Merc 5.7 MPI low idle

[mjfink]

So, this is the 2nd part of the problem I had today.

After the rebuild of my top end, the mechanic noticed that someone had "rigged" my throttle cable (by bending something) to increase the idle speed. He fixed it, but now the idle is too low, the boat dies out at idle (which makes docking very exciting).

He pulled out the IAC muffler when he was in there and said I need to get a new one (and ordered it for me) and said that may be why it's idling too low (because the muffler isn't on the IAC). The engine makes a loud "sucking" noise at idle now because that little muffler is missing (which also, incidentally, may be the cheapest part Merc makes!).

I'm just wondering, anyone else have a problem like this? Any words of wisdom to share?

 

[mjfink]

Re: Merc 5.7 MPI low idle

And kicking the 2nd part of my problem back to the top as well. The IAC muffler just came in, so I'm going to give that a try, but I don't understand why the computer isn't correcting this; I thought it "knew" the right idle speed and would adjust the injectors to control the idle. Well, either way, I'll put in the IAC muffler and let everyone know if it has an effect on the problem. But if anyone has any good guesses, please, by all means, guess away. I'm doubtful the IAC muffler is going to fix it, the throttle cable was rigged once to increase the idle, I'm guessing that means the problem pre-dates any clogging (and the eventual removal) of the IAC muffler.

Thanks!

 

[Don S]

Re: Merc 5.7 MPI low idle

How about an engine serial number, guessing is a waste of time if all you plan on doing is throw a part at each guess. Do you have an OEM service manual and a scan tool?

 

[mjfink]

Re: Merc 5.7 MPI low idle

I do have the OEM service manual; but no scan tool. I'll take a look in there and see if there a diagnosis for low idle.

Engine Serial number: OW370643

The IAC muffler is gone (it was destroyed, when we took the engine apart to fix the heads it just fell to pieces), so, I know I need a new one of them (and have it coming in this week). I'm just thinking that the IAC muffler is probably not the problem; and wondered where I should look next. My guess is the IAC sensor/valve assembly; but I'm not getting any SC beeps.

Thx!

 

[muc]

Re: Merc 5.7 MPI low idle

Your problem could be the IAC, they can go bad and not set a trouble code.
And they are a common problem.
But at over $133.00 it’s an expensive guess.
Might want to hook up a scan tool and run the IAC test.

 

[Don S]

Re: Merc 5.7 MPI low idle

I'm not getting any SC beeps.

What are SC beeps?

 

[Don S]

Re: Merc 5.7 MPI low idle

If you are planning on sticking with EFI boats, and doing the work yourself, you might as well get a scan tool. One like this http://www.rinda.com/marine/mtspec.htm#techmate will do everything you need, and can pay for itself with only a single visit to a shop. Trust me, codes aren't everything, and to be able to read what is going on while it's happening gets expensive when a dealer is doing it.

 

[mjfink]

Re: Merc 5.7 MPI low idle

What are SC beeps?

Sorry.. Smartcraft beeps.

I'm conflicted on getting a scan tool. I know I should have one, but, darn, they are expensive! And, I'm thinking, if I'm in for that much already, should I just get the PC version (I have a laptop on the boat already).

I'm going to try the guess of IAC valve, this is a 2006 motor that's never had it changed (it's still black like the engine), so, the way I figure, it's a reasonable place to start and the valve I have there is already long in the tooth. If it doesn't fix the problem, I'll keep the old valve as a backup.

I'll report back my results, but, in the meantime, if there are any other places you'd suggest looking, please, by all means, give me more parts to buy and throw at the problem.

 

[mjfink]

Re: Merc 5.7 MPI low idle

Trying to be a good online citizen..

I replaced the IAC today, problem immediately solved. The one on there was original (it was painted black like the engine). I couldn't see any particular "problem" with the one I pulled off (looked clean). The new one instantly fixed the issue, turned the key, engine started and idled pretty much rock solid. Had another problem, but totally unrelated to the idle problem, that seems to have been a nice easy fix.