What may be the cause?
 

I notice that my 87 300E will idle very rough if I am in stop and go traffic after an extended period of time. The car acts as if it will conk out and at the same time white smoke coming out of the exhaust. Engine also feels like it is missing if I give it gas. But everything goes back to normal as soon as I maintain constant speed again. Clogged injector? Need new fuel filter? Any thoughts?

White smoke?
 

You may have a blown head gasket. Check your oil/water.

No oil in water. White smoke only when car was running rough. Smoking stops when the engine smooths out.

elau

If it were my car I would make sure the cap and rotor are good. I would also pull the plugs and see if you have a lean running condition. How old is your O2 sensor?

White smoke is often the telltale indicator of water/coolant entering the combustion chamber (head gasket). But a lean running engine depending on the temp can also give off a bit of what appears to be white smoke.

A very lean engine is not good. Have you noticed any unusual higher operating temps?

Check your plugs ....

Haasman