5.0 mpi mercruiser

[brucegregory]

I have a 5.0 mpi mercruiser with a bravo 3 drive. we pulled the motor to get the transom fixed .while the motor was out we installed new water pump manifolds and we ran motor on the ground it ran fine. now the motor is back in the boat it is being stupid. turning the key on gies the motor 12volts but the fuel pump doesnt come on until 15 seconds later and the horn also comes on. the fuel pump never cycles off. when trying to start the motor it acts like the timing is off and if it does start it runs bad, i think the ecm failed its the 555 ecm. the computer wont connect to the ecm

 

[alldodge]

Welcome
What is the motor serial number?

turning the key on gies the motor 12volts but the fuel pump doesnt come on until 15 seconds later and the horn also comes on. the fuel pump never cycles off

Clarify this statement

Turn key ON and the pump does not turn on?
Does this mean it doesn't turn on for a couple seconds and then turn off, or does not turn on at all?

Does the horn turn on when key is first turned on or after 15 seconds?

Once fuel pump does turns ON it does not turn OFF? Meaning it keeps running until the key is turned OFF even if motor is not running?

 

[brucegregory]

serial number om530789 key on no pump or horn for 15 seconds then the pump and horn come on and never cycle off. the pump and horn only shut off when key is turned off.it will kind of start but runs bad, 40psi fuel pressure i have checked grounds and of course power i have a hot battery and i have a old dash panel for testing and i plugged it in and that key swith does the same thing

 

[alldodge]

Your motor
https://www.mercruiserparts.com/5-0l-mpi-alpha-bravo-0m300000-thru-0m5

The horn is most likely coming on because the ECM sees a problem. Not hearing the 2 beeps means the ECM didn't pass power up test

Guessing it runs bad because the ECM can not read it's sensors. First look at 5V reference.

The 5V can be measured on the MAP, Crank PS and TPS. Pic also shows Cam PS but your motor may not have it. If voltage is not right at 5V then this could be the issue. Motor does not need to be running but can be

 

[QBhoy]

id start simple. given the intrusive nature of the removal and back in again...it's quite likely that you have a loose connection somewhere. I would start with the battery (you may have already), then any battery isolator or switch. then the ignition feed wire and around the main relays on top. The most common cause for the fuel pump to behave weirdly or intermittently would probably be to do with the kill switch/neutral safety switch wiring network perhaps. have a look around things like this, I would advise. buy yourself a vessel view mobile for $200 and you'll get lots of data and any faults she throws up. well worth the money.

 

[brucegregory]

id start simple. given the intrusive nature of the removal and back in again...it's quite likely that you have a loose connection somewhere. I would start with the battery (you may have already), then any battery isolator or switch. then the ignition feed wire and around the main relays on top. The most common cause for the fuel pump to behave weirdly or intermittently would probably be to do with the kill switch/neutral safety switch wiring network perhaps. have a look around things like this, I would advise. buy yourself a vessel view mobile for $200 and you'll get lots of data and any faults she throws up. well worth the money.

i have a laptop and it wont connect to check for codes. i find that odd. the next thing i will try is to see if the ecm has power and a ground. since it delays 15 seconds before the horn and fuel pump come on i think it is a ground problem but the ground is connected to the back stud on the bell hsg.i tried a set of battery cables just to make sure and a different battery. what makes no since it ran fine before motor was removed and it ran fine on the ground, the problem is the boat has the motor way down in the hole and it is really tough to get to anything. the generator in front of the motor dont help. on one side is hot water heater and batteries and stbd side has gen. batt. and sewage holding tank. i am in no way wanting to remove the motor

 

[QBhoy]

i have a laptop and it wont connect to check for codes. i find that odd. the next thing i will try is to see if the ecm has power and a ground. since it delays 15 seconds before the horn and fuel pump come on i think it is a ground problem but the ground is connected to the back stud on the bell hsg.i tried a set of battery cables just to make sure and a different battery. what makes no since it ran fine before motor was removed and it ran fine on the ground, the problem is the boat has the motor way down in the hole and it is really tough to get to anything. the generator in front of the motor dont help. on one side is hot water heater and batteries and stbd side has gen. batt. and sewage holding tank. i am in no way wanting to remove the motor

Mmm. After what you’ve said there. Could be perhaps an ignition barrel related thing. But I also know that if the safety kill switch or neutral safety switch is activated, you will get no fuel pump. Is it possible that you may have a poor or shorting wire in this particular system ? I’d strongly lean towards one of the above to rule out first. If not…got to be something with poor continuity or contact somewhere