IDLE SPEED ADJUSTMENT
See Figures 1 and 2
One of the merits of electronic fuel injection is that it requires so little adjustment. The computer (ECM) does most of the work in compensating for changes in climate, engine temperature, electrical load and driving conditions. The curb idle on the fuel injected Nova and Prizm should be checked periodically but not adjusted unless off spec by more than 50 rpm. If you are a compulsive tinkerer who feels that the idle can be adjusted, therefore it must be adjusted, please don't.
If the idle is too far outside its normal range, the computer will see it as a problem (too much or too little air being taken in for the speed of the engine) and attempt to compensate by changing the amount of fuel being delivered. This can lead to wild surging and extremely bizarre idle behavior, making the car virtually undrivable. This is one area in which the old rule of Don't fix it if it isn't broken, applies.
On 1988-90 models, the idle speed adjusting screw is located on the side of the throttle body. You can find the throttle body by following the accelerator cable to its end. The adjusting screw may have a cap over it to prevent casual meddling. If so, pop the cap off with a small screwdriver.
With the engine fully warmed up, properly connect a tachometer. Make sure that all the electrical accessories on the car are turned off and remove the cap over the adjustor screw, if there is one. Start the engine and use a screwdriver to turn the screw. The idle speed should be as shown on the underhood Emissions Label or in the Tune-Up Specifications chart at the beginning of this section.
If for any reason the idle cannot be brought into specification by this adjustment, return the screw to its original setting and follow other diagnostic procedures to find the real cause of the problem. Do not try to cure other problems with this adjustment.
The air/fuel ratio burned within the engine is controlled by the ECM, based on information delivered by the various sensors on the engine. It is not adjustable as a routine maintenance item. The easiest way to check the air/fuel mixture is to put the car through a tailpipe emissions test. Whether or not this is required in your area, it's a good way of putting numbers on the combustion efficiency of the engine. The engine can only burn so much fuel; if too much is being delivered, it will show up on the test as unburned hydrocarbons (HC).
Putting the car through this test once a year from the time it is newly acquired can provide an excellent baseline for diagnosing future problems.