Hi i have a '79 300sd,.. and recently it has gone down to about 20 degrees. and i have no luck at all starting in the morning. i am sure my glow plugs, and relays are fine. (i think) it starts at pretty cold temps. but i can't start when i leave it over night.. i try cranking it over til the battery goes dead. the other day i noticed the inline fuel filter had a vegetable oil like consistency, so i assume it was the diesel fuel. so then i bought some diesel fuel additive to put in the tank. work fine for that day. now it doesn't work anymore. so here are my questions:

1. is there a way to check if any of my components are bad? i.e relays?

2. i heard people put some petro in the tank does that work good? (they have no.2 diesel over here)

3. any advices?

thank u ahead..

ned