26sonic

i have a 1998 502efi and noticed since i had it from brand new that if when i'm docking i need to to hit the throttle on and off real quick that it stalls ? this happens in forward or reverse . the idle is fine in gear or neautral . any ideas?

Dennis Moore

In my humble opinion (IMHO) I think some of the Mercruiser EFI engines needed a little larger hole inside the throttle butterfly for larger boats. A large boat or a high pitch prop drops the idle speed dramatically when shifting. The idle air control valve can't respond quick enough to keep the engine from stalling in some situations (the newer 496/8.1L engines have a different system for idle control that responds faster).

A larger hole will help to supply more air to the engine at idle with less dependency on IAC movement speed.

The best procedure is to install a scan tool and observe the IAC count when shifting in the water. If it is above 40 at an idle (neutral) you will need to increase the hole size.

The engine should be getting enough air through the hole in the butterfly so that it idles at 800 rpm in neutral, at zero IAC count, with approximately 8 degrees of spark advance. This will allow the IAC to open and the timing to advance as soon as the engine is shifted. This will help maintain the correct idle rpm and keep the engine from stalling around the docks.

The size of the hole will vary on certain Mercruiser engines. For example; the Blackhawks had high pitch dual props that needed a larger hole in the EFI throttle butterfly to idle smoothly when shifting, so do the Bravo III dual prop drive EFI engines.

This is one of the difficulties of building an EFI engine and having no idea if the engine is going to end up in a 40 foot boat or a 21 foot boat. You've got to hand it to Mercruiser, these engines are pretty flexible as they come from the factory!

Sincerely
Dennis Moore
Mercruiser Master Technician

26sonic

dennis on this 1998 502efi engine i installed a az speed and marine air cleaner (k+n) , ecu reprog., msd, ignit, , adj fuel reg., and added after market exhaust. i have 26 ft sonic with 24p bravo 1 with bravo 1 drive.

Dennis Moore

Good for you!
Doesn't matter, the IAC, the air hole in the butterfly, and the timing advance still control the idle speed. The marine air cleaner, the fuel regulator, the MSD and the exhaust system don't have anything to do with your idle speed. The reprogramed ECM still adjusts the IAC count and timing advance for idle rpm just like it did before it was reprogramed. This info will still show up on a scan tool.

Call up ASM and ask them what your IAC count is at an idle, they won't know because every boat is different when the engine is idling in gear, depending on the load on the prop and weight of the boat.

Good luck with your problem.
Sincerely
Dennis Moore

26sonic

dennis if the hole is to small in the butterfly how would you inlarge it and to what size ?

26sonic

ttt

Shooter

I know this might sound stupidly simplistic, but I had a suspected IAC problem with high idle. I hit it all over and all passages with carb cleaner and fixed it.

Thunderstruck

Does the motor surge to 1500 rpm or so when you restart it and then settle down after 5 seconds or so? If so you do not have enough air at idle. The IAC tries to count up and open the butterfly but it maxes out without being able to correct the problem. The max IAC setting causes the rpm at idle to be high when you restart the motor until it can reset the IAC (based on rpm) back down until the motor idles at the correct speed. The above post is correct regarding the bigger hole in the butterfly but my old 1994 502 EFI had an adjustment screw that I was told to adjust to open the butterfly slightly that I would try before drilling holes.

BTW I had the same problem with an ASM ECM and so did two other friends with various setups from ASM. NONE of us ever got them fixed.

Breathe Later

That adjustment must be made to the screw for the secondary butterfly.