1991 GMC R3500 Ignition Timing Adjust Cost

Ignition timing refers to synchronous operation of the engine mechanical and ignition components. In order for an engine to be considered properly timed, the engine crankshaft and camshaft must rotate synchronously, and the ignition system must fire the spark plugs at precisely the right time. If this does not occur, many older vehicles allow for adjustment of the ignition timing to correct the issue.

Ignition timing begins with the timing of the engine camshaft and engine crankshaft because ignition timing is set according to the position of the engine camshaft. The engine crankshaft and engine camshaft or linked together by a timing belt or chain, so they always rotate in perfect synchronization. As the crankshaft turns, the pistons in the engine move up and down, and the engine valves must open and close at the right moment of the piston. This is achieved by mechanically timing the camshaft to the crankshaft so the camshaft will open the valves and close the valves at the right moments. Since opening the engine valves allows fuel to enter the engine cylinders, the camshaft also controls the moment of ignition by use of a distributor gear, camshaft synchronizer, or camshaft position sensor. No matter which is used, the timing of the ignition system will always be relative to the camshaft, through distributor systems can be mechanically adjusted.

The most common symptoms of improper ignition timing will vary be the type of ignition the vehicle uses. For distributor type ignition systems, the vehicle may run, but it will run too fast or slow at idle, even after warmed. The vehicle may severely misfire and hesitate to accelerate from a stop, or the engine may have these same symptoms with increasing severity at higher RPM's. Also, if the ignition timing is severely off, the engine may not run at all. This applies to both distributor type ignitions and computer controlled ignitions, and is the most common symptom for improper ignition timing with computerized ignition timing systems. Computerized ignition systems tend to cause a no-start condition when ignition timing not as the engine control module expects, because the camshaft position sensor has failed or the engine timing belt or chain has broken or jumped time. For OBD compliant vehicles, the check engine light will be illuminated, and OBD trouble codes will be stored in the vehicle.