I have a 1992 gmc jimmy on a water access only island. it stands out in all weathers in the Vancouver area of BC. The vehicle is only used at weekends and when parked I disconnect the battery negative terminal. it would not start recently, jusy cranked on a fully charged battery. A week later it started up just fine so i gave it a run and then stopped and started it a few times, about 2 hours later it would not start again ...so a week later the same situation, started fine but then a few hours later had plenty of crank but would not fire up.. any suggestions ? thanks.

For future reference disconnect the positive lead if you are not going to use it for a long period of time.
Using a multi-meter, set the dial to read Vdc.
Take a reading of the battery voltage and note it. If it is low you'll need to charge it to it's full capacity in order to perform a load test.
Take another voltage reading to verify that it is fully charged.
Using a load tester, put the battery under load for 30 seconds and release the load switch. Read the meter on the load tester to see if the battery is still good or if it needs to be replaced.
I use a KAL-Equip 4260 [100 amp] load tester- it's capable of checking the charging system as well.

If the alternator is bad, you'll need to replace it with a new or rebuilt- or if you are mechanically skilled in this area of an auto's electrical system, you can purchase a rebuilding kit at any auto parts store.

If you're budget cannot afford buying a load tester and charger- bring the battery and the alternator to your local auto parts store and they'll test them for you- free of charge (at least where I live)

Cranking and not even trying to fire is usually a fuel pump failure. First be sure it is quiet at the vehicle area. Battery is fully charged. Set in vehicle, radio & a/c-heater off. Turn key to on/run position for 3-5 seconds there should be a faint sound of electric motor (fuel pump) running. Turn key off for 10 seconds & try again. No sound of it running? Then next simple check is to have a person crank the engine while you hit the bottom of the fuel tank in center area with a hammer. Repeatedly hit it with solid hits (not too hard, you do not want to damage the tank) while cranking. If the engine starts the cause is the fuel pump motor failure. This requires to remove the fuel from the tank & lower the tank to replace the fuel pump. You must get at the level below 1/4 tank to safely handle the tank.
If your engine did not start & you do hear a fuel pump 3-5 second prime then you could have a ignition failure. The most common is the coil wire corrosion in the boot and the tower of the coil. Check this at the distributor also. Also check all spark plug wires for corrosion. Best to replace these parts if corroded. If you do not replace them next best is to wire brush and seal with wheel bearing grease or ignition dielectric grease. Use grease on new parts you are replacing also. Sometimes the grease you add causes the boot to seal & air in the boot pushes the boot back off a little. The boot needs to be released of the air in it. Slip a very small screwdriver under the boot while pushing it on then there is a way for the air to escape.

Chris
ASE Master certified sense 1979