I have a mercruiser 502 serial im guessing is OK173702. It is in a 98 powerquest. Cold it runs like a champ, good oil pressure, no surging. After the engine is up to operating temp, guage shows 160 and I come into a no wake zone the engine will hunt. I usually try to pump it slightly off throttle just for sake of thinking its gonna stall. It isnt enough to make the tach fluctuate, but you can definelty hear the engine varying in rpm. If the no wake zone takes awhile to get through the engine almost seems loaded up, trying to take off it feels flooded, can tell its not responsive and sounds loaded. Usually giving it more throttle or coming off the throttle and then back on will clear it up within 5-10 seconds but doesnt seem right. I have noticed alot of black on the transom and under swim platform after every use. Any ideas what to look at, or being fuel injected am i stuck to having it scanned?

As with just abt any electronically controlled fuel injected eng, the scanner is the way to go. Having said that, no harm in looking for obvious items like unplugged sensors or damaged wiring. Another thing to look for would be a cracked vacuum hose for the MAP sensor if so equipped.

you say its a 502 ? is it a stock mpi ? if so my guess would the IAC and yes a scanner is your friend

Yes stock 502, hasnt been touched. Ive only put about 4 hours on it after purchasing so I dont know if it was a previous issue or not. Unfortunately all the shops around here are booked a month out for scanning :/

Fuel regulator is on cool fuel module hanging off the left side of the engine down by the oil pan. There should be a vacumm line connected to it with manifold vac. Dont remember exactly where it connects to the intake but verify you have good vac supply to the regulator. Checking your fuel pressure would be next logical step to me.

How old is the fuel?

Thanks for the SN, cool fuel MPI and MEFI 1
Pressure should be 43 psi at the rail. Schrader valve located under the plenum just behind the thermostat

I concur, double check the vacuum hoses. One of mine was plugged with gunk.

IAC motor is relatively inexpensive to try out.

Might be time to have the fuel injectors cleaned.

Idle hunting, from what I’ve seen, is usually timing or sensor related unless you have a bad vacuum leak. To diagnose it correctly, you need a Merc Techmate or Rinda diagnostic terminal. The IAC “may” cause some hunting if it’s stuck, but they really don’t go bad.

Things to try...

Pull the throttle body and IAC, and clean the chamber that the plunger sits in. If it’s full of gunk, vacuum can get goofy and throw everything off.

Put the system in timing/service mode and verify timing - 8 degrees BTDC if it’s off, it will surge (you can do with a paperclip at the diagnostic connector... sorry... you’ll need to look-up).

Fuel pressure usually makes it run soggy until
upper RPM. If you’re running around with it, more than likely it’s all good

If doing the above doesn’t net anything, you really need the diagnostic tool to see what the ECM is seeing.

Fresh, Ive put fuel in it 3 times this spring already