DIESEL CRANKING SPEED ACCURATE TESTING METHOD

MODELS: 1978-81 C1500 AND 1983 CABALLERO W/RPO LF9 - 5.7L DIESEL ENGINE

Cranking speed is extremely critical for a diesel to start, either hot or cold. Some tachometers may not be accurate at cranking speeds. A way to determine cranking speed and check tachometer accuracy is to perform the following procedure.

1. Install J-26999 compression gage into any cylinder.

2. Disconnect the injection pump fuel shut off solenoid lead at the injection pump or harness connector.

3. Install a tachometer.

4. Depress the pressure release valve on the compression gage.

5. With the aid of an assistant, crank the engine for two or three (2 or 3) seconds to get the starter up to speed, then without stopping, count the number of "puffs" at the compression gage that occur in the next 10 seconds. Multiply the number of "puffs" in the second period by 12 and that will be the cranking RPM (speed).

Example: 10 seconds = 1/6 of a minute 1 "puff" = 2 RPM RPM = (X) "puffs" x 2 x 6 or RPM = (X) "puffs" x 12

Tests conducted show that diesel engines start hot at around 200 RPM which is below the 240 specified in the Service Manual.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.