I have a 1985 Mercedes 300D all of a sudden it is starting very hard. I live in Florida the temp is 70 now so don’t think it is a cold start problem. I checked the glow plugs per the manual one was bad changed it out still have the problem checked the relay am getting 12 volts at all plugs. I changed both of the fuel filters no change. The problem is it will start after about 20 or more seconds of cranking with the accelerator the floor then a plume of smoke comes out. After it runs for a bit it will start with one crank if I let it sit for 4 or more hours the problem starts over. It has always started the first turn of the key.I have done a search and have not found any threads that described this problem. Any help will be appreciated.