So after losing my first engine with 700 hrs because of 2 bad injectors I install a rebuilt 350 mag MPI long block. With all original peripherals from the old engine.

Original Engine Serial Number M350MPIB3 W042371


On first start up the engine idols perfectly. But when trying to throttle up anything over 1500 to 2000 rpms the runs rough surging drastically.

So first had mobile marine mechanic out with mercruiser computer he had no answer to the problem.

So another mechanic came out and suggested to disconnect the TPS and wow the engine throttled up with no hesitation. I replaced and check voltages at the new TPS and checked out fine. Yet the engine still will not throttle up.


Things that have been completed


1 New cap and rotor

2 New oil pressure switch

3 Plug wires tested and verified firing order

4 New Plugs

5 Monitored fuel pressure 40PSI

6) All injectors tested

7 Checked compression all 90 psi low but engine has only run for maybe 1 hr so I hope break-in will improve compression.



Questions

1 Thinking it is electrical problem

2 The TPS must be seen by the ECM or it would not run better when disconnected

3 Could it be a bad map sensor? I tried disconnection map sensor and it started but immediately stalled

4 Could it be a bad ECM?

5 Does the TPS need some sort of calibration for ECM to see it properly like in some cars?

6 Could a rebuild be far enough off where the original ECM can't run it properly?


Thank for any Ideas

Joe

if its a flat cap dist try turning it one way or another i use a cut open cap while running to get it lined up

This is concerning !!!!! Get another psi tester to confirm !!!

Yes I am also concerned original engine was 180psi with same gauge. But I think the low compression would affect the the max power output but not the rough running condition. Since when TPS is disconnected it throttles up smoothly.

New TPS?
Tip: if you can find it Amazon or Walmart (online, many vendors)...
...you can return it if needed.

Ayuh,...... Welcome Aboard,...... Lack of fuel killed the 1st motor, so I'd start there,.....

Start at the fuel tank, through the dip-tube, anti-siphon valve, the fuel lines, fuel filter, all the way to the injectors,.....

Yes already replaced TPS and confirmed it working correctly. Checked output voltage .6 at closed throttle and 4.75 at wide-open throttle

Modern engine 'break-in' is not much of a thing anymore.

The exception being FT cams.

Ring seating is done very quickly if everything setup correctly...bonus if you gey a bore plateau hone.

Modern rings are very smooth...modern bores are very smooth...they use cross hatch in the bores to lube.

Ring/bore break-in is now more of a cleaning action. Micro-burrs coming off and getting cleaned away etc.A lot of this can be avoided during bore cleaning and setup. Green Scrubby bore finish etc

As SB mentioned...90 psi is not just low...you don't have an engine. Do you increase PSI if you put oil in the chambers with a squirt lube gun? Maybe you got fuel washdown and messed up the rings?

Cam timing off= low compression

Any way to check without tearing front end down?