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Dear rlstites,
Hope you're well. The gas tank (Volvo Part #3507456) is made of high-density polyethylene (PEHD [Swedish designation, as marked on the tank] or HDPE [US designation]). HDPE can be fused (melted) - and the crack closed - by using a low-wattage soldering iron (20-25 watts). Few glues or sealants will "bite" into HDPE, because it resists most solvents. That solvent-resistance makes HDPE an excellent material to hold gasoline.
I "welded" a polyethylene lawn-mower gas tank, which leaked at the seam between the tank's upper and lower sections. When the tank was made, apparently the heat applied did not properly fuse the two sections. By applying a 20-watt soldering iron to the seam, I "welded" the HDPE.
If you wish to try to weld the cracked tube: (a) be sure the tank is empty of gasoline; (b) flush out fumes with plenty of compressed air; (c) cover all openings to the tank with plastic sheet and rubber band the plastic sheets into place.
Touch the tip of the soldering iron to the HDPE, and keep the tip on the crack only long enough to soften the HDPE. HDPE will not liquefy as does solder. There's no need for HDPE to liquefy. There's no need to put pressure on the soldering iron: heat alone will cause the plastic to fuse.
By the way, a new gas tank costs about $1,300(US) at a Volvo dealer. You should be able to get a used tank at a salvage yard for a small fraction of that amount. A tank from any 1993 or later 940 will fit, as will a tank from a 1993 or later 960, without multi-link rear suspension.
Hope this helps.
Yours faithfully,
Spook
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