DIESEL HARD HOT START SERVICE INFORMATION

Diesel engines may exhibit hard starting characteristics when they are shut down for some period of time after being fully warmed up. The cause is almost always lack of fuel or lack of heat to ignite the fuel.

The first step is to detemine if it is a heat related condition or a fuel related condition. Normally this can be determined by looking at the exhaust when the engine is being cranked.

Large quantities of white or light blue smoke coming out the exhaust is fuel vapor that did not ignite. If you see lots of fuel vapor, the condition is most likely insufficient heat. If there is little or no white or black smoke, there is insufficient fuel.

The Service Manuals contain information regarding the diagnosis of diesel engines that are hard to start. Review the appropriate manual for diagnosis information. The following are items that are the most often overlooked, or items that were not contained in the Service Manuals. This listing is by no means a comprehensive list of all items that affect starting.

Insufficient Heat

The following items affect the heat and must be correct. Refer to the Service Manual for procedures:

1. 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 procedures:

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

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

c. Install a tachometer.

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

e. 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 10 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.

2. Dynamic Timing - If timing is retarded beyond specifications, hard starting may be experienced.

3. Compression - low compression may be experienced as a result of stuck rings. Stuck rings can be freed up with the use of a top engine cleaner. Refer to 'Diesel Engine With Stuck Piston Rings" Bulletin 82-I-188, File Group 60, #67, Dated July, 1982.

Insufficient Fuel

1. Cranking speed - if it cranks too slowly, high injection pressures will not be reached.

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.