The engine cooling system for the DeLorean operates at 15 psi. This makes an expansion tank P/N 101944A (also commonly referred to as the Header Bottle, item #35 on the illustration) a necessity. It would not be practical to use the radiator tanks for this purpose as is often the case on conventional cars.
This tank is under pressure when the engine is running, so it must be capable of handling this pressure and the 200+ degree coolant. The factory original plastic tanks have the disturbing habit of breaking down and exploding after years of heat and hard use. When new, the plastic tank is close to a milky white in color. If your tank is brown and/or shows cracking/crazing at the end towards the rear of the car, you are on borrowed time.
One of the most popular upgrades for DeLoreans since the 1980s has been a metal header tank. Both aluminum and stainless steel are popular materials for these. The cap attachment latching area on the aluminum tanks tends to wear from the steel cap, so if the cap does not tighten properly or has issue sealing, it may be necessary to replace an aluminum expansion tank at this time.
DMC offers a stainless tank that mounts the same way as the original. P/N 101944A
A reminder: The cap on the header bottle must also be capable of holding 15 psi. Incorrect or old caps may allow coolant to leak out under pressure and cause overheating. A new Header Bottle Cap P/N 101977 is cheap insurance.
By Warren Wallingford with edits by James Espey, DeLorean Motor Company (Texas)
Updated 6/8/2016 DAS [edits, links]