Aasved

I have twin EFI 502's 1997. One one them started having trouble idleing and when I start to increase the throttle it starts bouncing up and down substantially. It also has trouble starting unless I give it some throttle. It will run at high rpm, howeever when I let off the throttle it wants to die. Note: all this starts happening after it gets warm. It sounds and runs perfect when first started.
I took it to a local marine shop (and they said there was nothing on the computer but a old map sensor code. I swaped the sensor witht the other engine before I took it to them so I doubt it's that, But aflter waiting two weeks to get the test done I replaced the map sensor just to eliminate that.
Here are some ohter things I have checked.
Distributer cap, plug wires, Looked great!
All grounds on throughout both motors.
checked Fuel high preasure. Seems good, but dont know the preasure it's supposed to be at.

Doubt it's a gas issue as the other engine runs great.

Without any codes I dont know where to go from here without throwing money at parts. Does anyone have any suggestions.

Thanks in advance!

jeffswav

Sound like the IAC, (idle air control) its a stepper motor that controls the idle. It has no feedback so it would not throw a code.

articfriends

iTrader: (1)

I second the IAC, swap them from side to side and see if the problem follows, you won't spend a dime then until you are sure!!!

Griswald

and, you can get a replacement IAC at Autozone for about $25. Pretty much the standard part for most GM cars in the 90s. Part # AL25?

Aasved

Thanks I'll try that today and see if that works.

Chico&Zeus

Just replaced mine on the starboard motor, same issues... after warm up, surging idle, stalls if rpm's are kept too low, etc. Got the part from Checker (O'Reilly) it was MasterPro part # 2IAC2 and it was $26 which is about 1/4 of what the Merc part costs. This is all on a '99 502 MPI Mag.

It took about 10 minutes to pull the air cleaner, unbolt the throttle body, remove the IAC Motor, clean the throttle body, reinstall the new IAC motor and reinstall the throttle body and air cleaner.

Took it out this weekend and problem is fixed. Under $30 and hardly any time wrenching is my kind of boat repair!!!

kawzracn442

I just fixed this problem on mine. 2000 502 MPI, mine has a sidemount Whipple supercharger so the symptoms were quite dramatic. I checked and replaced everything that you are all talking about. Finally I removed the upper plenum and replaced the gaskets on everything on the intake side of the engine. Problem solved. I had also sprayed massive quantities of carb cleaner all over everything looking for a vacuum leak to no avail. Hope this helps.

30ctsutphen

if you get it fixed post up a reply son others have a definate answer

Aasved

A little update. I actually tore into the boat a little more since the marine repair shop I was dealing with was pretty worthless. So my buddy and I fired it up and see there is water leaking form the head. So long story short..have two fresh 502 long blocks waiting at the machine shop to be picked up. One would think that the mechanic I originally took it to would have see water spewing out of the head. But who am I to criticize? I didn't see it either.