Minimum Air Rate... Why are you doing this to me? It's for your own good.

The IAC or air control valve is in charge of letting a measured amount of air into the intake manifold for the purpose of controlling the idle speed. The ECM takes this into account and compensates accordingly. The min. air rate is an adjustment meant to bring this system back to baseline. And if this baseline is out, then the rest of the idle control will also be out. See why right here ...

 

THEORY OF OPERATION

Minimum air rate is the lowest amount of air needed to keep the engine idling. In other words, when a vehicle is at idle it takes a certain amount of air to go through the throttle plates, into the intake manifold, and to the combustion chamber to maintain combustion together with the fuel. The minimum air rate, therefore, is the minimum amount of this air going passed the throttle plates needed to maintain idle. The minimum air rate will vary according to the manufacturer. It will even vary on the same engine, year, make and model due to the different engine wear patterns and driving habits of the vehicle owners. An engine in the north will wear out differently than one in hotter climates. For this reason, the adjustment procedure explained in this article will try and address the real world situations found in the field, and such procedure might differ slightly from other publications. Minimum air rate, engine load, TPS sensor and the IAC are very closely linked. All these have to work together to accomplish the goal of keeping proper idle.

The basic ECM idle control strategy is as follows. At the moment when the engine is cranked, the ECM will go into cranking enrichment mode, which means that it will increase injector open time and pulse them in groups (all together). This is because at cranking time is when the engine needs the most fuel. While all this is happening, the ECM retracts the IAC so as to let the greatest amount of air into the intake manifold to feed the large amount of fuel going into the cylinders, brought about by the extra injector open time. As soon as the engine starts and revs up to a certain minimum RPM, the IAC is extended to lower the engine speed. This is the reason why any properly operating engine, when first started, has to rev-up to about 1200 RPM and then down to proper idle speed. All this happens fairly fast. Once the engine is idling any load placed on the engine will be met by the ECM with an increase in IAC steps (IAC retracting) to let more air into the intake manifold, therefore, compensating for the extra load. If the minimum air rate is not properly set, the engine will occasionally stall or set a TPS, IAC or idle speed code due to pneumatic (air) fluctuations in the intake manifold. Any variation in vacuum, as in an unexpected engine load (power steering pump, cooling fans, A/C compressor, etc will cause severe drivability problems on a misadjusted minimum air rate engine. As a final note, keep in mind that the IAC operation is factored into the ECM’s fuel delivery factor. This means that as the IAC valve lets more air in (to compensate for a load placed on the engine) the ECM also increases the fuel delivery as well, by increasing injector pulse. A great misconception in the field is that the IAC is a form of vacuum leak. This is true, but the IAC created vacuum leak is always compensated for by the ECM with added injector open time (extra fuel).

ADJUSTMENT PROCEDURE

Minimum air rate adjustments should be performed every time the engine is stalling due to lack of idle control or out of adjustment throttle plates. The fastest way to know if a minimum air rate adjustment is needed is by extending the IAC valve to its full closed position. This will prevent any air from going past the throttle plates and therefore just letting the engine run on whatever air gets through the engine through the throttle bore and not the IAC passage. If the engine stalls a minimum air rate adjustment is definitely needed.

Follow these steps in the order presented here to perform a minimum air rate adjustment.

NOTEIn order to perform a minimum air rate adjustment the throttle bore has to be fully cleaned and de-carbonized. Any adjustments will not work in case of dirty throttle bore.

1. Start the engine and let it idle making sure correct engine temperature is reached.

2. Stepper type IAC - Using an IAC electronic actuating tool extend the IAC to its fully closed position and disconnect the electrical connector. Duty cycle type IAC (Ford) – Remove the IAC and/or plug the hole leading to the IAC air passage. DC motor type (Cadillac) – Retract the IAC so as to eliminate it form controlling the idle and disconnect the electrical connector. In essence this first step consists basically of eliminating the IAC from controlling the idle and affecting the adjustment. An engine stalling indicates the need for a minimum air rate adjustment.

3. Using an appropriate tool (allen, torx, etc) back off or extend the throttle plate screw to the point where the engine is about to stall but not letting it stall. For the most part 600 RPM will work out quite well for most engines, since this low engine speed is bellow any idle specification. An engine with mechanical problems will have a hard time idling this low. If severe engine mechanical problems are present, idle adjustments may not be possible. However it is always possible to adjust the throttle plate screw to a higher RPM setting, on worn down engines, so long as it is bellow the desired idle PID on the scan tool. Desired idle is the engine speed (RPM) value that the ECM will always try to match through the IAC control.

4. At this point the minimum air rate is already set. However, by adjusting the idle plate screw the TPS might have come out of adjustment and will be necessary to re-adjust the TPS base voltage level. Adjust the TPS by doing the following—leaving the IAC disconnected loosen the adjustment screws at each side of the TPS and slide the TPS back and fourth until the correct TPS base voltage signal in achieved. Then tighten the adjustment screws. On nonadjustable TPS there is no adjustment necessary. This type of TPS usually has a broader voltage signal range for its base voltage (Ex – 0.45 to 0.85 volts). If this base voltage signal does not fall with the specified range, the TPS is probably defective and replacement is needed.

5. Once the throttle plate screw and the TPS is set to proper specifications, SHUT the engine off, RE-CONNECT the IAC motor, and START the engine back up. It will probably take a couple of engine crank tries to get the engine to start properly, since the ECM will have to adapt to this new settings (Idle re-learn).

6. Perform IDLE RE-LEARN procedure. This process is fairly quick on some engines and would just consist of driving the vehicle at 35 to 50 Mph for about 3 minutes and then let idle for 10 to 15 minutes. This procedure can always be implemented to see if correct results are met. On a number of vehicles, an IAC idle reset should be performed through the use of a scan tool. Consult your information system, scan tool, or service manual for this information. Most European, Chrysler, Asian, and some GM vehicles have scan tool idle reset procedures. As a rule Ford vehicles reset the IAC adaptive memory by disconnecting the negative battery post and joining it together with the positive to discharge the electronic PC board internal capacitors (DO NOT SHORT THE BATTERY POSTS as this could cause harm to yourself not to mention the vehicle). This is also the way any adaptive memory is re-set on Fords any time work is done to the fuel control system. The same procedure could also be applied to older GM vehicles.

The main concept to remember when dealing with idle speed problems is the engine load perceived by the ECM, whether real or otherwise. A short or open circuit in any of the ECM load inputs (A/C pressure switch, P/S switch, brake switch, etc) will cause an idle problem. This procedures could be considered part of a complete tune-up and should eliminate any engine stalling problems due to incorrect idle settings.

 

 

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