i just put a timing belt and thermostat in car it still reads cold as i drive it

?cooling system properly purged of air after t-stat replacement? many systems have special procedures that can take quite a while.If the gauge reads cold but the engine is 'cookin' you most likely have air in the system around the sensor ,it needs hot LIQUID to make it get a true reading. Also re-check all your timing marks one tooth off can run ...but poorly and with ...problems and YES ..check the overflow for residue as previously stated.Did the radiator cooling fan ever come on?