Cars & Vehicles Auto Parts & Maintenance & Repairs

1989 Corvette Won't Idle & Poor Performance

    First Things First

    • Before anything else, check to make sure that you don't have any vacuum leaks and that your throttle position sensor and idle air control sensor are working and properly adjusted. Check your ignition timing to ensure that it's within factory spec and to ensure that you have a constant spark. These failures can easily cause a rough idle and loss of performance, and they're common enough to warrant checking before anything else.

    Fuel Problems

    • A clogged fuel filter won't necessarily cause a serious drop in pressure, but it will cause a reduction in fuel volume. Ultimately, it is this reduction in volume that causes the engine to drop power, and the rapid fluctuations in pressure that can result from a clogged filter will cause a loss of idle quality. Of course, you may also have the opposite problem; your engine could be getting too much fuel if one or more of the injectors are hanging open and constantly leaking fuel into the engine. This latter will result in a constant or intermittent miss and a raw fuel odor in the exhaust.

    Limp Home Mode

    • Fuel injection systems typically operated in what is called "closed loop" mode, meaning that the engine's sensors form a loop of information regarding air going into the engine and the quality of air and fuel coming out. If any of the engine's sensors fail, the computer will go into "open loop" mode, a very conservative state of tune wherein the computer resorts to its best guess on how the engine should run. The result is a noticeable loss of power and possibly idle quality.

    EGR Stuck Open

    • Your Corvette uses an exhaust gas recirculation system to feed used exhaust gases into the intake to reduce combustion chamber temperatures and thus nitrogen oxide emissions. The EGR valve regulates how much exhaust gas goes back into the engine, and carbon buildup in the unit can gum it up and stick it open over time. This will ultimately result in a loss of performance, bad fuel economy and a rough idle.

    Spark Failure

    • A malfunctioning electrical system or intermittent spark will also cause the above-described symptoms. A loose or frayed ground strap, burned or split plug wires, bad ignition coil or failing alternator can result in reduced ignition power. "Spark scatter" happens when ignition timing randomly jumps up or down a number of degrees, and it's usually the result of a worn and stretched timing chain. A small-block Chevy's distributor gets its motivation directly from the oil pump, so spark scatter can also be the result of a worn-out oil pump or distributor drive gear.

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