I have twin 1995 502 mag efi gen 5. One blew up and couldn't find a bland new long block but merc said that the gen 6 was the replacement. 3 months later and 18k in parts and labor and still not running correctly. Now I finally think I'm good and put it in the water and the new motor will not go over 3000 rpm. Ran great just stops at 3 k. My past mechanic says it's the computer , does that make sence? I spoke to merc and they say I can't update the computer that I have though, any suggestions before this boat ends up on the rocks

You should have a MEFI 2 controller here but I would need your old motor engine serial # to verify. You need to hook up a scan tool to see if you are pulling any fault codes and verify correct sensor outputs along with other diagnostic information , if you can not even communicated between the computer and the scan tool then its could be a good possibility that your computer is bad. I would also think about having the ECM tested before you buy a new one. They are not cheap plus you have to do a conversion to MEFI 4 if you are buying thru Merc. MEFI 1 thru 3 are NLA thru ALL marine OEM's. With that said you can buy a reman ecm from a few places or get yours remaned. Merc will want to sell you the MEFI 4 conversion kit.

I think your problem is the mechanic and not the engine.

Old engine serial numbers of586127. New long block numbers 883283r5

18 thousand in parts and labor? ....Where did the money go? .I agree with Griff's statement above

lack of knowledge from installer seems to me....