1990 GMC R1500 Suburban 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.

Ignition timing failures for computerized vehicles will nearly always require immediate repair. This is because the vehicle will likely fail to start, run, or drive. For vehicles with distributor type ignition systems, the ignition timing should be brought back to the correct setting according to the manufacturer immediately as well, but for other reasons. Incorrect ignition timing can lead to carbon build-up, poor fuel mileage, overheating and detonation, as well as the drivability mentioned.

Ignition timing adjustment is conducted as a normal tune-up item, so the interval for checking the timing is far greater than adjusting the timing. Since oil on older vehicles is changed more often, around 3,000 miles, the timing will be inspected just as often. Certainly the timing on a distributor type ignition vehicle should be checked and adjusted after no more than 50,000-75,000 miles.

Diagnosing incorrect ignition timing for a distributor type engine is simple, and only requires a timing light. The timing light is connected to the number one cylinder spark plug wire and the battery terminals. Then, with the engine running, the pulsing of the ignition wire on the number one cylinder will cause the ignition timing light to flash. When the light flashes on the crankshaft pulley or harmonic balancer, the light will illuminate a line or arrow that corresponds to a gauge on the timing cover. This will allow the technician to identify where the timing is currently, and adjust the timing if needed.

Ignition timing adjustments require the timing light to remain connected, the engine running, and the distributor hold-down fasteners to be loosened very slightly. Once the engine is warm, the distributor will be slowly rotated to achieve the exact angle of ignition in relation to the crankshaft pulley, and the hold-down fasteners will be tightened once again.

We do not recommend adjusting timing by vacuum pressure, by ear, or by idle speed. Vacuum pressure can be incorrect even if timing is correct, and idle speed is set after timing is corrected. Using these methods will result in an engine running below optimal efficiency, and the engine temperature may be too high or low in older engines.

When adjusting ignition timing on distributor type ignition system, there is a high probability of electric shock due to worn ignition wires or distributor cap. The wires should be inspected for wear and tear before attempting to rotate the distributor to adjust timing. Also, the vacuum or mechanical advance should be tested before adjusting timing, as these will prevent the distributor from maintaining proper timing.

Timing a distributor type ignition system is simple, and typically only requires a wrench, timing light, and a helper. As long as the crankshaft pulley is properly aligned, the vacuum and mechanical advance is tested, and the ignition wires are in good condition, this is a breeze for almost anyone.