its wires bad.  The injector can leak or clog. The injector coil can open  1 ohms 8v,  10 ohms 16v.
If the motor stalls for lack of fuel, I can attach a fuel pressure gauge to the fuel rail  or filter banjo test port (6mm tread) and see if pressure drop below 30 PSI?
The fuel pump can lose power, 12vdc, so I monitor the power at the rear bumper pump connector for 12vdc minimum running.
The worst case test of fuel pressure, is  up a hill , wide open throttle.

4: Loss of air supply, at idle can stall you .
No engine runs with out air, (can't burn fuel without oxygen)
A hot engine only has 1 air path,  the ISC so if it sticks closed,it will never start  and stay running, with out drivers vast right foot actions.  (the bleed screw is more air, but is small)
The  8V has  5 paths of air but 2 are critical, the  IAC Thermo , and  ISC electric . Look here about IAC valve, etc
8V dash pot on Throttle body left side is dead causing slow cold rpm during the first 10 seconds of cold start warm up.
Both engines IAC(thermal) are open below 150F coolant temperatures.  If not, cold starts will be hard.
If motor stalls, hot, the Loss of air can cause this (a sticking EGR main is more common fail) but the ISC can fail to open ,starving engine of all air.  Clean them both.
If the TPS switch side, does not report IDLE , all idle controls go dead. But usually only stalls in A/T trans DRIVE with A.C on full bore. Never on M/T.
If the stall is easy to correct with a fast easy right foot, the Air supply is lacking (feathering the gas foot with 5% throttle proves this)
If it is difficult to feather IT, suspect the EGR as failing.
On a hot engine ,the ISC must open or the engine will stall.
The 8v TBI races with excess air, the 18V MPI stalls if that air is unmetered (hot) (vacuum leaks caused)
At red traffic  lights the ISC holds the idle at 800 RPM,  if the ISC sticks, motor may die, if it sticks  coasting or idling.
If the TPS idle switch side,  is dead (open), the ISC is inactivated by the ECU at idle.

The ISC is left and the EGR is  right, below. Clean the EGR AND ISC LIKE HERE.



5: Running too rich and flooding or fouling the plugs with wet gasoline and carbon deposits (shorts them electrically).
Can be caused by  above stated #1, 2,3,4 , or ECU bad or ECT , IAT bad, or 100% clogged CAT converter or a cracked exhaust manifold or header.
Rich is common when the ECU goes to FAILSAFE  mode because of too many sensor failures.  Or Backup mode (worse).

Stuck open fuel injector(s.) Dribbling or even clogged (due to confusing the ECU, dead cylinders can set the 02 sensor lean, and the ecu goes max rich... in error)
Engine vacuum not steady at idle (test vacuum ,it must be near  a steady 19 inches of HG) (less on old motors and less if idle is wrong)

5B:  The O2 sensor is bad, will cause lean or rich fuel mixtures at idle or , flat easy cruising. only.
 The 02 is not used, accelerating. or at wide open throttle.
5C: The ECT if open circuit  or the wires fall off it,  An open ECT signals -39F coolant, and EFI goes MAX RICH. flooding engine. All the time, as does a bad low thermostat.
The ECT unplugged reads 6000 ohms at 32f and 300 ohms, on a fully hot engine.(see my chart)
5D: Fuel pressure running way to high. (bad Fpreg or other problems related  there are at least 4 tests for the FPreg,  leaking diaphram and 3 pressure points using vacuum)


6: To much Air.  (or vacuum leaks)  both engines, G16a and b.
Unmetered air  is going to cause a lean state.  (he 02 sensor can correct small error but not large or when accelerating from idle, it will be lean)
Small leaks are ok, pin holes?
The ISC and IAC and bleed screws, all use metered air. (hoses not cracked)
The 8v TBI engine, does not ever go lean, with air leaks, it just races. (MAP systems)
The MAF MPI engine  must measure all incoming air, if there are leaks behind it to the engine, the hot engine will go lean, that is if the 02 sensor can not correct that.
But the O2 is off line accelerating, so metered air must be accurate.
(Air  sneaking past the MAF , illegally is unmetered air)
The ECU will report a dead MAF or MAP
It will never report weak.
The 16v MPI system is the most difficult because any leaks at all are bad. end to end.
Many hoses to leak and check.
Some are so bad the mechanic uses a smoke machine to find them.
We pinch all hose running to find leaks or inspect each carefully . (inspect the MAP hose only)
 GM says tap gently on the side of the MAF idling and if RPM changes, suspect a bad MAF.  It's easy to test.
The MAF must report (air meter) correctly at all times. the 8v uses a MAP, it too must be connected and working.

7: The TPS switch is bad (all motors 1.6L). (not the throttle angle TP resistor  POT side, but the other IDLE switch side)
Usually this will not stall any engine. If it does, there are 2 problems.  But with and A/T trans car IN DRIVE, and A/C on,sure it can stall.  M/T mostly not ever.
The switch must be calibrated and tested .
The switch is easy to test , by using an ohm meter.
It must read less than 300 ohms at idle.  (It goes to infinity , off idle, if not?, it's bad or never calibrated)
At idle, the switch must be closed!.  It must be calibrated (see TPS icon),  after  its  fiddled  with or replaced.
Its not the sensor if car stalls parked or parked in neutral, idling.
 
8: Any EFI sensor or actuator (EGR?) failing .  EGR STUCK wide open or cracked open leaking air ,see  #6 for cracks near all EGR devices and tubing.

9:  Many things can stall the engine. DTC's errors and FAILSafe modes can stall the engine, as will BACKUP mode.
Both Failsafe and for sure backup mode, the spark plugs will turn black and stalling in eminent.
If the ECU sees, lost spark, it cuts fuel NOW.
  • TPS switch signaling , wrong, showing not idle at idle. (on A/T engines in drive and A/C on, this can stall you)
  • Engine not signaling hot (ECT) (must be above 150 deg. F)[normal is 180 to 195F].  [ bad coolant thermostat ] (or installed wrong !!) cause Rich AFR TOO !
  • ECU sees loss of CAS/CMP signal. (Distributor CAS, cam or Crank , as the year and case applies)  Set the DLC Diag jump and see if you get code 41 or 42.
  • ECU sees loss of Spark Feedback ( tachometer brown  wire)  from the Coil Suppressor module. The ECU sets error codes on all cars for that, CMP or CKP sensors.
  • The newer ECUs, may show   bad RPM (scan tool only) reading at 0 RPM cranking or P03xx errors. of these sensors fail.  CODE 41 AND 42 too.

10: If the RPM drops below 300 ,  the engine will stall,  the ECU opens the ISC wide, as rpm falls below 750 RPM,  and prevents all stalls, normally.
If the ISC is working ,then the stall is caused by bad fueling or spark.
As the ISC adds air ,the MAF/MAP sees that and adds fuel "to match",  if spark is missing , it can't burn the fuel, and stalls.
If the ISC opens and spark is good, but fuel pressure is way low, it will stall.
If fuel pressure is like 60 PSI you will never stop the flooding and stalling. (bad Fpreg, or pinched fuel return line)


11: Total Exhaust blockage by CATcon melting inside or Offroad damage. (some CATS can break up and plug an exhaust muffler)
It's pretty rare but can get so bad the engine will not start. or starts and stalls fast.
Cat back pressure is 2 PSI max at 2500 RPM, measure that at the 02 dismounted bung hole, !
The vacuum test ,it is easy to see a bad melted closed CAT (catcon)

12: The EFI system when confused or has bad sensors, will drop to Failsafe. mode  and will be rich , retarded timing, and RPM limited,  this may foul the spark plugs and the engine will stall. (so will ECU backup mode)
There are no idle controls in failsafe mode. and for sure never in Backup mode.
Most failsafe actions  cause/ set (throw) DTC errors.  In failsafe, the engine will run rich and lack power and full RPM. (and huge carbon monoxide emissions)
ECU Backup mode  , causes he ECU CEL lamp to be dead all the times. and loves to flood.
Failsafe sets retarded timing, and limits RPM,  the engine goes way rich, black smoke out the exhaust and lacks 1/2 its power.  Compustion belches out tons of CO, carbon monoxide.

more?

13:
Loss of Battery  POWER?  ( I have seen batteries short while driving, voltage dropping to 10v or lower, instantly)
I could add 1 more reason, total loss of  electrical power, but you should know that, if the head lamps don't work, the engine will not either.  (if both head lamps are dim , then the battery is low)
Check battery voltage , it must be near 12.6v resting key off, and about 13.3 to 15vdc running.
This includes bad grounds on the engine and at the battery connections.
The engine bay  fuse block is notorious for corroding and killing power,  especially the one on the in engine bay right finder monster fuses.
LOOK underneath the block.
Power loss, causes instant spark lost (first) and this is a VIOLENT cut in power, and even more violent, as it cuts back in , gas foot heavy
Spark loss is classic, in its behavior.
Smooth loss of power is fuel pressure, (pull fuel pump relay to see that)
Violent loss is spark or ECU killing spark,.

Please do NOT throw parts at the engine ,but DO test them.

I'm sure I missed a cause of stalling, but this page covers most.
Bad engine,late cam timing due to cam belt slip, burned engine head valves, a flat cam (lobes damaged) and over 50 more deep engine problems, cracks in the water jackets, etc.
Broken cam, broken cam cog. ,stripped out cam or crank keys. 
(yes, check compression and look a the cam, for damage to it.) broken valve springs, missing keepers.
endless this topic, near.
Commentary:
It could be a bad ECU, but you will have other symptoms. 
The ECU usually only fails for 3 reason, 2 really.  Caps bad, acid damage from them (random fails) and bad injector drivers, NOID lamp dead or stuck on.
Pull the codes from the CEL lamp.  ( it has a record of all failures, in memory).

Tricks:
Mash gas pedal to the floor and CRANK, to unflood the engine. This is called SECRET WOT unflood mode,  WOT = wide open throttle.

Key on , and off 3 times, to hear the  prime fuel pump sounds. (not on any 89/90)
Rap the Fuel pump Regulator with a screwdriver handle.  (Rap is not a song/jive, it's an action) this can cure lean or flooding problems, (temporary)
Pull the FPregulator vacuum hose, see no fuel here at hose,  if you do ,it's bad. (this regulator) Smacking it, can unstick it.
Smack the Fuel tank, if the Pump won't run at key on, for 3 seconds, or you think it is running weak.  All 89/90 and some 1.8L sports only get pump action cranking.
Smacking the side of  starters works for NO CRANK (a weak soleniod)  (smacking parts is no cure, it's just a clue)
Check spark with a timing light if motor ever  fails to start. The engine can't run without spark. (and Otto cycle engine) if way off? the cam timing belt slipped.
Check  DTCs at any time, for DTCs , keyon, cranking 5 seconds or running, in fact leave the DLC DIAG jumper in place and see the ECU tell you why the motor died.
Look at the spark plug tips to see  if the motor is flooding.  
White/cream/tan and dry are normal or lack of fuel  is white.  try SPRAY CAN test fuel .
Black or brown spark tips, and wet and stinking of raw fuel is flooding of the engine.
Try Secret WOT Unflood mode. or pull the fuel pump relay to kill fuel pressure and unflood motor. (wide open throttle)
Try extra air by  adding 5% throttle  with the the right foot and start car,  this adds extra air and  a weak air supply engine (ISC stuck?) may allow starts. Gives you a clue.
Not cleaning the EGR main invites hard starts and stalls at all red  lights.
ECU: Brain,  what magic does it do, to prevent or correct stalls (lots !) and does no constantly ! Dogmatic to the extreme. A true Servo it is.
  • It sees RPM drop below 750RPM and will advance timing,(spark ) a bit or if the drop is excessive will open the ISC valve more. (called idle control loop (servo))
  • If by chance the above fails, it opens the ISC more , until it's at 100% open . "idle speed controller actuator"
  • If this fails, the ECU will just keep the ISC open or in some firmware versions (years) will hunt the idle, up and down. (this is good, as it might unstick a stuck ISC)
  • The ECU many not be able to get AFR correct and injects fuel based on air flows. so if fuel pressure is low (or horrid low compresson) It will fail.
  • The ECU also fails in Failsafe mode, and Backup mode, to even attempt to use idle controls.
  • If the Dash pot (cold start fast idle device) fails, all the above fails. If the IAC fails on a hot engine, the engine races and there will be no idle controls.
  • And 50 more reasons, but are just 3 in physics,  bad engine, bad spark ,bad fueling,  mostly too lean but gross rich misfire too.
Super simple diagnosis?
The below drawing can help find why engine stalls, engine at 150 psi  compression or more and a closed EGR main valve, and no melted CAT.
In no order,  fate wins every time.
  • Bad fuel (includes water in fuel or diesel mixed in?)
  • Bad injector(s) leaking, or clogged. It's interal Filters clogged.  NOID lamp test failures?
  • Fuel pressure not at spec.  30 PSI; not 60 not 10.  (FPreg tests passing?, and not leaking fuel from wet side to vacuum side)
  • Bad spark for any reason or timed wrong. Tuneup time.!
  • Lies to the ECU, bad sensors, ECT, MAP, MAF.  (the ECU is a computer and as such, the inputs need to be correct or will inject fuel wrong)
  • Loss of 11v to 15v at ECU at the ECU power pins. Check battery voltage running.
  • CEL lamp off driving. (but on, key on, not started) Check for DTC errors. (1996+ scan the PCM,  I ignore this light,mostly)
  • Rust in tank clogging the main fuel filter   , over and over and injector internal screens.
  • Bad ECU Cap.'s.  a $5 fix.
  • Wrong gas cap on tank causing it to build vacuum and stalling the pump.
  • ISC valve stuck closed. (clean it) cold start air valve needs to work on any cold engine.
  • Engine coolant not reaching 180F+ and holding, bad thermostat   or installed upside down?

There is a binary rule for failure here.
  1. ISC is dead.  ( can be watched with scope or duty meter for "life")
  2. Engine runs bad. (or both) but gross misfire tops that list.
The A/T trans car,will stall ,way before the M/T trans, because the drivers foot on clutch cars, is more smart !!! (knows how to match load to RPM naturally)
ceation: 3-9-2010
v7, 3/17/2011 , edits, and needs lots more, subject is complex.  (2014 more links, lots more)