502MagMPI Surging at Idle when Hot
 

Hey fellows,
I have a 1997 Merc 502MagMPI.
The engine seems to have developed a surging problem at idle when she's hot. The problem has come and gone 3-4 times this summer and today it was to the point were she would stall below 1000rpm.
The strange thing is that I can launch the boat, turn the key, she fires, and idles at 600rpm when she's cold. This engine has always started quickly when cold.
Today after she cooled down some at the dock, I turned the key and she fired and idled fine again. It did cough out some black smoke but then seem to be fine. When it was surging I think I smelled gas like it was running real rich.
So do I have a bad sensor within the EFI system?
Or do I have Fuel problem?
I'm thinking I should start by changing the temp sender to the computer.

I'm betting the IAC is at fault - you need to scan it and get a IAC count and the IAC Follower count.

A bad IAC would make sense.
I don't have a scan tool.
I do have a code tool, but that is only going to show faults.
Would the ICA show a fault?
I have extra parts from my 7.4MPI. I wonder if it would be the same IAC?

An IAC failure would not set a fault, but I don't think it's the IAC. It sounds to me like a rich condition is causing the problem, and the IAC is trying to keep it from stalling.

I would check for codes, check the ECT sensor, check fuel pressure, and other possible causes of overfueling.

Dave

OK Dave, I can do that.
So if she thinks she's cold, she'll run rich?
Thanks.

I had this problem and it was an IAC, just my .02

Thanks for the input guys. Please keep it coming...:cool:
Even if I replace both the IAC and ECT, we're only talking $110-$120.
Is there any setup for a new IAC or do I just screw in a new one?

Don't forget IAT :)

How much does that one cost?:p

Here's part numbers to save you some time.
GM and AC-Delco respectively.
IAT 25036751 213-190
ECT 15326386 213-928

From memory, I think they're around $40 each.