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Merc 5.0lt issue PLEASE HELP

B

Browneee

New member
Hi there, I’m new here, I’m writing from down under in Australia...

I have a Mercruiser 5.0 thunder bolt 5 ignition serial number 0L046028 in my Bayliner Capri circa 1998/99.

please bare with me as I try give you ALL the info I have to give regards my issue.

Boat will crank, fire and run smoothly. It will run for awhile and then stall/stop...

originally I thought fuel issue (contaminated), I replaced water/fuel separator filter, flushed/blew out fuel lines. Pulled the electric fuel pump off and tested that it is working on bench with another battery and pulled the little filter in the carburettor which was clean as a whistle. Put back together and it did the same thing... Cranked, fired and idled smooth then stalled/stopped.

i’l add when I say stalled/stopped in did not cough, hiccup, splutter. It stopped like it ran out of fuel...

fuel is a week old and tank was full, I syphoned 20ltr off to see if there was any crap in the tank, none...

I pulled the fuel pump connector from the main engine harness and substituted a battery so pump would run on its own constantly, placed a volt meter into the fuel pumps harness to see if it was getting power at start/crank.

The boat started and ran once again no problems, volt meter indicated during the crank cycle it indeed got 12/13 volts. After motor had started key returned to run/on the volt meter reads zero... hum so fuel pump not powered after start, that would explain running out of fuel as that’s exactly what it’s doing. Using all the fuel in the carby bowl.

Now on/in my engine wiring harness this positive wire is purple and yellow, which is connected to one of the terminals on the starter motor (gives power at crank). Then goes to fuel pump and choke. Also connects to one side of the oil pressure switch located next to the oil filter and pressure sender. Then on the other side of this oil pressure switch the wire changes to purple and goes to the exciter terminal on the alternator...

This is where this **** turns to white mans magic for me... after the crank cycle and engine is running with key in run position. Obviously the power is not provided via the starter motor terminal anymore, it’s provided from the alternator via the oil pressure switch.

if I’m correct above this leads me to think the oil pressure switch is faulty ?

this also leads me to ask the question does the oil pressure switch act as a safety device/switch when it senses no oil pressure and cut power to the fuel pump to save engine.

also I will add yes the engine is full of oil, it’s clean and looks like good.



HELP cause with out changing the pressure switch, which mean I have to go find one I’m guessing...

also I will add, my oil sender to the dash panel gauge is providing a reading...
 
Last edited:
The Oil pressure switch may be your issue.

Disconnect the two wires at pressure switch (purple and purple with yellow) and jump together. If this solves your problem then replace oil pressure switch.

(NOTE: this bypasses the oil pressure switch and will apply power to pump all the time key switch is in run position> ONLY FOR TESTING)

The Junction of the PURPLE wire you refer to is the 12+ from Ignition on. That wire although appears to only go into Alt it goes to all 12+ when key is in run position. (choke, coil, gauges etc)

I dont have my merc manuals on this PC so I cannot post a schematic at this time.
 
kghost said:
The Oil pressure switch may be your issue.

Disconnect the two wires at pressure switch (purple and purple with yellow) and jump together. If this solves your problem then replace oil pressure switch.

(NOTE: this bypasses the oil pressure switch and will apply power to pump all the time key switch is in run position> ONLY FOR TESTING)

The Junction of the PURPLE wire you refer to is the 12+ from Ignition on. That wire although appears to only go into Alt it goes to all 12+ when key is in run position. (choke, coil, gauges etc)

I dont have my merc manuals on this PC so I cannot post a schematic at this time.
Click to expand...

Hi there, thank you for taking time to reply, I have a few days off coming up and will try this. I have wondered if that might do the trick to problem solve.

regards R
 
those pressure switches are common failure items....if it is bad, get an extra one for your spare parts box...
 
Ok quick question to confirm...

11# has TWO electrical connections and that is for what ? (Pressure switch) which kills the power to fuel pump

13# has ONE electrical connection and that is for what ? (Oil pressure sender to dash instrument)
 
Item 11 is a N/O switch, One wire is 12+ in the other is 12+ out.

The oil pressure switch is Normally open (Open connection internally)

When oil pressure reaches the switches set point (Non adjustable) the switch internal contacts close allowing 12+ to pass through.

Note: when cranking the 12+ to pump comes off the starter solenoid.

Item 13 is a (Snap switch) most likely for an alarm if oil pressure is lost.

Item 7 is the Oil pressure sender to gauge

http://www.marineengine.com/parts/m...9999/wiring-harness-and-electrical-components
 
kghost said:
Item 11 is a N/O switch, One wire is 12+ in the other is 12+ out.

The oil pressure switch is Normally open (Open connection internally)

When oil pressure reaches the switches set point (Non adjustable) the switch internal contacts close allowing 12+ to pass through.

Note: when cranking the 12+ to pump comes off the starter solenoid.

Item 13 is a (Snap switch) most likely for an alarm if oil pressure is lost.

Item 7 is the Oil pressure sender to gauge

http://www.marineengine.com/parts/m...9999/wiring-harness-and-electrical-components
Click to expand...

..........
 
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