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).
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.
NOTE: In 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.