Motor will turn over but will only run a few seconds.

A dirty carburetor or something is restricting fuel flow. When you hit the gas pedal, the throttle plate opens, allowing a lot of air in, the engine vacuum ...
10 answers · Top answer: Then stop pressing it and committing autocide!

Running out of gas can damage the fuel pump. Need to do a fuel pressure and fuel volume test to confirm pump is working

Questions here are better with full 'year make model' instead of generic '318'.

A few generic ideas.

1. Check fuel pump pressure.
2. Run injector cleaner through the fuel system 'Extreme Clean' is a good one.
3. Clean the throttle body.
4. Does it have a MAF (mass air flow sensor)?

More ideas here

https://www.google.com/search?q=1989+Dodge+318+stalls+and+surges+when+starting

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It's losing one of two things - fuel or electricity. I would check and or replace the fuel pump and fuel filter first.

Replace the alternator, do not wire directly. If it's not working, direct wiring is just a waste of time.

Sounds like a fuel supply problem. Does it pop through the carberator as it it is dying, like a lean back fire? If you drove it up a hill would it die before 35 or 40 mph? If the fuel demand excedes the fuel supply the engine will die, but would restart after cranking the engine for awhile. Possibly a plugged fuel filter, fuel line, rotten fuel hose, ( sucking air ) or tired fuel pump. Hope this helps.

Try putting a little gas in the top of the carb. Then try to start, if it starts then it's a fuel problem, clogged filter, lines, carb or fuel pump. If it doen't start with the gas then check for spark, disconnect a spark plug wire put on a spare spark plug, ground plug threads to motor and have someone turn over motor. You should see a spark between plug and electrode. Also you could check compression of engine, rings could be siezed and no compression will mean no start. If low or no compression, pour wd-40 or other lubricant into spark plug holes and let sit for a couple of days. Hopefully that would unsieze rings.

73 was an awful year for smog devices.
You could just look at any 1973 Dodge diagram, because a motorhome is no different.
But my advice is to take all smog stuff off. It did not work well in 73, and harms engine. You don't have to test anyway.

if you can't hear the fuel pump cut in for a couple of seconds, check the fuse. Check the relay, or replace it with a known good one. See if you have power to the fuel pump. If you have power, your fuel pump is in need of replacement.
When you run an electric fuel pump out of gas, you should always cycle it about 10 times before you try to start the engine. That is, turn the key to on for a few seconds, and then off, and repeat. You need to get some fuel up to the engine.

it should be mounted on the lower right front of the engine. it's held in place by 2 bolts