It's entirely possible for the relay to energise and switch as normal, but if there is burning on the contacts, although it will show 12v on a meter it won't deliver any guts so to speak, when you put under load, quiet normal.
If not too badly done they can rescued with DE-OXIT, but it's bloody expensive stuff, especially in quantity, so new relay is cheaper, you could always rig a few lights up to tell you if you lose power either as an output from relay to pump, or as a switching feed to relay from ecu, bit of messing about but tells you what is what if it conks out.
tabetha