Electronic Fuel Injection (EFI)  Troubleshooting  (ECU)               
SideKick & Trackers. 89-98
The purpose of the EFI system (spark and injection) is to recover the lost 25%  efficiency of Carburettors and to keep the air clean (keeps the CAT running, as much as possible)
Some studies show 35% efficiency over carbs.!

My page does not apply to hacked or modified cars. Stock cars, not with CAI mods, or any other gross EFI modifications, or removed CATS or EGR.
There are scan tools that allow you to see live data and how most things are working or not. 
If you have 1996+ USA car,  all scan tools made support this Sidekick
The EFI system is totally dependent on inputs. (sensors)
The #1 input ,is called a constant; the ECU expects 30PSI fuel pressure running, if not near there, the ECU may  fail to inject properly, the factory spec is clear on this.
The Injectors must work and not leak and the spark must be hot (blue/white) and timed correctly.

The number one failure complaint is poor MPG  (15-20mpg)  and that is really bad. {19-30 MPG is EPA normal under good driving condition and non crazy driving.}
If car is running lean, jump below:

In many cases, the motor will not hold closed loop . ( this is why the gas mileage is bad)
Closed Loop is goal at idle and light cruising, and never happens wide open throttle (WOT)  or decelerating (cut fuel) or flying  up tall hills.
Close loop drops out normally, if you move the throttle pedal fast , then after you steady your foot, close loop returns. (if not, its busted)

Here is a list of steps to take. if this happens to you. including the PRO shop action and DIY action (Do It your self or shade-tree action)
The First Problem or Compliant :
Motor runs rich, it stinks of raw unburned fuel at the tail pipe or even black smoke emitting from the same. 
My Spark plug tips keep fouling (black carbon, wet or both).
Or the  motor runs lean, bogs, and can even stalls.

LOOK HERE for other symptoms

Most likely the  motor (ECU) is in Limphome mode or O2  sensor is stuck measuring LEAN for ever. But motor  is rich.
The exhaust is cracked near the front 02 sensor render it useless. (full or part of the time)
Most people ignore the horrible MPG because running rich makes plenty of power or don't notice the Limphome mode symptoms. (lost power)
The ECU will throw DTC errors if it has bad Inputs.

This ECU can not  (mostly) report sensors or actuators that are no longer accurate , it can only report DEAD ONES, and only a  few.
The newer ECU 96+ do report , better,  EGR tests, lean , too rich, and others, see "monitors" under OBD2 tech.
This old system has little redundancy and that is the core design flaw of  most EFI systems.
The ECU can only believe a sensor is good, unless its dead .  It believes the sensors with blind trust.

The ECU runs in 3 states (besides  cranking, cold start, warm up , hot  open loop and closed loop).
  1. Sensors good in Closed loop  ( when I say sensors here, I mean all inputs and actuators and fuel pressure) For Maximum MPG and clean Air.
  2. Sensors dead ,  DTC (errors) stored , CEL glowing.  (may go to limphome or just run lean or rich )
  3. Sensors  way out of calibration (offset,biased, inaccurate)  but the ECU knows something is very wrong but can figure out what ! Limphome Happens
There is a dead sensor mimic mode but I consider that just a feature of Limphome.  Using the mimic mode one can drive home , with MAF pulled ( a trick with a weak MAF)
And  a "BACKUP  mode" ( ECU processor is Dead !) there are no documents for that mode , anywhere but is real. It is BAD ECU detected, now in back up (rich and low power)
Most  N.American Sidekicks with the CAT missing (zero back pressure) will cause the EGR to fail ,when that fails (code 51) then the EFI will go to limphome. 91+ years.

CAT Missing? melted inside, rodded out, test piped, blowout, or just somehow not there.

Givens:  (preconditions)
There must be no air leaks in the exhaust system forward  of the CAT (converter) . No cracks, no gaskets bad or missing! Remember; Exhaust blows and sucks !!!
There must not be any illegal induction leaks (air leaking to this vacuum or up stream of the MAF , from the MAF to the intake valves). {Un-metered Air}
The CEL lamp is off running and you have corrected all DTC errors logged.    (DTC = Diagnostic  trouble codes)
Hot motor , idle set to 800 RPM with top of TB bleed screws all accessories off.
Water (coolant) at 180 F {82C} or higher ( use a real meter to measure this, DO NOT guess and a IR thermo gun is best.)
Working with no tools is quite impossible.  The tools are free,  think about that?  I will try my best to avoid tools, for this page.
If the MPI injectors are bad, the EFI system will never work right or make any sense.  The ECU may hunt and go quite crazy.  {EFI weakness #1}
The same with bad fuel pressure. {EFI weakness #2}  Low fuel pressure is so common and it seems no one ever checks it.
The EGR main valve must not Leak (valve blocked open by carbon, or fresh air leaking past its gasket)
The Catalyst exhaust converter (CAT)  must not be melted. Check exhaust back pressure.
The motors sensors must be operation.
The Engine must be viable , with good compression and steady vacuum (no pulsing vacuum gauge needles allowed) The engine must pump air perfectly !
A pro starts with the Gas Analyzer at the exhaust tip but a DIY person , works with his noses ,eyes and a DMM.

Steps:  (16v only) 8v  later:  (96+ cars substitute the meter readings for OBD2 scan tool readings, ok?)  {an Air density system , MAF based}
The engine is HOT: (if not hot , 180F, stop)
  1. {PRO}Connect car tail pipe to 4 or 5 gas analyzer.  See end of  this page , to understand the possible causes. Check gases carefully
  2. DIY, use your nose and look at the spark plug tips to discover burn richness.  not easy. we know now for sure the motor is running very rich.
  3. {PRO} Attach a DSO scope   {DIY us DMM} to the O2 Sensor and see if is reading rich or lean.(off site graphic)
  4. Almost zero volts is lean,  0.9v or near 1 volt is rich ( a new O2 sensor is a must to start diagnosing rich) .  If the O2 Sensor swings; then the EFI is in closed loop.
  5. DIY: connect a good digital voltmeter (DMM) to the o2 sensor (back probing it) and make sure meter has an digital analog bar on the bottom of the LCD screen.
  6. We are now  connected to the FRONT O2 sensor, with either a DMM or DSO  (meter or scope). (requires  back probe leads and back probing techniques)
  7. This sensor can do 3 things, show rich full time , lean full time or it  will swing (proving normal  closed loop operation)
  8. The ECU trust the 02 100 % , at idle and steady light load cruise. if the sensor is biased(bad) the EFI will inject poorly under these conditions.
  9. If the O2 sensor reads rich then it is reporting the true facts,  we can lean the mixtures artificially by opening up the plenum (induction manifold) by causing a leak , and one place to do  that are the nipples and hoses (1/4") connected to it.  I use the EVAP nipple.  This will send illegal air (unmetered on ,16v only) to the engine, add more leaks as needed and the O2 sensor must start lowering its high voltage 0.9v   (900mV)  and reducing shows that not only does the O2 function correctly but that it can SEE this leaner condition occur,  you might even trip off the CLOSED loop mode and the 02 might starting swinging from .1v to .9v.
  10. If you can not reduce the rich reading O2 in step 8 , the O2 is either bad (MAF offset too high) or the fuel pressure is way too high , measure the fuel pressure now.  See my pump pages for spec and ways.
  11. Rich always, can be caused by ECT stuck at 10,000 ohms instead of 300 HoT. Missing/bad  thermostat or stuck open or the wrong one. (common on 15year old cars).
  12. Rich can be caused  by a stuck in limphome mode.
  13. If the injections are too lean at W.O.T (wide open throttle ) then the MAF may be weak. 16V Cars only.
  14. To understand the O2 sensor you must understand command authority.
  15. Rich/ lean can be caused by a bad MAF (stuck high or reading more than 20% higher than reality will cause very rich condition)
  16. Now we pretend that the 02 is reading lean, but motor is running rich.  !  (there are 4 states considering 02 agrees with reality or not)
  17. Try to add propane gas to the suction side of the air cleaner (filter off , idling)   This illegal (not injected) fuel will make the motor go richer and the 02 sensor must see this  and make the Lean 02 reading rise in voltage, if not then the 02 is bad or you have a massive AIR to  exhaust leak. Exhaust leaks are never tolerable by the O2 sensor.
  18. I will now reverse the rules and symptoms, here  for the  motor running lean. (actually is lean running)
  19. IF the motor is lean ,then the motor will want to bog.  (a bog equals long  hesitations,or loss of power or back firing into the induction (hiccups))
  20. This can be caused by low fuel pressure (measure it , see pump link above line 9) you can measure the 02 sensor and it will read lean (low and stuck)
  21. Or be caused by a dead/weak MAF (stuck lower low by 20% of reality) or a massive induction air leak. (induction leaks bypass the MAF and defeats its whole purpose)
  22. Again add propane gas (raw) to  the air cleaner suction and see if you can move the readings. or even bring ON CLOSED LOOP (CL)
  23. Keep in mind if you have an air leak in the plenum (induction) and you add propane,  the motor will instantly race. (lean + fuel = race). Keep RPM in mind here.
  24. If you can not raise the lean running 02 reading then the 02 is bad or you have a massive air link in the INTAKE.
  25. If you had a massive air leak in the exhaust, near the 02  this front sensor will go NUTS. (lean 02 and huge EFI rich, or will cycle like mad, lean then rich)
  26. This can be caused by low MAF, induction leaks ,low fuel pressure and or bad fuel pressure regulator or a weak fuel  pump or low power feed to said pump.

All statements on the page assumes you have checked to see that your motor pumps air.  (it's really not an engine ,it's an air pump to all ASE mechanics)
Do a Compression test,  and vacuum test. Do it now or you will chase you tail ,just like a puppy.
Look for:
170 PSI minimum all 4 cylinders with wide open throttle (WOT)
idle 800 RPM , hot,  19 inches HG vacuum.  the needle the gauge is NOT wigging at all. Steady as rock , hands and foot off that throttle ok?
if not ?,then the motor is bad.
Bad motor means , engine pumps air perfect (no stuck open EGR) no melted CAT and perfect steady vacuum at  idle.
Not checking fuel pressure KEYON is a real bad idea.  This is the 1st check after checking , bad motor, bad spark.
The ECU assume fuel pressure is at spec. at all times. There is NO fuel pressure monitor or sensor. The ECU can not warn you , in most cases. (96 and later , maybe)
Checking all sensors and actuators for connections and damaged  wiring now, is the first step.  INSPECT all  ECU and EFI wiring for damage.
Missing sensors. and OEM parts.
Hacked wiring in the EFI system is a  no GO.
Missing EGR or (leaking any any way shape or forum that includes a bad EGR base gasket) EGR mains , love to stick open as you come to a stop sign .
A Missing Cat con ,kills the EGR dead.  (the live test of the EGR will prove that point)

This information above will allow you to hone in on the causes of rich or lean motor.
The 8 valve motor behaves quite different with similar malfunctions ,which I will cover next.

Keep in mind both gross  lean and rich cause misfire and power loss, and fuel wasted.
16v Induction air leak rules :
(all leaks during cold start running cause a too fast idle)
Hot (all below are hot) 16v only.
  1. Leaks in the plenum all cause lean, (ECU + O2 try to correct this and is seen in LTFT as huge plus percentage of fuel added, and especially at idle)
  2. The 4 idle controls leaking all cause  high idle and not AFR leanness  (IAC , ISC, throttle plate and bleed screw)
  3. Any breach (leak) from the Throttle body air horn to the MAF all cause lean. (The ECU can compensate for tiny leaks,seen as LTFT showing add fuel)
  4. Any PCV hose , or IAT sensor air hose cracked, will cause lean.
  5. A dirty MAF causes lean. (lost power) especially W.0.T.
  6. There must not be any air leaks (abnormal) from the engine intake valves, all the way too the MAF. 
All air leaks show up worse at idle.
The ECU can not compensate for leaks, when running in normal open loop mode, at all, because the 02 is not used then.
Accelerating is one such time. and off idle, tip-in acceleration can be real bad, with air leaks.

The 8V Running  Rich or Lean Diagnosis. ( a unique speed density EFI system)   "runs off the MAP"

The same things happen as per above but on this motor any induction air leaks  Naturally cause a high idle and no loss in power or CL status.
That is correct.
This motor eats air leaks for breakfast,  it acts just the same, as stepping on the gas (air) pedal.

So adding extra air will only raise RPM and never change O2 readings. (on a working 8v system !) Induction air leaks on this system cause high RPM with full power.
In fact do that, pull the EVAP  nipple and see the RPM rise.  (if it don't rise , you are ON TRACK , the MAP is dead or fuel pressure is low)
If it does not ,then the fuel pressure is very low or the MAP is dead.
The same tests on the 02 sensor are as follows:
8V top fails: (Rich)
  1. stuck in  limphome (lots of reasons)
  2. Fuel pump regulator  FPR, is  stuck closed fuel pressure on the moon, even as high is 60PSI (BAD)  30 PSI is normal.
  3. Fuel return line pinched (tank to pf reg output)  60PSI. Horrid MPG.
  4. FPR stuck open , causing low fuel pressure,  a hopeless case.
  5. Exhaust leak (slams O2 to lean for ever) Motor to rich as RESULT. horrid MPG. (or cycles, from gross lean to gross rich over and over)
  6. MAP stuck at  MAX  pressure (min vacuum) (bad MAP or bad vacuum source to it)  bad hoses or clogged,  The MAP never fails, just the path. (vacuum)
  7. ECT open. (reads very cold all the time and is a lie) motor stuck in cold start mode for ever, rich. (wires fell off it is top reason)
  8. Thermostat bad. (coolant never attains  180F ,or cant hold it there driving)
  9. IAT stuck open (reads gross low temperatures (8v  will be most effected with  rich)  (wires fell off it is top reason) 
  10. O-rings leaking around (x2) the Injector. (rich , flooding and hard to start rich)
  11. O2 sensor bad  or its heater, all waste fuel, install a new one for $20
  12. Bad ECU

In all cases a dead or weak MAP sensor ( our vacuum lines clogged or leaking to MAP):
  1. If the O2 reads lean all the time 0.1v , the exhaust  may be cracked , or O2 is bad. and Engine is rich as piggy.  (a bad O2 may be causing the rich condition) (or stuck MAP)
  2. If O2 reads rich all the time  0.9v , because  the O2 sensor is covered in a blanket of carbon. ( due to many possible causes) but Engine is running rich.
  3. O2 reads rich and engine is rich.  Bad MAP, high fuel pressure, stuck closed Fuel pump regulator on front of TB. (throttle body) or injector seals bad.
  4. In each case you must try to test the sensors or replace them.  Replacing the O2 is the first step.
  5. The adding of propane gas to the air cleaner may change the  02 sensor readings, if the RPM rises with propane ,your induction AFR may be too lean.
  6. The map is easy to test.
  7. Checking fuel pressure is critical on EFI systems.
In general: (8v)

Lean: (bogging)
Bad MAP, fuel pressure low,  Bad O2.  ( most exhaust AIR leaks cause a rich motor but not not all , due to a design flaw in all 02 sensors)
O2 sensors can read wrong due to the carbon blanket effect , Bosch has white papers all about it, just imagine covering the sensor with your palm,  and now the o2 is blind to the presents of oxygen.  The O2 will signal rich. (falsely and is not bad) This is the O2 sensors greatest weakness.  After correcting  bad (rich) AFR , the O2 will clean itself and once again operate. 
Bad MAFs are very rare. as is, weak or out of calibration MAPs)
Fuel pressure what is the fuel pressure, and what is the voltage to the fuel pump , only 8vdc? bingo the problem.
O2 sensor or bad MAF/MAF .
or Illegal induction air leaks. (16v)

Rich: (stinking and low MPG)
Bad / weak MAP,  high fuel pressure (out of band) , Bad 02 sensor  or an Air leak in the exhaust forward of the center line of the CAT converter.
Leaking Injectors (inside the valve is stuck open) or the  seals on the outside leak (look at it idling, it is viable)

If the MAP error is over 20% it  is bad .(note it has a range of valid outputs for a given vacuum input)

Bottom line. If you lie to the ECU bad enough the 02 sensor can not save you.    If the O2 sensor is bad, there is no hope at all.

CLOSED LOOP fuel trim.    The  old GM cars work like (similar ) to our OLD ECU cars pre 96 years
The 1996+ and newer cars use OBD2 and  make these checks real easy .

Keep in mind all 02 sensors are designed to only operate at Stoich.  14.7:1 Air fuel ratio  or AFR for short ( or chemically perfect combustion !)
Stoich is short for stoichiometric, the chemical perfect burning of the fuel using the exact amount of air.  Think Perfect burn or best MPG (close).

To understand the Closed loop and how the ECU controls Air Fuel mixtures one has to read about block learn and fuel trim.
There are 2 variables in the ECU that store SHORT term fuel trim (STFT ,aka : integrator) and long term fuel trim (aka : block learn mode).
Integrator and BLM are only GM terms. So we will try to avoid them. and use the new standard SAE terms.
The STFT changes many times a second and LTFT changes every second or two.  These terms are SAE.org terms,  a standard of the industry.
See these pages , from my  favorite authors.
The first is by Jeff Hartman,  a famous motor books author ( the best , in my opinion)

One fine page on topic, few better , IMHO.

My rule, is to not trust any old 02 sensors, at all , I just put in a fresh one for $20 and now I can trust the swinging 02 and all trim readings.

The ECU has up to 16 PID's that are the storage values for LTFT.
LTFT is long term fuel trim,  it stores the Fuel correction values, for many running conditions of the engine. (idle is 1)
This EFI  works the same way !

The LTFT is the coarse adjust for fuel trim.   Slow,  every second. (not so slow huh?)
The STFT is the vernier adjust for fuel trim.   Fast.  (many times a second) I ignore this, because its too fast to watch.
If the STFT rails (128 is the center)  to 0 or 255 ( 8 bit storage word in ECU memory)  1 byte to geeks. 
 Both values are 1 byte and are a major part of the AFR total equation. 

The total equation uses tables, ECT, MAF/MAP , RPM, Load, as other factors for AFR. But the O2 is king of the mountain and rules the roost.
128 means , I (ECU) am not adding or subtracting fuel, I am using fuel tables and they are working perfectly now. (O2 says so. and I believe it says ECU)
When the Integrator STFT, is less than 128, the computer is subtracting fuel from the base computation.
The 0BD2 car shows zero ,0, for perfect AFR, that is , its not correcting fuel trim at all.  (eg. the MAF is doing a perfect job on air flow)

These rules are only OBD1 rules  "128" etc, OBD2 reports Percentages using any scan tool.
My OBD2 car runs about -3% at idle hot. (a very good reading)

Above 128 the ECU is adding fuel ( engine is lean)   STFT:
0=   -1% full
128= nothing added or subtracted.
255= +1 % fuel

0= -100%
255 +100%

Open loop is when the ECU uses the Total AFR equation minus the 02 readings, and in which case STFT and LTFT in Not applicable.
The ECU drops the usage of the 02 , when not at closed loop. (eg. WOT. fast acceleration, up tall hills or  at deceleration mode, foot off gas pedal in gear coasting"cut fuel")
Any car with bad fuel mileage , you may find the LTFT dead, or stuck or reading badly for the driving conditions.

The o2 sensors may be stuck lean, and AFR is really very rich. LTFT in this case would show minus huge numbers. or near -100 (OBD1) or -34% OBD2
On new cars they limited the extremes of LTFT , (and DTC'S when HIT) on OBD2 cars.
The new rules are Long +Short > +43% or  lower than -34% (or ether  over 20%)
or STFT short below -19% or long trim alone below -16%  (exhaust leak near 02 or 02 bad? or full pressure on the moon (60psi?)
DTC 171 or 172 are the warnings for above #

My example:
Lets pretend the 02 sensor  (aka: Oxygen Sensor) shows lean and the STFT went to 255. ( it will do this very very fast, 1 second or less)
This happens because the ECU keeps adding fuel (guessing)  and the STFT rails (in my example) and the AFR is still too lean. (the o2 sensor says so)
If the STFT rails,then the ECU then moved the fuel to 255 rich and the motor was still too lean, so it sets the LTFT to 129. and resets the STFT to 128.
The hunt then again starts over looking for Stoich (14.7:1)  or simply looks for the 02 sensor to switch. It really is just a switch (Narrow band sensors).
Then the ECU hunts for Stoich some more and repeats this guessing, until it finds the crossover point.  ( the O2 sensor, toggled rich for the first time)
That point where the 02 reading finally shows the cross from lean to now rich.
The ECU now tries to toggle back and forth from left and right of Stoich,  but just a tiny amount (a secret amount).
This looping is what makes the 02 swing and cross counts. over and over never ending.  (look with any scope to see what the 02 is doing?)
Things go wrong:
If lets say the ECU keeps railing (255) LEAN, the STFT then the LTFT will keep climbing (adding fuel)  at some point depending on year of car. the LTFT will get near the RAIL
and hit a threshold ( lets say 220)  [I'm guessing , because the actual value, is a secret]  but if the 220 LTFT is hit , some ECUs (after 1996) will throw a fuel trim error DTC.
On older cars,( pre 96) it may just drop into  limphome, for a time or until car is restarted.  ( big secret 2)
On older yet,cars  89? There may be no threshold at all (ouch) , an the LTFT rails and stays there.  Rich as a pig , most likely.
If the 02 sensor was lying all the long time. then the stupid ECU rails and runs rich all the time.  << good reason to buy and keep a fresh O2 sensor (front) in your car.
There is more to this Algorithm then my simple example(  eg: ECU purposefully goes rich and lean extra amounts to keep Catalyst hot and active(needs fuel to do that and oxygen).
The ECU purposefully wastes fuel to keep the CAT running.  but the average AFR is "STOICH".
But forget that, all we care? , is can I hold closed loop and is that loop centered around Stoich.?

If the 02 is new and the EFI is in closed loop ( see 02 swings and cross counts ) then we know all is well at idle and cruise.
The only way to prove it?, is with 4/5 gas analysis.

Other factors play into this
One is command authority.

OBD2 scans (many)

OBD1 scans

Jargon,  words,  WOT is wide open throttle and all others, used in the SHOP

rev4; 2-4-2013 added OBD2 cross references... .too much jargon...