Originally Posted by tx911

My brother bought a 502 mag from someone off this site, it was complete minus vst and accessories. His boat got damaged in July and that is when he decided to upgrade the motor while the other one was out, the boat had a 330hp 7.4lx that ran great, just wanted more power.

The marina that is working on the boat ran a compression check and all cylinders where within a few pounds of 165psi. They started the motor on the ground and it ran, but will not idle on its own. They say after you get it over 1500rpm it sounds fine, it just dies at idle. They said the injectors where bad, so they where sent to a injection service place in Austin, and all injectors where within 1% of each other. then they said it was the ECU, so my brother bought another one off oso, they said it was not right either so they sent them both to merc for a reflash, motor still does the same thing. I called them today and talked to the shop foreman, he says they have checked the ignition and for vacuum leaks, and there are none, he also claims the fuel pressure has been checked and it is fine.
He did not go into detail about what all they did to check the ignition, fuel, or vacuum, and I did not want to grill him about every detail as I am not actually the paying customer. He thinks there is something mechanically wrong with the motor like a wiped out cam lobe or something, but I think he is grasping at straws. The motor has 52 or 57 hours on it according to the scan tool, and I trust the person the motor came from, and he says the motor ran great. Everything he has told me about the motor thus far has been totally above board, like the fact that the injectors in the motor where new.
Does anyone that made it through this rediculously long post have any ideas I can tell the marina to try, because the only other option at this point is for me to drive 6 hours one way and pick up the motor to fix it myself. My brother does not want the marina to tear it down for obvious reasons, and I think there is a simple awnser they are missing. Thanks for any suggestions or ideas.

Originally Posted by txtraveller2002

I'm the brother here....I just got off the phone with the shop, and they found that the injector electronics were wired backwards. They stated that it was wired according to the Merc shop manual, and that evidently the manual is wrong. Thot sounds rather suspect to me, but oh well, it is idling correctly now. I can't imagine that every mercury published shop manual would A) be incorrect, or B) not have a technical bulleting sent to certified dealers. I may be wrong on that, but I'm certainly not paying for poor troubleshooting skills/publishing errors. Anyway, thanks for all the replies guys, and I have definitely found a different shop to take my business to in the future. Not a shameless plug here, but Bob over at Full Throttle Marine was extremely helpful over the phone, (he posted in this thread also). I'll absolutely go there from now on.