1991 Mercruiser 5.7, Thunderbolt IV ignition. Would run fine for hours, sometimes for 20 minutes, then wouldn't idle, and would often stall out completely. Giving it extra throttle would restart the engine, running rough enough to get back to the dock. (Stalling a couple times on the way.)
I figured I had possible fuel pressure issues, vapor lock, carb or ignition. Took the boat out with a timing light attached and the trigger zip tied, but couldn't get the issue duplicated! Ran just fine. Still couldn't pin it down as electrical or fuel.
So, checked fuel pressure, carb is brand new, double checked float level. Replaced coil. Re-routed my fuel line to go alongside the raw water hose that comes off the crank driven pump. Took it back out, did it again after 2 hours! Hooked up my timing light again, started engine...and ran fine the rest of the day.
In the garage at home, I went inch by inch through the engine wiring harness....removing the wrap as I went, inspecting everything. Sure enough, the purple wire that goes from the alternator to the coil? About 3 inches from the coil connection, inside the electrical tape wrap, was a 1/2" section of the wire that had a split in it. Actually looks like the wire had become hot at some point, close to melting. The wire itself was intact......but I went ahead and made a new wire, the entire length.....4 hours on the water today, all rpm's, couldn't make it stall or run rough. Thinking it might be a good idea to just make a whole new wiring harness for the engine, it is 22 years old.
I figured I had possible fuel pressure issues, vapor lock, carb or ignition. Took the boat out with a timing light attached and the trigger zip tied, but couldn't get the issue duplicated! Ran just fine. Still couldn't pin it down as electrical or fuel.
So, checked fuel pressure, carb is brand new, double checked float level. Replaced coil. Re-routed my fuel line to go alongside the raw water hose that comes off the crank driven pump. Took it back out, did it again after 2 hours! Hooked up my timing light again, started engine...and ran fine the rest of the day.
In the garage at home, I went inch by inch through the engine wiring harness....removing the wrap as I went, inspecting everything. Sure enough, the purple wire that goes from the alternator to the coil? About 3 inches from the coil connection, inside the electrical tape wrap, was a 1/2" section of the wire that had a split in it. Actually looks like the wire had become hot at some point, close to melting. The wire itself was intact......but I went ahead and made a new wire, the entire length.....4 hours on the water today, all rpm's, couldn't make it stall or run rough. Thinking it might be a good idea to just make a whole new wiring harness for the engine, it is 22 years old.