My volvo s70 1998 doesn't want to start at all in very cold weather. What should I do?

I would try cleaning the Electronic Idle Air Control Valve with a trusted Electrical Cleaner then a Non-Conductive Lubricant.

If it cranks but does not start, you could try starting fluid sprayed into the intake to boost fuel supply.

check the radio fuse for burn

check the distributor for cracks. lightly spray it with water. see if the issue reoccurs.

hard starting in cold weather for a diesel is a sign of problem injectors or battery not full charged
take it to a specialist diesel shop for a diagnosis of the problem

Volvo V70TDI 1998 starts slowly at -2 glow plughs are working, still got with fumes after the car stated;
What to do??

mines got same problem

perhaps you need a new cap and roter a new spark plug wire set. they don;t like moisture. also check to see if you have any broken or leaking vacuum hoses. the worse case senerio would be that your head gasket is leaking--at cold temps a little antifreeze will enter until everything warms up and it seals shut due to metal expanding. but since you say it's cold weather only--this probably wou;dn't be the case. good luck.

Need new spark plug wires/cap/rotor.

weird this one, has a transistor playing up but where ??