502 MPI idleing problems
 

Went boating yesterday, and a couple of times the SB engine would start up, and then run @ a HIGH idle, ~1200-1500 RPMs, and I would have to shut down the motor and start it back up and it would idle fine! :(
Does anyone have an ideas what's going on? Motor's are stock, with about 325 hrs on them.
Thanks,
Dean

Re: 502 MPI idleing problems
 

Probably the Idle Air Control Valve is sticking. Something similar happened on my 502, eventually it would not idle at all. The valve had stuck closed.

Re: 502 MPI idleing problems
 

Dean,
As cdscarab said, it's likely the IAC valve. This is an electrically operated spring and seat valve, (kind of looks like a thimble with a small shaft, overlaid by the spring). It allows more or less air in the injection system based on engine temp, manifold pressure etc. A new one will run $50 to $90. Easy to install, there are 2 small screws holding it in place. The electrical plug in has a vinyl/rubber boot protecting it thta is oval shaped. You'll find it located on the air inlet at the front of your injection manifold. If you have a Merc scantool the IAC actuating is one of the perameters checked as you scroll thru. I've had mine read from 140 down to -0-. I believe that reading is milivolts. The HP engines have a blue IAC and the non HP mercs have them in black. I think they are all black if you buy them from a GM dealer. I don't believe there is any difference in their efficiency, weather you buy it from a Merc dealer or your local auto dealer.
You'll hear the IAC work when you first fire up. You'll hear a loud hissing for a while and then it quiets down. That's the sound of the air bypassing the valve. If it's not working right, you either have a lean condition, or rich (too much fuel) and thus the high idle. When you shut it off and restart it may be actuating itself and curing the problem.

Hope this helps,

Steve

Re: 502 MPI idleing problems
 

Steve, the number you see on the scan tool is actually called a "step". There are 255 steps from closed to fully open.

As for the idle fault, prolly a sticky IAC pintle. Make sure you check the seat area for carbon build up after you remove the valve. The engine controller seats the valve at every shut down and then counts back a predetermined number of steps to prepare for start up. If the seat is contaminated, the ECM can't seat the pintle fully and so it takes the new, partially open position as "0" and counts up from there. This can lead to a high idle condition. If the seat is carboned, clean it with carb cleaner before installing a new IAC valve.

Re: 502 MPI idleing problems
 

Dean.
The guys are on the right track. I had to replace one of mine while it was at the shop. The IAC controls the amount of air that enters at idle. At speed the engine will run fine---it just won't idle. You can check by placing you thumb over the air inlet hole (the one you had to grind on you air intakes), you should be able to hear and feel the air flow. You will have to pull the Plentum off and the IAC is mounted on the underside. Mercruiser part # is 805224A1. Not a major job! You will need new Plentum gaskets also. Let me know if you need any help.

Dail

Re: 502 MPI idleing problems
 

Thanks, for the info guys!

Steve,
Can I go buy this unit @ the local GM dealer? I wonder if would be cheaper than buying thru Mercrusier?
Thanks,
Dean

Re: 502 MPI idleing problems
 

Hi Dean,

Yes you can buy it from a GM dealer. Suprisingly I didn't find it much less expensive. In a pinch I bought one (black) from the local Sea Ray dealer about a year ago and paid about $70. Same exact part, just not blue HP paint.

If you have difficulty finding one, let me know and I'll pick one up at MarineMax in Pompano and send it to you.

Steve

Re: 502 MPI idleing problems
 

Steve,
Thanks for the offer, I'm going up to Callan Marine/ Mystic Power boats this week or next, and I'll check with them about getting one.
How's the Black Thunder running? Are you trying to sell her?
Thanks again for the offer,
Dean

Re: 502 MPI idleing problems
 

Hi Dean,

The Black Thunder has a new owner. Really nice guy who spends 6 months in Florida and the rest in Philadelphia. He formerly had a 42 Sonic that was destroyed when hurricane Wilma knocked down the west building of Marina One in Deerfield Beach. Boy we sure had 6 great years with that boat and will miss her. We're enjoying cruising ALOT slower in our 540 SeaRay. My speed need is met by our 11' Boston Whaler Sport tender which runs 27 MPH with two of us, now that's exciting..... That's stretching it a bit actually, with the 25 Merc timmed to the moon, it's barely 27 MPH....
Will still be driving the Unlimited Hydro O Boy! O Berto/Miss Madison this summer. Take care Dean, let me know if you need any help.
Steve

Re: 502 MPI idleing problems
 

Steve,
Congrats on the new boats! I really like the looks of the new Sea-Rays, especially the new colors they are using! :D
Good luck this season,
Thanks again for all the info and help offer.
Take care,
Dean