Gday there.
i'm an aussie fella new to this forum but am seeking some good advice.
I have a mercruiser 454 mpi with a mefi1 ecm.
I recently purchased a new gm454 long motor and swapped all my old bits over.
Problem occurs between 2000 and 3000 revs...misses badly, tacho goes berzerk..then over 3000 (eventually) seems to be fine.
Had a scan tool on it and read error codes 33 (map sensor) and 42 (ecm). managed somehow to clear 33 (but not 42).
With 33 clear seemed to run ok through rev range however next day was back to bad state again.
removed map sensor wires and reconnected and seemed to work ok again for a while.
When i say ok i mean it got up on the plane and boogied along ok but at about 1500 revs could still hear slight miss from engine (popping sound).
Things to note:
Firstly- this problem was also evident on my old engine, so i doubt its in the engine itself.
Also- when i had old engine on scan tool, i noticed the distributor advanced to about 40 when the missing was evident (2000-3000rpm) and then dropped back to about 25-30 (over 3000rpm). According to the manual this is over thetop and so i replaced the distributor..wishful thinking..
Any ideas anyone??

If your ecm is coding out find another to try. If your timing was that sporadic it has to be that. If it was that high your knock sensor was probably pulling out timing causing more problems.

You may also have some play in the dist shaft causing erratic signal, also ck for oil leak into the cap area.

Code 42 is not ECM, it's IC (Ignition Control). The fact that you were unable to clear it means that is an active code. You may have a bad connection at the Ignition Module or ECM, a bad module, or a shorted or open circuit in the IC circuit.

To troubleshoot this condition you will want to refer to a factory service manual.

Dave

Thanks guys.
Trying to look for someone local who can redo my ecm.
I thought the knock sensor retards dist not advances??
My ignition control module would have come with the new distributor so am assuming it's ok. probably best not to assume.
Still trying to sort the prob.

I just replaced a 502 and I'm having the same problem. I was told by a reliable source to replace the entire dist and the problem would likely be fixed. Apparently they tend to de-magnetize. Frustrating as hell........runs fine in idle and in neutral, but when trying to plane out, same issue.......2800-3000 rpms and craps out. I'm changing the dist tonight, I'll let you know if it helps.