Idle video screenshot

Idle control – fixing rev hang

Does your Falcon have a high idle?  Do you have rev hang for a few seconds after a quick stab at the throttle?  Does your car stall when you come to a quick stop?  Read this article for some fixes to resolve this fairly common problem.

Idle 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 (between the two halves of the BBM)
  • Old, cracked or disconnected vacuum or PCV hoses
  • Modified, bored out throttle bodies with poorly sealing throttle plates, or drilled throttle plates are also a common cause of rev hang issues.

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

Falcon 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 on an EA-AU Falcon:

  • With a fully warm engine, disconnect the Idle Speed Control (ISC) valve electrical connector.  This is located on the throttle body
  • 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 a problem where your engine idle is too low, or does not raise when you turn on your A/C or turn your steering wheel, you may have a faulty ECU.  A common fault with EF Falcon ECUs is an ISC circuit failure, preventing the ISC from getting a signal.

To check if this is occurring, use a test light, or volt meter 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.  If you suspect wiring, back probe the loom to the ECU to confirm the cause.

Adjusting Your Tune

If you follow the steps above and have everything working correctly, 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 – We can adjust the base idle RPM speed the engine targets in gear, in neutral, with the A/C on or off
  • Idle fuel and ignition advance – We can adjust the maps to fix a rich idle.  These can affect rev hang and cause hunting when a cam has been fitted
  • Dashpot – We can adjust how the EEC controls return to idle airflow
  • Cold start enrichment – We can control how much additional fuel the ECU adds when the car is cold (choke)

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 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 cammed Falcon 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!

10 replies
  1. Avatar
    Sheridan says:

    What if you’ve got a rev hang issue but no vacuum leaks, a different ECU doesn’t change anything, a quick stab of the throttle causes rev hang for a few seconds, coming to a stop causes rev hang for a few seconds, the idle doesn’t rise with the steering or A/C and the car drops from 1200rpm to 500 rpm dead cold unless it went straight to 500rpm when started even when dead cold.

    What would be the issue here?

    Car is a manual swapped NC V8 that has had everything replaced except for the ISC.

    Reply
    • Avatar
      Jason Bolger says:

      As per your other comment Sheridan I would fix the neutral input wiring & then check your base timing before looking at replacing the ISC.

      Reply
  2. Avatar
    Drew says:

    I have this issue with my XH ute , I think it is the oil breather hose but I can’t find where it connects to the inlet manifold , any help would be appreciated muchly

    Reply
  3. Avatar
    Justin Horsburgh says:

    Hi,

    Extremely informative and helpful information on topics that would otherwise be far too complex and overwhelming to tackle easily. Thank you for your generosity in sharing your valuable knowledge.
    To add to your topic;
    Idle control – fixing rev hang
    (Idle Air Leaks), I’d like to also add a specific part to check for vacuum air leak- brake booster. It can be small enough to affect rev hang, and possibly not be noticed in brake performance, even though there is certainly loss.

    Cheers

    Reply
  4. Avatar
    Andrew Augustine borg says:

    i have an one ton au ute that revs really high about 2500 when you start it and barely drops below 2200 rpm when engine warms up .i can hear a hissing sound near the front of intake manifold and front fuel injectors like air getting in or a bad vacuum leak around this area. Could this be why the engine is revving so high and also tends to overheat a bit . Have tried disconnecting isc switch and choke assembly on throttle body to no success and hardly makes any difference . Any help would be appreciated as your info is very informative as i am a bit out of touch with working with engines as i seem to get older unlike my younger years Cheers

    Reply
    • Avatar
      Jason Bolger says:

      Definitely, a blown intake gasket or loose bolts will cause it to idle high. Sounds like a new gasket is on the cards. Cheers, Jason

      Reply
  5. Avatar
    Ben says:

    Hey guys, thanks for doing this very helpful write up, just wondering if the base idle procedure is the same for manual transmission? Thanks!

    Reply

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