In a diesel engine, a glow plug preheats the combustion chamber so diesel fuel becomes easy to ignite, according to the University of Tennessee. Since a diesel engine's fuel-air mixture is ignited by the compression of the piston--not a spark plug--cold diesel fuel will not burn readily, but hot fuel will. Worn glow plugs can cause such problems as hard starting, rough running and dirty exhaust.
Hard Cold Starting
Several diesel mechanics concur that cold engine hard starting is a classic symptom of bad glow plugs. This can be especially pronounced on a cold morning. A bad glow plug will not heat the combustion chamber, and hard cold starting is the result.
When an engine with bad glow plugs finally does start, it runs very rough until it heats up. Misfiring occurs because the glow plugs did not heat the combustion chambers sufficiently. As a result, the fuel does not burn well until the engine reaches operating temperature to completely burn the fuel.
Dense White Smoke
Dense white smoke when the engine is cold is sign of bad glow plugs. By the engine having incomplete burning, raw fuel is allowed in the exhaust. The raw fuel may ignite inside the exhaust system, which shows up as dense smoke coming out of the tailpipe. When the engine reaches operating temperature, the smoke goes away, since the fuel is now being completely burnt.
Combination of Symptoms
With bad glow plugs, all the symptoms may occur at the same time. First, the engine is very hard to start. When it finally does start, it runs rough and dense white smoke is seen from the exhaust pipe. Both rough running and white smoke disappear when the engine warms up.