Idle control – fixing rev hang

Does your car idle high?  Do the revs hang for a few seconds after a quick stab at the throttle?  Does your car stall when you come to an abrupt stop?  This article contains fixes to resolve this fairly common problem.

Air Leaks

The most common cause of a rev hang issue on an EA, EB, ED, EF, EL or AU Falcon is an air leak on the intake manifold.  This could be caused by a number of factors:

  • Damaged intake manifold gasket or loose intake manifold bolts
  • Damaged BBM gasket or loose BBM bolts (ie. the two halves of the BBM)
  • Cracked or disconnected vacuum or PCV hoses
  • Modifed throttle body – bored out throttle bodies with poorly sealing throttle plates, or drilled throttle plates are a common cause of rev hang issues

To identify the possible cause, use a can of aerostart and spray around all seals on the intake side of the engine.  If the idle changes when you spray a seal, you have found the leak.

Base idle adjustment

The second most likely cause is a poorly adjusted base idle.  This should only be adjusted after checking for air leaks as above.  To correctly set the base idle:

  • With a fully warm engine, disconnect the Idle Speed Control (ISC) valve electrical connector
  • Adjust the throttle stop screw on the throttle body so that the engine idles as close to 500rpm as possible
  • Reconnect the ISC valve electrical connector

Low idle or no idle assist

If you are having the opposite problem, where your engine idle is too low or not raised when you turn on your A/C or turn your steering wheel, you may have a faulty ECU.  A common fault with EF ECUs is an ISC circuit failure.

To check if this is occurring, use a test light, volt meter or oscilloscope to measure the voltage across the two terminals of the ISC electrical connector when the engine is running.  If there is no voltage, you may have an ECU or loom fault.  Backprobe the loom to the ECU to confirm the cause.

Adjusting your tune

There are a number of engine operating parameters we can alter on a J3 Chip that can help with return to idle.  These include:

  • Idle speed – the base idle RPM speed the engine targets in gear, in neutral, with the A/C on or off
  • Idle fuel and ignition advance – these can affect rev hang and hunting when a cam has been fitted
  • Dashpot – this is how the EEC controls return to idle airflow
  • Cold start enrichment – this controls how much additional fuel the ECU adds when the car is cold

J3 Chip Options

We can program a J3 chip to suit any modifications made to your engine or driveline, including fuel system, transmission, camshafts, boost etc. See our range of J3 Chips below.

 

 

Deleting the ISC

Deleting the ISC or installing a restrictor plate is a common reaction to address idle issues.  We at T.I. Performance do not recommend this for the following reasons:

  • No cold engine idle assistance – meaning the car will not idle properly at cold start
  • No idle adder to prevent stall – coming to a stop could mean a stall and loss of vacuum assist in the braking system
  • No idle adder for steering assistance – prevents stalling when driving around in a car park
  • No idle adder for A/C – helps keep the A/C efficient when idling

With a mechanically sound car, addressing the issues outlined above, a good tuner should be able to get any car to idle steadily with zero rev hang, regardless of the mods done to the engine.

Sample idle video

Here is a sample video of the cold start idle of our modified EF Fairmont on a cold Melbourne morning.  The car is fitted with a Wade 1521a camshaft, 42lb injectors and a Powerdyne BD-11a Supercharger.

Anything else?

If you have any other suggestions or ideas that have worked to help with idle control in your car, leave us a comment!

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