Mercruiser 454 MAG MPI weak!! help!
 

I just recently bought a 99 Baja outlaw with a 454 mag mpi. The exhaust manifold leaked some water into one of the cylinders and did some pretty good damage. The engine needed to be rebuilt or replaced. I ran across a 2001 used mercruiser 454 mag mpi that came out of a Caravell interceptor and ended up buying it. The motor hadn't been run for 2 years. The old engine ran 68 MPH at around 4800 rpm with a 23 pitch 4 blade prop. After swapping the engines the boat will only run 48 MPH and only turn 4150 rpm..?? Engine will rev up fine and sounds fine. But even coming up on plane it is more sluggish than the old motor. On top of that i can move the throttle back from wide open about 3 inches before it reacts. Could it possibly be something as simple as an adjustment on the throttle cable? Or should i be looking more in the direction of a clogged injector..? The interceptor is the same size as my outlaw and the guy said his boat ran 68 and would also turn around 4800 rpm. From what i've read the engines hp are the same from 99-01. If anyone has any ideas please let me know! Your help is greatly appreciated!!

easy enough to check the throttle linkage. I would check the timing did you give it a full tune up?

A friend of mine with the same symptoms was having issues with his knock sensor.

l would start with the basics.pull the spark plugs and check them, compression check, spark check, firing order. dump the water sep filter make sure theres no water or alcohol separation. make sure everything is plugged in on the engine were its supposed to be

I had a local shop do the swap. They did do a tune up. I'll start with the linkage and timing. ....after I see if it has any codes I guess. Thanks I'll let y'all know what I find

You are most likely not going to find any codes, the self diagnostics of the early merc ECU's is not very good. I think these guys have you on the right track, however you have to check all the basics first.

Not enough fuel or too much timing will cause det that your knock sensor will see, weak spark can also cause issues. The computer will then turn the timing back and you will lose power. If you have a scan tool that can read timing and knock sensor activity this will let you know what is going on. You can also leave the hatch popped and have a buddy read the timing while under load. Anything over 3000 rpm and your engine should be running at total timing, which I think is 35 degrees for your engine. This test usually only works under load, when the engine is more prone to detonation. Be sure to set the base timing before doing the test. Jump pin a to b on your diag connector and set the timing at 8 degrees 1800 rpm.


As mentioned above a faulty knock sensor is a real possibility, they are relatively fragile and exposed when the engine is not mounted in a boat. But test, don't guess :D

any new updates. i have a similar prob

I believe he had a 454 MP I and not the mag. He is rebuilding the mag..