The '88 240 has a black radiator cap and the '92 740 has a grey cap. I read on the boards that thr red cap is much better.

Are these caps interchangeable? What's special about the red cap?

Should I replace mine to the red one?

If you look, you'll find the black cap labeled "75KPa", and the grey one "150KPa"; that's the relief pressure(75KPa=~11psi). I don't have a red one or a green one, but I assume they're different pressures, too.

Without knowing exactly why the designers used a particular pressure, I'm hesitant to change.

Never seen a red cap. Aftermarket? The cap color indicates the pressure they will hold. Black is 75kpa. White(diesel) 100kpa. Grey cap, 150kpa. The grey caps have a tendency to crack if overtightened. The green cap(150kpa) is the only replacement Volvo sells and is suitable for all models, and is reinforced to prevent cracking.

The green cap is the latest generation to the best of my knowledge.

I had the grey cap on my 88 740T, and I kept blowing up radiators and hoses... I pulled a used black cap from a crash, installed it, and have never had cooling problems again.

T

The grey cap has been replaced by the green cap because the design of the black and grey caps makes the pressure vent easy to seize shut. A good grey cap would serve you fine, too.