Engine Idle Quality

There has been some confusion over the exact function of the IACV (Idle Air Control Valve) and it's role in maintaining a smooth idle speed. In fact it is the EEC-V PCM which controls engine idle, not the IACV. The IACV should really be called a 'Throttle Air Bypass Valve' because this is all it does.

The EEC-V receives signals from the IAT and the ECT for the temperatures of the inlet air and the engine, from the TPS for the command position of the throttle, from the MAF for the actual amount of air drawn in through the inlet manifold and the Engine RPM is calculated from the CKP input. It also receives an input from the Air Conditioning clutch cycle and the Power Steering Pressure (PSP) switch (to compensate for the drag of the compressors) and the Park Neutral switch to show the command state of the automatic gearbox when fitted.  

From all these the EECV computes the LOAD on the engine and before each cylinder fires it computes:

These are 24V PID addresses for use with the OBD2 software and reader - other engines may differ.

The nearest approximation of the conventional 'Choke' is OPEN LOOP when the HO2S sensors are cold, but they have heaters in them to bring them up to temp very quickly - when the PCM switches to CLOSED LOOP the engine itself is still quite cold. If you block off the Idle Air Bypass Pipe at this point the EECV cannot maintain the engine idle - combustion is too fragile. However, once the engine is at full operating temperature the programming can maintain idle by varying the spark advance and the pulse width - the heated cylinders are much more amenable to these adjustments. Blocking the bypass pipe like this is just one demonstration of how powerful the EECV is and how it can maintain engine function under quite extreme conditions.

Because engine control is holistic, the chances of an EEC-V controlled idle being effected by a dirty IACV is probably 10 or 15 to 1. It is always worthwhile cleaning it on the EEC-IV 12v engine variant where this was quite common, but much less so on the newer engines.  Experience shows that a failed IACV normally causes stalling rather than a poor idle.

Diagnostics

So, if you are experiencing a poor quality idle on your 16V or 24V engine then the following order of diagnostic work should be undertaken :-

1. An air inlet leak causing unmetered air to enter the inlet manifolds. These can be split or disconnected vacuum pipes, a loose EGR pipe, faulty EVR, split/loose air bypass pipe, split inlet O rings especially after disturbing the inlet system. In particular there is a plastic vacuum T piece located underneath the central throttle body of the inlet manifold which is very prone to failure (Two of the Ts are 4mm and the other is 6mm) and is no longer available so you may have to make your own replacement if this has fractured.

2. Faulty EVAP system, loose or faulty petrol filler cap. Make sure the filler cap is ratcheted tight.

3. Lost TPS position. Check for P0122 TPS Sensor Circuit Low, and on the DATA page check the % figure for the TPS. Static the TPS should show 16% approx. If the reading is lower, then the EECV has lost synch with the TPS. Fuelling will be poor, idle effected by lurching or surging. On autos, you may also notice that the gears are being selected with a thump, both when stationary and on the road. If the DTC and the percentage reading is seen then force a resynch by disconnecting the battery for half a minute.  Don't forget the radio codes before you do this, however. After the initial poor running while the running parameters are relearned, the engine idle and autobox changes will be greatly improved.

4. A 'Hanging throttle' - engine delays at higher revs when accelerator is released, then revs die suddenly, revs stick high and engine races irregularly without touching the accelerator - is probably caused by a sticky throttle butterfly valve, particularly on the DOHC models. Grime and burnt oil (from the PCV system) can gather round the throttle plate and cause it to stick slightly open. Clean the inside of the throttle body with good quality carb cleaner and an old toothbrush.

5 . Faulty connections on or flatlining upstream HO2S (Lambda) Sensor. These are the ones located on the engine side of the Catalytic Converter - those after the Cats do not affect engine performance.

6. Faulty MAF. Where the element or conditioning electronics has failed a MAF will not produce a meaningful reading to the PCM. This will result in a surging idle, and probably also jerky, thumping autobox gear changes and a P0102 Diagnostic Trouble Code.  The MAF will show typically 0.000 to 0.005 lbs.p.m. at idle or KOEO

Where the element is failing or corroded the MAF may show too high a reading. This drives the Long Term Fuel Trims too lean.  This is detailed on Faulty Maf

7 . Faults with the Ignition modules or wiring (on 16V engines in particular), but the EI module on the 24V has been known to split at the rear.

8. A slightly racing idle may be noticed on occasions when the petrol tank is nearly empty. Idle is corrected when the vehicle is next filled up and this is not a concern. However, a persistently high idle, surging or racing, allied with thumping into gear [autos] or dragging on the clutch, bulking in reverse [manuals] then see (3) above.

9 . Quite Rare - incorrect installation of the 24V camshafts during an engine rebuild. It has also been known with the 24V for the cam chains to jump sprocket teeth when the timing chain has become very badly worn.