Diesel Engine - Exhaust Smoke Explanation

83ford21 Article No. 83-4-16

DIESEL ENGINE EXHAUST SMOKE - 6.9L - "NORMAL" CHARACTERISTICS


LIGHT TRUCKS 1983 F AND E SERIES

Unnecessary service on 6.9L diesel engines is being made because of mis-diagnosis involving the visual appearance of the exhaust smoke.

The following is a description of what is "normal" and expected exhaust smoke for a vehicle with a diesel engine. Diesel exhaust smoke can be classified into two categories according to the color of the smoke.

The first category is blue-white smoke.

^ Blue-white smoke may be observed at engine startup whether the engine is up to operating temperatures or not. This start-up smoke will be observed at all ambient temperatures and should last no longer than a minute after the vehicle is driven.
^ When ambient temperature is below 50~F (10~C),
blue-white smoke can return after engine warm up due to extended idling. This is due to the combustion chambers cooling down during periods of extended idling time.

Heavy blue-white smoke will also occur when the engine is operated at wide-open throttle (accelerator pedal to the floor) with the transmission in neutral or with a lightly loaded vehicle in any transmission gear setting. The smoke is a normal characteristic for a diesel engine with a light minimum-maximum governor spring in the fuel injection pump. This results in the following characteristics due to the engine operating above its rated full load speed (3,300 RPM) in a no-load or lightly loaded condition:

NOTE: In a no-load or lightly loaded condition, the engine speed may reach as high as 3,700 RPM.

^ Heavy blue-white smoke.
^ Fuel injection pump governor "hunting", resulting in high speed engine RPM surging.
^ Engine sputtering or misfiring.

These conditions can be avoided by not exceeding the rated RPM in a no-load or lightly loaded condition.

The second category of diesel exhaust smoke is black smoke. Black smoke occurs whenever the engine is working hard. The engine works hard when it is going up a steep grade, pulling a trailer, carrying a heavy load, or during heavy acceleration. More black smoke will be observed when operating the vehicle at higher altitudes. If black smoke is observed while the engine is idling (at low altitude) or under normal driving conditions, the cause should be diagnosed as soon as possible.

OTHER APPLICABLE ARTICLES: None
WARRANTY STATUS: "INFORMATION ONLY"