Can't just guess with the information you provided so far. You also really need to include the engine size when discussing a running problem. It's rather relevant. Your truck didn't come originally with electronic ignition, but the GM HEI distributor could easily be dropped into a 350 c.I. Is that what you have?
How does it act when it quits running? Does it sputter and die out slowly over several seconds, or does it cut out instantly as though you had turned off the ignition switch? How long before it will restart? When it does restart, do you have to crank on it a while or will it fire right up?
February, 20, 2012 AT 10:50 PM
Engine is 350 V-8 with GM HEI electronic ignition. It sputters and then dies. Hard to restart sometimes
February, 20, 2012 AT 11:40 PM
First you must determine if it's fuel or spark you're losing. If you can keep it in the no-start condition long enough, check for spark first. If you have a tachometer on the dash, a clue is it may go to "0" while the engine is still coasting to a stop. That would point to the ignition system primary circuit. The most common cause is a failing pickup coil around the distributor shaft. The wires can break off from the flexing caused by the vacuum advance mechanism moving it back and forth, and corrosion can cause the wires to break off right where they're connected to the coil. If the coil itself has the bad connection, it will often show up when it gets warm and work again once it cools down. It normally takes at least a half hour to cool down.
If the tach stays up as the engine coasts to a stop, you either have a fuel problem or something in the ignition secondary circuit. If spark is missing, suspect the ignition coil. They can be intermittent. As I recall, the ignition module typically fails permanently, but as with anything electronic, it's always possible for it to fail intermittently too. If you must replace the module, be sure to use the heat-sink grease that comes with it.
If you have spark but the engine won't restart, look down the throat of the carburetor and work the throttle to see if a pair of nice streams of fuel squirt in. If not, you're running out of fuel. A common problem that's hard to diagnose is a plugged or collapsing fuel pickup screen inside the gas tank. The typical symptom is it will run fine for about ten miles, then gradually start bucking and losing power until it just won't run. It will take five to ten seconds of cranking to refill the float bowl with gas before the engine will start, and it usually has to sit for five or ten minutes before that works. Those few minutes are needed for the screen to stretch back out or for microscopic debris to migrate away and let gas through.