454 MAG Hard Start When Warm

[Mrahe]

I have a 1998 Falcon 2500BR with a Mercruiser 454 MAG EFI. Since I got it a year ago it has always ran a little rich (black soot on back of boat and slight smoke at times). Decided to install new spark plugs and used NGK BPR6FS plugs. Took it to the lake for the weekend and ran for about 5-6 hours with no issues.
Since I got the boat the temp gauge never worked (always said as cold as it could read) so decided to change the temperature sender for the gauge(did nothing with the one going to the ECU) and while I was doing it decided to install and new 160 degree thermostat. When I went to install the thermostat I found there was not one in the housing so had been removed by the previous owner.
Now the boat starts and runs great cold but when it gets to about 150-160 degrees it starts to miss and sounds terrible. Also, when warm it is very hard to start- cranks for ever and then when it does fire sometimes doesn't stay running long and sounds terrible like running on half the cylinders.

Anyone ran into this and have any suggestions what to check??
TIA

 

[alldodge]

What's the serial number?
Start with basics, compression test, check for spark on all cylinders

What's timing set at and was is set using base timing mode?

What is the fuel pressure?

Have you checked for codes?

 

[Mrahe]

What's the serial number?
Start with basics, compression test, check for spark on all cylinders

What's timing set at and was is set using base timing mode?

What is the fuel pressure?

Have you checked for codes?

SN: 0K192243
Have not checked spark or compression when warm but everything is great cold, runs fine and plugs looked perfect before changing the thermostat so that it warms up now.
No idea on timing setting or how to find this out.
Did not check fuel pressure because cold it runs great but can check into that.
Have not been able to check codes yet.
How does the Merceuiser EFI system work?- does the system read the MAP sensor at all times or just when warm? Any other sensors I could look at and get ohm readings on?

 

[alldodge]

Your serial number shows, cool fuel, and EST Dizzy, Gen VI and MPI

Need to find out if the issue is fuel or spark. You will need to test to find out.

On the front of the fuel rail under the plenum there is a cap over a Schrader valve. Remove cap and connect fuel gauge. Turn ignition ON and you should hear the pump run for a couple seconds. Might need to bleed air out of the gauge, but see what the pressure is by cycling on/off a few times.

Should see around 40+ psi

All sensors inter act with the ECM. When cold it gets more fuel, when hot less. Running rich most the time prior this can be a fuel pressure, manifold air pressure (MAP), engine coolant temp (ECT) sensors.

To read codes you need a 12V resistor LED or a voltmeter with a tone setting, and a paper clip. Connect a jumper (paper clip or other wire) across pins A and B, and connect LED or meter across E and F. Turn key ON, and watch for flashes or tones.

A flash, short pause, flash, flash is 12 and all good. Flashes/tones will repeat 3 times and move on to next code if there is one

 

[Mrahe]

Actually just got done testing codes this way. I immediately get 12 and that is all it repeats so appears no codes. I will get a fuel gauge from the shop tomorrow and test fuel pressure next.

Thanks

 

[Mrahe]

Your serial number shows, cool fuel, and EST Dizzy, Gen VI and MPI

Need to find out if the issue is fuel or spark. You will need to test to find out.

On the front of the fuel rail under the plenum there is a cap over a Schrader valve. Remove cap and connect fuel gauge. Turn ignition ON and you should hear the pump run for a couple seconds. Might need to bleed air out of the gauge, but see what the pressure is by cycling on/off a few times.

Should see around 40+ psi

All sensors inter act with the ECM. When cold it gets more fuel, when hot less. Running rich most the time prior this can be a fuel pressure, manifold air pressure (MAP), engine coolant temp (ECT) sensors.

To read codes you need a 12V resistor LED or a voltmeter with a tone setting, and a paper clip. Connect a jumper (paper clip or other wire) across pins A and B, and connect LED or meter across E and F. Turn key ON, and watch for flashes or tones.

A flash, short pause, flash, flash is 12 and all good. Flashes/tones will repeat 3 times and move on to next code if there is one


View attachment 367973

Finally got time to revist this. Looks like fuel pressure is right around 39-40 psi. I did find that when I installed the new thermostat it looks like this small line that tees off of the line going to the fuel pressure regulator came off- would this possibly cause the issue I was seeing?

 

[alldodge]

That would cause it to use more fuel due to keeping the fuel pressure at max even at idle. The port on the Plenum has a very small hole in it. So it should not effect starting when warm so much

Start motor and place in base timing mode. To do this need to put the jumper between pins A and B on the DLC, as when checking for codes. Don't install the LED. Connect timing light to number 1 plug wire (port side 1st plug)
When in base timing mode and at idle,
Timing 8* BTDC
Plug gap .040

Check the distributor carb and rotor inside it for corrosion

 

[QBhoy]

I have a 1998 Falcon 2500BR with a Mercruiser 454 MAG EFI. Since I got it a year ago it has always ran a little rich (black soot on back of boat and slight smoke at times). Decided to install new spark plugs and used NGK BPR6FS plugs. Took it to the lake for the weekend and ran for about 5-6 hours with no issues.
Since I got the boat the temp gauge never worked (always said as cold as it could read) so decided to change the temperature sender for the gauge(did nothing with the one going to the ECU) and while I was doing it decided to install and new 160 degree thermostat. When I went to install the thermostat I found there was not one in the housing so had been removed by the previous owner.
Now the boat starts and runs great cold but when it gets to about 150-160 degrees it starts to miss and sounds terrible. Also, when warm it is very hard to start- cranks for ever and then when it does fire sometimes doesn't stay running long and sounds terrible like running on half the cylinders.

Anyone ran into this and have any suggestions what to check??
TIA

Likely not too much difference, but should your plugs not be bpr6efs ?

 

[alldodge]

Likely not too much difference, but should your plugs not be bpr6efs ?

Agree long reach plug same as MR43LTS, and gap is .045