I don't have a meter on hand so it will be a bit to check the grounds. A friend of mine has a noid tester so ill try that tomorrow

Does the mercathode have anything to do with that?

no

Just to shine a little light on the subject. Our 502 efi engines are "batch fire". Meaning the ecm fires 4 injectors at one time unlike sequential were there all individual.

So reading you lost 4 all at once tells me you broke a wire in the harness some where. only 2 wires from the ecm fires 4 injectors. iirc there is a fuse under the ecm box lid for the injectors. since 4 still work im guessing its good. Google the mercruiser #16 manual. the wire diagrams and trouble shooting guide is section 5 I believ for the fuel.injection. I just got done reading it cover to cover ;-)

Hope this spreads some light on the subject

http://www.maxumownersclub.com/forum...cruiser/16.pdf

Im guessing J2-5 (on the ecm) wire (color Dark Green) is broken somewhere in the harness or at the ECM. 468 drives injectors 1, 4, 6, 7. Same ones you have that are dead

Look at page 501 on the above link. It is the injector wire diagram.

First check the wiring at the ecm and harness (that is the easiest). After that you will have to remove the top intake manifold and start tracing j2-5 (dark green wire) from ecm to injectors.

does the rev limiter shut off 4 injectors or 4 plugs?

At idle it wil rev past 3k
But when you try and take it on plane it is 2-2500rpm max like its under a big load

^^^??? So now you have all 8 injectors working?

Look at the merc manual #16. "troubleshooting". It will walk you threw step by step of what to check.

^^^still only firing on 4

check to see if theres at least 12v at the injector w/ the key on.then we'll go from there.