502 mpi hard restart
 

Did a mild build - rebuild two years ago with eddies fuel return to tank trick and a computer reprogram. Ran great for a couple months then became hard to restart and had to have it full throttle to start. Its been getting worse to start some times starts great but next time it almost starts at full throttle but not quite ( when it does start she always runs strong). There are days it has 43 psi on the rail and spark but wont take off. I have been throwing lots of parts at it on weekends but no luck. I have a MEFI pro scan hooked up to it but still confused. Any advice would be appreciated.

Full throttle is clear flood mode. Ie: no fuel (or very reduced) thru injectors.

Injectors were cleaned and tested at time of rebuild. I never could see fuel in the sight line to throttle body and both vents on vst held vacuum but i replaced the float seat and diaphragm anyways yesterday. Rail pressure seams to hold good. Started right up today and took for a ride when i got back on trailer it would not restart. Got home and vst tank was about 2/3 full but had a little pressure when i pulled the fill port plug.

How do you know it has spark when it won't start?

Used one of those inline spark checkers

someone else had a similar issue turned out to be the gas line from the tank to the filter head, it was collapsing internally

I can measure it for a new one this weekend but it ran about 69 mph yesterday parked on the trailer and would not restart and the vst tank was about 2/3 full.

where are you picking up fuel psi schrader valve on the rail ?

If you were coming off the coil, then it has to be fuel

Injectors are batch fired, I use a Noid light and check a couple when cranking to see if there firing.
Might be something simple as the Tach pulse is to weak for the ECM to see. If it doesn't see that the Dizzy is firing it won't fire the injectors

Yes. about 35 with vacuum and 43 without.

I didn't try the noid lights but the weekend before i unpluged the fuel pump wire and when i cranked it at full throttle fuel pressure held but a low idle it would drop fast. I replaced the ign module but not the pickup. yesterday the dash tach would move when cranking but didnt on the computer ( but even with it running the rpm on the MEFI scan is all over the board)

I could be totally off base here, but I always think of a leaky injector or two, when a warm restart is difficult and aided by clear-flood mode. I know you had them gone through, but that doesn't mean they are all still good.

Besides watching for rail pressure to drop is there any other test besides pulling them out and finding a shop to test them?

Looking at plugs might give clues about a rich condition in one or more cylinders. Wouldn't be a slam dunk for a leaky injector but it might narrow down which injectors are suspicious?

Also, this might be a stretch, but if you pull the plenum and use a borescope in each intake port, you might be able to see an injector tip leaky when the rail is pressurized.

Thats a good ideal. I tried that a week or two ago but i only have a cheap scope without a side shooting camera. Maybe its time to upgrade :)

I was just thinking about possible leaking injectors and when i shut it off Sunday it was only seconds before i tried to restart it with no luck. Could they leak that fast?

My knee-jerk reaction is probably not, but I guess it depends on the severity, and how many of them.

A trick I did on my car was to pull the fuel rails up 6" or so (leave fuel lines connected), pull the injs and install them in the fuel rail, power up fuel pump and see if any leak. Not sure this will work on your eng or not. My injs had little clips to secure them in the fuel rails.

The MEFI-1 injectors definitely don't have retention clips so you could wind up shooting the injectors out of the rail once it's pressurized, with a mess of fuel spray. I'm not sure about the later MEFI 3style but I think the same is true for those.