MAJOR issue with mercruiser mag mpi

"Have I got a mystery for ever

"Have I got a mystery for everyone! I've got a 350 Mercruiser mag mpi motor here (2000 era) that is missing about 80 to 100HP. The motor has recently been rebuilt and runs great except for the lack of power.
Here is some history: Its in a 22' walkaround coupled to a bravo III drive that used to get 4600 rpm's. Last year it underwent surgery (complete rebuild) after losing the stbd manifold and ingesting water. It was rebuilt by a local motor service center, with a good rep might I add. Upon reinstallation and fine tuning everything seemed well until water trials, where the lack of power was discovered. Max rpm was 3700. The fuel system was drained and cleaned, new fuel pump and all new injectors and rails. New distributer cap also. Now rpm's are down to 3250. The engine has 6 hours on it now. Compression check shows 178 to 184psi +/-2. Spark plugs had not been gapped and varied from .022 to .010 but are now set at .045. Plugs showed no sign of fuel starvation.
Bottom line, the motor runs like a top. Starts right up, idles beautifully, revs smooth (in neutral). In gear idles great, doesnt stall spit or sputter, and increasing the throttle is smooth but lacks power. The boat barely planes anymore and struggles just to get 3200rpms. Advancing and retarding the timing (mechanically) has some negative effect (expected). This boat is trailer kept so has no bottom growth.
I can't figure out why a motor that runs so well could have such a major malfunction unless it has the wrong heads or cams. The rebuilder denies any rebuild problems and backs the engine against mechanical failure but insists that the problem is with the computer or fuel. ANY IDEAS?!?!?"

"I`d want to know exactly what

"I`d want to know exactly what was rebuilt and what was replaced during the overhaul."IF" they are the ones that put the spark plugs in without gapping them, makes you wonder about the other work.
It does seem odd that some other problem crops up at the same exact time that you rebuild
motor. Mind you, stranger things have happened.
Sounds like it`s time for a diagnostic check."

"There are places that have Ma

"There are places that have Marine Engien Dyno's that can be used on the boat, i agree with barry would question the heads, cam, cam timing(off by 1 or more teeth will cause the lower power). One of the easy things to check is fuel pressure underload at WOT, also if the injectors were incorrect or updated and the ECM wasn't, that could also cause the low power."

"Thanks for the imput guys. A

"Thanks for the imput guys. Actually the plugs were put in by the shop that "tuned" the motor, "updated" the computer, and had the fuel system replaced. Unfortunately they are the only local Mercruiser shop and will NOT be receiving any more work from here. Is there a place that I could send in the computer or does it have to be monitored on the engine? Another idea would be is there a way to check the serial numbers on the block and heads to see if the engine rebuilder just pawned off an old 350? Possibly not a marine engine? Unfortunately the remanufacturer only listed "new pistons, rings, bored .030" over, machined heads, 350 mag rebuild". They didnt give part numbers or part manufacturer. Really kind of upsetting since they have always done first rate work for me till this so I'd rather think that it was an ECM problem. Would a computer problem really cause such a huge loss in power and still have the motor running so smooth? Thanks again!"

"the head numbers are inside w

"the head numbers are inside where the rockers are but the engine serial should be on a flat pad about 1"x1.5" infront of the RH head if the block wasn't "decked" or milled, all it will eventually tell you is if it's a 2 bolt or 4 bolt main bearing cap, the auto/marine differnce then is steel or brass freeze plugs."

Rev limiter?
Cam was changed?

Rev limiter?
Cam was changed?
Cam timing way off

"Ayuh,... Did the Shutters ge

"Ayuh,... Did the Shutters get Burnt,+ drop into the Y-pipe,..??"

"Bondo:

Good answer oh &#34

"Bondo:

Good answer oh "mighty wizard" of problem solving. Just kiddin'. "

"Definately could be an ECM pr

"Definately could be an ECM problem or the injectors. The MEFI1 thru to MEFI4 are not programable as such, tune ups are not possible unless the complete software is erased and new mapping and software is installed. Only one place I know of that does this, check out mefiburn.com. The efi system on Mercrusiers is open loop, no O2 sensor, so the computer never knows if the engine is getting enough fuel. That is why the injector sizes have to be correct and the fuel pump pressure is critical. Injector sizes have to match the ECU.
If you want to work on an EFI system, you MUST HAVE a scan tool. All faults are stored in the ECU memory and will put the ECU into the reduced power mode which reduces the pulse width to the injectors. With the scan tool, you have to clear the stored faults and then reset the ECU to full output."

"Thanks for the replys everyon

"Thanks for the replys everyone. Chris, have you any experience with the mefiburn.com system? I checked that out and it looks like a real nice set up. I haven't been able to get back to the boat recently so no update there. I'll probably be pulling the motor (again), tearing it apart and checking everything that has been brought up here. I'm just stumped with it. All I can figure is that the reman outfit really got one over on it, or hopefully just accidently gave up an automotive 350. I've done business with them for a while and never had such a problem. Thanks again for the help, hopefully something will show up."

"I wouldn't be tearing the

"I wouldn't be tearing the motor apart again just yet. If you have good compression and good oil pressure, mechanically, the motor will be OK.

Check the fuel pressure under load, should be about 40 psi (check service manual for correct psi) the back pressure controller could be stuck open. There is a procedure in the service manual to correctly service the pressure controller.

You will need a scan tool, Part #94050m, from http://www.rinda.com/marine/marine.htm .

MUST HAVE, to set ignition timing, ECU MUST be in the service mode and this can be only be done with the scan tool. You will need the service manual so as to complete the correct order of dianogistics on the ECU.

The new injectors MUST HAVE the same part numbers as the old ones that were removed. Calibrated to the ECU.

Simple faults, like a faulty switch or low oil in the out drive oil bottle will put the ECU into the reduced power mode.

Knock sensors incorrectly installed is a common problem after re-builds."

"good afternoon... new here wh

"good afternoon... new here while looking for an answer to something else and tripped over this thread. be advised that if the builders used the merc scan tool to set the timing, that may be your issue. in order to set the timing you have to use the scan tool and select " service mode"... then you set the timing. if your guys then forget to get OUT of service mode, the motor will remain there and will perform exactly as you describe."