Would a 1987 Volvo 240 B23F fuel injected engine gain more then 25 percent in fuel economy if hydrogen was pumped through the air intake?

No. There may be a slight increase in performance, but too much hydrogen can easily mess up the effective octane rating of the fuel. That fools with the engine performance - not always for the good. Timing goes wack because the fuel burns too fast and all that. And the engine will meter about the same amount of fuel even if hydrogen is included in the air stream. There might be a slight increase in fuel economy (because of the effect of the hydrogen in making the engine "run with more power" under the circumstances, but it isn't a cost effective solution to the problem of improving fuel economy. If the inquiry isn't about fuel economy but about performance, nitro (nitromethane) will work a lot better. But it will mess with the performance curve, too, and tuning will be required. Additionally, it burns a lot hotter than pump gas. That makes it hard on your engine.