The 2 pump thing is partly a warm weather vapor lock thing, yeah.
That said, the D-jet 140s worked pretty reliably with the gravity fed single under car pump next to the tank.
But then they moved the tank closer to the axle/farther from the back of the car & gave it an expansion chamber for the USA market in addition to the usual charcoal can (in 140s this was a mess of stinky hoses & lived in the trunk).
The k-jet cars ran at higher pressure than the efi cars ~70-84 psi, depending. The ‘74-‘76 ‘recall’ tank iirc originally had a sort of side-feed spout like the 140s (been so long since I replaced one).
Starting in ‘77 (& to retrofit the earlier cars) they combined the sending unit & lift pump & return into a baffled bucket that only lets sloshing fuel in gently (tho at the bottom maybe over hot pavement).
Some of it is a cost compromise, some of it is needing to come up with a safer tank farther from the rear of the car, some of it is higher operating pressure of the CI/K-jet, some of it is to prevent vapor lock with minimal frothing, stirring, total volume thru-put & heating of the fuel with what they had to work with at the time.
Without going to a bladder like a Prius / airplane & single pump/return-less system with digital control, they did the best they could with what they had at the time more or less?
The filter lives in the engine bay on the CI cars partly because the pump/accumulator takes up the whole under car tray, but it’s not on the exhaust side or likely there much hotter than hot pavement?
On the MBZ CI cars the tank is a above the two large external/easily serviced quality Bosch main pumps in series more like the D-jet 140s were behind the back seat trapped between layers of metal out of the cabin oriented vertically, w/expansion chamber shielded from sun or hot pavement.
This works better than the Volvo sorta band-aid lift pump + slightly overworked main pump & metal tank over hot pavement. But the Volvo wagon has a flat load floor w/tank fwd from the back of the car & solid axle too & is a cheaper simpler car that forgives neglect better in the USA market.
Turbo k-jets cars the 700T (or moral equivalent) lift pump works well & keeps the main pump from being so overworked. Don’t let the tank run low/into the red on the gauge & it’ll last indefinitely on the cool cloudy weather no hot pavement cars.
The quality of the pre-86 lift pumps or those generally in the 240s leaves a bit to be desired.
When they went to EFI on the 200 series they ran pretty low pressure (2.5 Bar vs. 3 bar from the get go on even the N/a the 700s) / keep the work load down on the engineering compromise 240 tank.
Without significantly altering the tank design, or returnless digitally/precisely controlled somehow, 2 pumps it kinda has to be for optimal operation with the CI /generally in some ways in the 200 chassis w/‘77+ style tank.
1995-only 940t uses the 850/70 style single in-tank pump in the USA market, but the tank/baffle is also different & plastique.
I still prefer gravity fed/external pump like the MBZ or 2 pumps in the 240 chassis if you’re using the ‘77+ style tank to hacking a single onto that poor sender & return line setup to really froth & stirr it up in that confined bucket/sender setup, tho.
People love to hack weird stuff into these cars or think one (especially domestic junk) pump is some great panacea tho?
Idk why Volvo even bothered offering the cars in unusually hot climates?
