502 MPI intermitant no start when hot.... not fuel prob
07-11-2017, 06:41 AM
#21
Registered
FIXED !!!!!
Boating and had many hot restarts with no problem.
So the scenario is this: The distributor ignition module screws were rusty which indicates moisture at the least or heat from a bad ground contact at the module. The module would always give spark and injector signals even when it would not start; that was confusing. The no start seems to be caused by the 5V reference dropping off which carries the 400+ rpm signal to the computer to begin the start sequence. The resistor in the module would get so hot it lost it's ability to maintain the 5 volts and so the computer never knew to start up. It seems that when this was happening the 5 V could be lost while the engine was running but not shut off because the computer had synced up with the primary trigger therefore stay running until shut off but not restart because the 400+ signal was gone. This is what made troubleshooting this thing so tricky. The primary trigger through the pickup and module never quit so there was always a signal which made the testers I use say it should run and run it did, just not start up again after the 5V was gone.
I used Artic Silver #5 for the thermal paste. Top line stuff used for computer heat sinks and should far exceed anything this module will need. The original white stuff was nearly none existent and probably more of why the module failed.
Boating and had many hot restarts with no problem.
So the scenario is this: The distributor ignition module screws were rusty which indicates moisture at the least or heat from a bad ground contact at the module. The module would always give spark and injector signals even when it would not start; that was confusing. The no start seems to be caused by the 5V reference dropping off which carries the 400+ rpm signal to the computer to begin the start sequence. The resistor in the module would get so hot it lost it's ability to maintain the 5 volts and so the computer never knew to start up. It seems that when this was happening the 5 V could be lost while the engine was running but not shut off because the computer had synced up with the primary trigger therefore stay running until shut off but not restart because the 400+ signal was gone. This is what made troubleshooting this thing so tricky. The primary trigger through the pickup and module never quit so there was always a signal which made the testers I use say it should run and run it did, just not start up again after the 5V was gone.
I used Artic Silver #5 for the thermal paste. Top line stuff used for computer heat sinks and should far exceed anything this module will need. The original white stuff was nearly none existent and probably more of why the module failed.
It seems as if I'm having the same issues with my 1997 502 MPI. Does anyone have a part number for this module at an auto parts store, or do I just bend over and assume the position from my local marine sealer?
