I have a 1999 Formula 312 fast tech with 2 454 mercruiser engines. One engine is giving me problems and I would like to know if someone has encountered this problem before. One engine under load goes into a cutting on and off sequence. I have changed rotor and cap module plugs and checked alot of other things. Nothing seems to be bad and the items that I changed did not help. Now again this only happens under load past 2000rpm. Can someone assist me on what to do next? Thanks.

I had that happen on one of my 525sc's. It ended up being the coil. Try changing the coils around and see if the other motor skips.

Do you have the ability to scan the ECU for codes? Did you chnage the fuel filter?

The above is all correct Im guessing that they are mpi motors if the above checks out ok also check the iac ( idle air control if that is bad it will seem to be doing what you are saying ) # one to check as mentioned above is the fuel filter and fuel press...and then if it has a vst (vapor sep ) check the float and needle and seat ...also you didnt mention if the cond of the plugs and if running rich lean ect....

Anything off of idle, the IAC has no effect even if it's defective.

oh ok sorry guess im wrong that if its stuck open its not allowing extra air in, im sorry guess im wrong and that wont effect anyting out of idle or that if its bad the motor wont go into limp home/ guiardian mode sorry im wrong



oh ps call merc tech and tell them also because they dont relize that

i just had the same problem so i changed the water seperator & then the fuel pump.
it was the fuel pump

sorry didnt mean to get pissy but just because something when working properly only effects idle doesnt mean that when its not working properly it doesnt effect other things ie the iac. been doing this a longggg time and ill be the first to admit that im not always right but ..........


anyone who says they are always right I wouldnt trust anyway !!!

this is from the merc tech manual


I know I know the iac does nothing out of idle

I had the same systems on my 95 454 merc. It turned out to be the coil.I didn't find it till after I changed everything but the stringers the engine was mounted to.I just happen to be working on it while it was getting dark and had the engine running.I noticed and heard a zapping sound and saw the sparks coming out of the coil tower and shooting to the wiring system.It was a pain the the transom because it would run good for about 10 minutes till the coil got hot and then start breaking up under load and then would die.Hope this helps.

Anthony