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

Sounds like the IAC to me, what are the iac counts on the scan tool? If ity was bad the number (I think) should be real high. The ecu tyells the iac to open so much, if it doesn't seem to respond I'm not sure it will throw codes but thats what I would be checking. I would also be looking for a new mechanic, Smitty

I agree with the other guys, sounds like a IAC or MAP sensor. Could be unpluged or defective, the IAC is a stepper motor it will not usually trigger a code.
Oh, and fire the mechanic

The ecm has no way of knowing the actual position of the IAC. All it knows is what it is commanding the IAC to do. The ecm doesn't know if the IAC is actually moving or not. It will not throw a code for the IAC not moving or moving irradically, or if it is unplugged for that matter.
It sounds like a simple problem. You also need to check the tps reading at idle, especially since the ecm has been programmed. There is a table to calculate the tps opening and closing percents. Make sure it is zeroed at idle and 100% at wot. (check with the key on and engine not running:lolhit:)
Good luck. Give me a shout if I can help.
Eddie

Thanks for all the replies. I just called the shop foreman at the marina, and he claims they have checked everything I suggest. He cannot tell me if the IAC was pulled and cleaned, but he did tell me that the reason they had the ECU reflashed was because it was throwing a map code. He said he would have the tech check the map sensor and verify it is the correct one. I also tried to explain to him that the IAC would not throw a code, but I think he is getting tired of hearing from me seeing how it is not actually my boat.

My shop is in the Austin area. Be glad to take a look at it or offer advice. Sounds like a simple fix. I doubt it's anything internal. Is the engine in the boat now? I have the capability to reprogram the pcm if needed to get it correct, although it shouldn't need that.

Bob Lloyd
Full Throttle Marine

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.

Glad they got it sorted out.

Bob

[QUOTE=tx911;3328696]The marina says it is not throwing any codes. QUOTE
What an ugly thing to say.

Based on that explaination I would never go back to that shop for service. The injectors are batch fired, 4 at a time, and the wiring harness has very little excess wire for things to be crossed up. They are telling you a complcated sounding explaination to justify a large bill for their bad diagnostic skills. RUN. Just my opinion.

I just repaired mine for the same issue. I absolutely guarantee that it was a vacuum leak. I checked for leaks everywhere and finally just pulled everything apart, cleaned, and installed new gaskets and it idles perfect. Glad you got it running.

Correct! Diacom has a nice "graph" funtion that shows the IAC level. If the IAC is not working, the graph will be flat lined. But, still no code.

The shop probably figured that had to make something up to cover a rookie mistake.

Just for future information, the IAC is a $26 part from the auto parts store so if your idle ever shows problems again, it's a common part that fails and for $26 and about 15 minutes time you can replace it and rule that one out.

Looks like it absolutely guaranteed wasn't a vacuum leak based on post #16:rolleyes:

Do you have the part number of the IAC from a parts store. Most parts guys around here wouldn't know how to look it up if you don't give them a year and make of the car/truck. I would like to keep a spare on hand for $26

MASTER PRO PART #21755 or 2- IAC2

I think that Checker, O'Reilly and a few other stores carry this brand. $26 vs. the merc part #805224A1 which is nearly $100

Thanks Chico&Zues, I drive right by O'Reillys on the way to work everyday, so I will swing in and pickup a spare. In fact I might go ahead and change mine in the off season, and then clean mine to use for the spare.
My garage at the lake is the place where all my friends come to fix there boats, and it seems like it's always the first day of a holiday weekend that someone has a problem. Have saved many Holiday weekends for my buddy's.

Sorry I tried to help. :snide: