Why does my idle speed SURGE ?    

This covers all EFI engines, because all use an idle speed  servo system  , a computer controlled, idle speed controller.

They all use a device (SAE.org) named the  ISC,  for idle speed controller valve. (and is just a simple actuator, no brains inside)  (when I say brain, I'm referencing the the Alan Turing brain (circa 1944)
I must state the obvious first, the ISC servo prime purpose, is to stop surge, but sadly it can cause, it !  (shocking yes, back in 1990)  My joke, is the Flux Capacitor was overloaded then?
The ISC keeps idle at 800 RPM at all times, (unless in idle up mode , @ 1000 RPM) The ISC will work perfectly , so long as it is not tasked to do the impossible. 
(what does impossible mean?,  that means, engine is  too lean or there are ail leaks)
If the ISC is 100% closed now , how could it close more? , to say correct an air leak (IT CAN"T) The  old year ECU's go quite nuts, and surge. ( to prove this, take the Dash pot on the 8v engine and unplug its vacuum line, behold SURGE) (cured in time frame 1992 to 1995)
A.K.A. IDLE  HUNTING !


First some simple concepts. HOT engine only. (I'm not doing cold here)

All ISC servos  will keep idle speed, at 800 RPM (common #)   under all conditions.  (all electrical loads (Alternator) Transmission shifting from park to drive (big load there) and even minor misfiring.
The PCM (the EFI brain) does not know why RPM goes out of control , only that it did and the PCM changes the modulation on the ISC to correct it in about 1/10th of a second, It can also tweak spark timing to correct tiny RPM errors.

There are many ways for surge to happen.:
  1. The Servo is confused.  (the servo gets confused, easy with both below hard failures.) In fact, the Servo can get so confused?, it can actually make the surge worse ! (this happens ,due to loss of command authority of idle) (some servos hunt others rail, all 100%  software logic)
  2. The engine is lean and so lean ,that the ISC can not correct idle , nor can you by gentle hand on the throttle. (engine may back fire, and  car driven, the engine feels weak.)
  3. The engine is so rich it misfires. (again lost engine power) (or any other reasons for misfire (a huge list of causes)
  4. The ISC can stick, rendering the ISC dead. and RPM is totally random and based purely on fuel mix and engine loads. (clean the ISC valve now)
  5. The ISC can be just ok, but the TPS is set wrong (bad calibration) making the SERVO dead. ( the PCM thinks engine is not idling but racing) see #4 above.
  6. The ISC can also be dropped off-line if the PCM is in Limphome mode (aka, Limp-in, or Failsafe) Scan the PCM and bingo see why.  (with same #4 effects above)
  7. Air leaks to any  EFI system with a MAP sensor will cause the ISC to go NUTS. see #1  (air leaks to plenum for  ANY reasons what so ever)  (MAF BASED EFI systems go lean and cause #2 effects.
To solve this riddle.? "collect EVIDENCE"
  • Is engine misfiring, (scanned PCM  DTC codes P03xx?) (feel it misfire, see engine shake, see RPM drop real low, to illegal RPM?
  • Is engine lean (Scan tool shows it is)?  (O2 shows it, LTFT shows this?) Got too lean DTC errors?
  • Scan tool shows closed loop dead at idle? (o2 dead)
  • Is one or more oxygen sensors stuck hard to lean or rich, and fail to swing at idle (bingo)
  • Is the ISC dirty , clean it yet?
  • Is the TPS calibrated, does it read correctly with the scan tool, live data stream?
  • Is PCM in limphome (scan it)
  • Are there vacuum leaks,  if engine hunts , from 900 to 1500 RPM seems there is a leak, due to not landing below 800, so find the leaks. (cracked hoses , vacuum)
  • Is the EGR main valve stuck open, (if it does RPM would be like 400 to 600 RPM) and might surge.  clean it yet?
  • Fuel pressure is very low, or very high. The fuel pressure must be in spec. range. and have a working FPR fuel pressure regulator.
  • The injectors must be clean. (and all firing) and pass the leak down tests and the balance tests.
  • Spark must be good. (on all cylinders not just 1)
  • And are all scanned sensors reading correctly (this is a skill set.)
  • For sure this list is not complete, which most books forget to say, after making a list....  I'll add one more... , is LTFT near -5% or stuck at the rails, -40 or +40%?  WRONG  "long term fuel trim"

Partings shot:
On newer cars the ISC is not programmed to hunt now, to make driving safer. (See $2 billion Toyota law suit on that)
The PCM if it sees RPM over 800 RPM and closes the ISC, just  waits for ever now (safer), It does not reset and repeat, it just waits , say for you to clean the ISV valve and  unstick it.  (I have a 1990 car that does hunt, with vacuum leaks)
The key reasons for hunting not caused by the PCM  ISC logic are:
Lost engine power or misfire. (way too slow RPM that added air can not cure.)
Vacuum leaks (engine races) ISC closes but its just not enough to cure this hard failure.
Lean Air fuel ratio, (engine runs too slow)
Or anything that weakens engine power, for any reason that the ISC can NOT correct.

The Loop (Servo) This is a gross over simplification , but is all you need to make it work.  The ISC takes fresh filtered AIR and adds it or subtracts it from the plenum. In this way, idle speeds are regulated.


There are 2 more lines of  program code, if  the TPS sensor idle switch is not closed, give up and wait for it to close. On newer cars the switch is gone and is now , waits for the TPS to go below 1volt. (signaling, IDLE NOW, and REGULATION) 
If the PCM is in limphome mode, suspend the loop. (until cause is corrected) 
Tacho (short for tachometer)is the Crank and Cam sensor , one or the other or both. (Your PCM decides that , not me) (on very old cars Tacho is 2 times the cam sensor speed (ECU math) on newer cars, it's just the crank sensor speed RPM)
In many cars, too fast or slow is NOT 800 RPM +-50 RPM.  The scan tool shows RPM at all times, just look, see what you have.
The brain above is a mutant.  (and is very retarded). LOL ! It follows instructions quite well,but has a bad habit of getting confused..... much too easy..... and is just one job in the whole set of jobs,, (spark, injection are 2 more jobs of the long list)


rev 3   ++++8-24-2014
By, I.Mudd


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