502 mag wont idle
 

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.

Forgot to add, my brother had to put on the VST and all the accessories, and had to pull the intake plenum, so if there is any possibility of messing anything up there let me know.

The idle quality is mainly controlled by the IAC (Idle Air Control). If the plenum was removed, the electric connection to this was disconnected. There are three connections in the plenum, IAC, intake air temp, and MAP sensor. Verify that the connections are all connected properly. What does a scan tool tell you for codes?

Sounds like Boat 1 is on to something. The IAC is just on the bottom side of the throttle body. If it is plugged in, it just may not be working properly. If something was mechanically wrong with the engine, it wouldn't rev easily past 1500 rpm. The IAC have a tendency to plug up with soot and carbon and stop working. Have them pull the throttle body, pull the IAC out and clean it with some carb and choke cleaner. Watch out if you try to operate the IAC if it is not installed. The pintle can come all the way out and then it is ruined.
I think you will find your problem there. Good luck. Give me a shout if I can help in any way.
Eddie

The marina says it is not throwing any codes. I actually told them to check the IAC today, and he seemd to think I was out of my mind. I will call him again in the morning and try to get him to look at it seriously.

I appreciate the replies and would really like to thank you Eddie for always going out of your way to help.

I just went through this on the dyno with a 454 Mag, ran good up above 1500rpm but died under that.....check and see what MAP is there, a Bar 1 or Bar 2 and see what the ECM was flashed for.....??

I had put a Bar 2 on mine for a Procharger set up and found out the MEFI 1 ECM could not be set for a Bar 2, went back to the stock Bar 1 and runs like a charm...just a thought...good luck with it. Jeremy

Your the customer, they are supposed to be the experts. If they can't fix it, go get a second opinion.

Went throught a similar issue last year when I rebuilt my motor and missed hooking up the MAP sensor connection, and it did pretty much the same thing. Have to make sure they are all hooked up.

The first thing I would tell your brother is to find a new mechanic. It sounds like a simple problem for a good mechanic. Probably just a bad IAC, MAP sensor or a wiring problem.

I have done electronic drivability on ford & gm vehicles for a few years,I think Eddie is correct just because the computer doesnt show any codes that just means it is not seeing anything outside of its programmed parameters if something were not plugged in it would pick that up. You should be able to ohm the suspect part (Iac) & find it a little out of range