My 2003 Mitsubishi Diamante will briefly cut out at highway speeds when the accelerator pedal is held constant the car has 189k miles on it

SOURCE: engine drops speed when I press gas pedal

Ignition distributor caused problem. Rotor element needs replacement.

SOURCE: Car won't start when it is wet

its your distributer cap, have someone look at it while you crank it and ask if they see any sparks, if its crack "just from age, or whatever the case" the plugs wont fire right causen it not to start, 20 doller project an it took me one hour, if its the problem you can do it yourself.. i think that may be your prob. it was mine and my 01 diamante ls wouldnt start in the rain or cold either

SOURCE: I have a question about a 2001 mitsubishi Diamante

thats a transmission problem, need to check your fliuid and if its ok bring it to a transmission shop for a diagnostic

SOURCE: 2003 mitsubishi diamante thermostat

You're going to lose some coolant, so have a bucket or other container ready. Follow the upper radiator hose to the point where it's connected to the engine. The hose is clamped to a metal housing which is bolted to the engine. Remove the bolts (not the hose) and lift the housing to expose the thermostat (may require a little prying with a screwdriver or similar tool). Note how the old thermostat is fitted (which end is up), lift it out of the engine, and then remove all traces of gasket from the housing and the engine. Install the new thermostat and gasket, replace the bolts, add coolant as necessary, and you're done.

SOURCE: Location of fuel pump on 2003 mitsubishi diamante?

bs it is in the back under the seat dont comment if you dont know
remove four screws pop plate off and there it is it took me 25 mins to change fuel pump beat that