454 Mag MPI
 

I have a 454 Mag MPI; bought it used with 68 hours. I put approximately 20 hours on engine. Ever since the first time, engine has had difficulty starting, rough ideling, and surging. I thought it was just bad gas, plugs and wires, but after fresh fuel and new plugs and wires, still having above problems. This weekend, I went to start the boat, and the audio warning system went off even before starting the engine, and the engine would not start, and when it started was surging, ideling rough and stalled. There was no problem with the oil pressure, temp or drive oil. I thought it could possibly be the computer, but without diagnostic tool, not sure. Other possibility could be the fuel injectors. Does anyone have suggestions or thoughts. Engine is getting serviced this week.

Re: 454 Mag MPI
 

check the VST if equipped you didnt state a year..... rough Idel and surgging could be one of many things on a mpi from iac to tps but if equiped with a VST system and without looking at sensor values Id start there

Re: 454 Mag MPI
 

Joe is right depending on the year. You may also have a bad IAC motor or a vaccum leak.. Jamie / Lakeside

Re: 454 Mag MPI
 

The engine is a 1999. We had checked for vaccum leaks and there does not appear to be any. What is VST? Thanks for the input

Re: 454 Mag MPI
 

VST system it an efi hybrid. It has a fuel resivoir like almost like a carb & 1 low psi & 1 high psi pump. the 99 I donot belive has this. I believe it was the earlier ones. I would think more IAC or TPS may need checking or even a map/baro sensor... Jamie / Lakeside :)

Re: 454 Mag MPI
 

here is a flow chart of what to check so your not wasting alot of time


I think donz133 my be right about the iac with out a scan tool its a shot in the dark and will take alot longer but follow the chart and it will take you in the right direction


there is more charts to follow when you get done here or have other symtoms

and yes they stopped using the vst in 1998

Re: 454 Mag MPI
 

Thanks guys! Appreciate the input. I have the service manual for the 454 Mag MPI. We are going through the flow charts for trouble shooting the problem. I let you know what we find.

Re: 454 Mag MPI
 

Diagnostic test showed that the intake air temperature sensor is faulty. We ordered a new sensor. Hopefully, that should resolve the symptoms. Thanks for your help.