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.

do yourself a real favor and take the money you were going to spend on all these sensors that you are guessing at and buy a scanner instead... THEN you can actually buy the RIGHT part the first time and the time savings ALONE will make it worth while. it makes not a single whit of sense to shotgun a problem like this when you can actually know the answer for real this time and every OTHER time something happens.

I hear you, but a scan tool costs like $400 and I have to wait until the weekend to trouble shoot.
I'm going to stop buy Ocean Performance after work and spend $110 for an IAC and ECT. I should be able to change those 2 parts in about 1/2 hour.
If changing those parts does not work then maybe I'll invest in the scan tool.

i got mine on ebay for less than you are about to spend.

After dealing with this maddening problem for the better part of two years, I finally got off my butt and ordered a new IAC motor and TPS from BAM. Removed and cleaned the throttle body, installed the new parts and now the GD surging is gone. I have a 2007 GM 502 but the intake, etc. is from my 2000. If I were you I'd just order the IAC motor, clean the tb real good and install the new motor (2 screws, very simple) and I'll bet you'll be good to go.

Rock

I picked up an IAC and ECT tonight.
I'll let you all know how things turn out.
Thanks for the input!

Well guys I finally got out on the water to test after replacing the IAC and ECT.
She ran perfectly and idles at 600rpm in or out of gear.
Thanks for the help.