New engine will not throttle up 350 mag mpi

[joefran]

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.
8 All done under no load just throttle.

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

 

[bondo]

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,.....

 

[joefran]

Well fuel rail pressure is steady 40 psi and if fuel was the issue, it would not throttle up with TPS disconnected

 

[kghost]

Are you testing on a trailer or in the water under load?

 

[joefran]

In water inside boathouse no load

 

[kghost]

Have you shifted into gear and tried to throttle up?

 

[kghost]

There have been similar posts here and for EFI and Alpha drive, there was mention that if the ECU does NOT think its in gear, It wont throttle up past ~2500 rpm.
EFI is not my area but just passing on what I have read in the past

 

[joefran]

Think I could manually hold the shift switch and try throttle up? Don't want to put stress on the boat cradle it is just up enough to hold the boat in place. I never remember the engine not throttling up before when disengaging the shifter. I think protection should not kick in until after 2500 rpm

 

[kghost]

One observasion regarding compression.

90 psi is a failed engine/cylinder.

A Fresh 350 should be around 175 psi

Your about half...Not good.

Also are you 100% firing order is correct?

 

[joefran]

All cylinders are at 90 psi and the engine would not run smooth at over 3000 rpm with the TPS disconnected if firing order is off. Has to be electrical with relationship between sensors and ECM

 

[AllDodge]

Unless someone "really" screwed up a fresh rebuild compression would be higher than 90 psi. I would suspect there is a problem with the test gauge or how compression testing is being done

As for the problem Reving above 2000 rpm as kghost mentioned, it could be the in-gear switch. You should most likely have a Bravo drive and therefore it will not have a shift switch. If you do have a Alpha and a shift switch you may also have a in-gear switch.

Disconnect the switch and see if this fixes the issue