Lack of voltage to fuel pump, intermittent, mercruiser 4.3 LH

[Kzreelrods]

1998 Mercruiser model MCM 4.3 LH S/N 0L690518 I'm having an intermittent stalling issue that is the result of a lack of voltage to the fuel pump through the oil pressure safety switch circuit. I've been working on this issue for a while, and being intermittent, it has been difficult to diagnose. Some days the boat fires right up and works flawlessly all day long. Often, and usually before the engine has come up to full temperature, it will stall out and refuse to restart. Sometimes it will persist long enough where I call it a day, and other times after 10-15 minutes the problem will magically go away and the motor will run flawlessly all day long. Seems pretty random. I have spark and there is never a flooding issue when attempting to restart. When things are good, and I jumper across the oil pressure safety switch, the fuel pump will run when the key is in the run position (eg. There is voltage to the oil pressure switch). When the issue occurs, the fuel pump will not run when I jumper across in this same manner (no voltage to the oil pressure switch). Here is what I've done so far. - mechanic rebuilt the carbeurator (initially he figured this was the issue... Whatever). - fuel filter was also replaced. No indication of water in the fuel. - I re routed the fuel tank vent line to ensure no low spots holding gas and causing resistence. - I replaced the oil pressure safety switch - I replaced the fuel pump - I replaced the fuel line leading from tank to filter... Also verifying there is no antisiphon valve in the system. - I replaced the ignition key switch. - main wiring harness connection was pulled apart, cleaned, greased, and re secured. At this point, the problem seems to be electrical and seems to be preventing voltage from getting to the oil pressure switch / fuel pump circuit. I'd appreciate suggestions on further troubleshooting. I know the starter is also involved in the fuel pump circuit as well, but not sure what to look for there. Starter is very difficult to see / access. Thanks... Erik

 

[Bt Doctur]

have you checked for another harness connector under the dash.

 

[alldodge]

1998 Mercruiser model MCM 4.3 LH S/N 0L690518 I'm having an intermittent stalling issue that is the result of a lack of voltage to the fuel pump through the oil pressure safety switch circuit. I've been working on this issue for a while, and being intermittent, it has been difficult to diagnose. Some days the boat fires right up and works flawlessly all day long. Often, and usually before the engine has come up to full temperature, it will stall out and refuse to restart. Sometimes it will persist long enough where I call it a day, and other times after 10-15 minutes the problem will magically go away and the motor will run flawlessly all day long. Seems pretty random. I have spark and there is never a flooding issue when attempting to restart. When things are good, and I jumper across the oil pressure safety switch, the fuel pump will run when the key is in the run position (eg. There is voltage to the oil pressure switch). When the issue occurs, the fuel pump will not run when I jumper across in this same manner (no voltage to the oil pressure switch). Here is what I've done so far. - mechanic rebuilt the carbeurator (initially he figured this was the issue... Whatever). - fuel filter was also replaced. No indication of water in the fuel. - I re routed the fuel tank vent line to ensure no low spots holding gas and causing resistence. - I replaced the oil pressure safety switch - I replaced the fuel pump - I replaced the fuel line leading from tank to filter... Also verifying there is no antisiphon valve in the system. - I replaced the ignition key switch. - main wiring harness connection was pulled apart, cleaned, greased, and re secured. At this point, the problem seems to be electrical and seems to be preventing voltage from getting to the oil pressure switch / fuel pump circuit. I'd appreciate suggestions on further troubleshooting. I know the starter is also involved in the fuel pump circuit as well, but not sure what to look for there. Starter is very difficult to see / access. Thanks... Erik

:welcome: to iboats

If the fuel pump will not run when switch is jumped then you lost 12V on the purple wire going to it. When the problem occurs you can still crank the motor and your getting spark. If this is all true then you have a bad connection on the purple wire feeding the switch, and there is two ways to fix it. Either start by pulling your harness apart and flowing the purple wire from the oil pressure switch back where it is spliced. Once found check connection and connectors and use a millimeter to ohm it. Wiggle the wires and connection to see if you have a break inside the wire or connector.

Other way to fix it, is splice in a new wire from pin 5 (purple wire) on the engine cannon plug and run it to the oil pressure switch.

The only other thing that puzzles me is the engine won't start when cranking. You should be getting 12V direct to the fuel pump on the purple/yellow wire coming from the starter. So the engine should start and later die after the key is released when the carb runs out of fuel.

Last thing, you should have an anti-siphon valve on the tank unless someone has removed it. It looks like a regular barbed metal fitting which the fuel line is attached to at the tank. There would be a spring and check ball inside. If yours has been removed you should get another one and put it in.

 

[Kzreelrods]

have you checked for another harness connector under the dash.

No... I'll check that. Thanks.

 

[Kzreelrods]

Alldodge... Thanks. I will do more snooping around that cannon plug and at least inspecting the wiring harness where the purple wire runs. I dread the idea of pulling it apart, but the idea of splicing a new purple wire in seems worth considering. How can I test whether the fuel pump is getting voltage from the starter when the starter is engaged? When this condition is present, I turn over the engine and it fires only momentarily and dies immediately when the key returns to the run position. Re the antisiphon valve... It is there but previous owner must have removed the guts. Will address later. Thanks. Erik

 

[alldodge]

Alldodge... Thanks. I will do more snooping around that cannon plug and at least inspecting the wiring harness where the purple wire runs. I dread the idea of pulling it apart, but the idea of splicing a new purple wire in seems worth considering. How can I test whether the fuel pump is getting voltage from the starter when the starter is engaged? When this condition is present, I turn over the engine and it fires only momentarily and dies immediately when the key returns to the run position. Re the antisiphon valve... It is there but previous owner must have removed the guts. Will address later. Thanks. Erik

You said in your first post that you were getting spark when the trouble occurred. If your getting spark then you have 12V at the coil.

You said in your first post that you would get 12V at the oil pressure switch when it was running and when the engine would not run you were not getting 12 at the switch. Your telling me you have a volt meter and your measuring the voltage.

Your current post says it fires then dies when the key is released, this sounds to be a different issue. If the fuel pump is running when cranking it would put enough fuel in the carb to have it run more then momentary. With the engine dying right away this sounds more electrical.

If you put a volt meter on the fuel pump or on the purple/yellow wire at the oil pressure switch, you should see 12V when your cranking the engine

 

[Bt Doctur]

if you follow that wiring diagram you will see the purple wire connects to the coil ,alt and pressure switch. You say you have spark so you have voltage at the coil. check for voltage at the alt,power there means your open from there to the pressure switch.

 

[Kzreelrods]

Thanks for the follow up. Yes, I have a meter and did measure voltage at the switch. Will also verify voltage at the fuel pump when starter motor is active ( next time the issue occurs).

I also see the purple wire has a connector or splice of some kind just before the alternator. I'll see if. I can find that location on the wiring harness and look for a bad / loose connection. I'll also do more testing at the cannon plug to see if the purple wire is giving voltage up to it when the key is in the run position. Will also take a closer look at the cannon plug for bad / loose connection on that #5 position.

May take a couple days before I can complete this, but I'll post back the results.

Thanks.

Erik

 

[Kzreelrods]

So... After I pulled the cannon plug apart and made sure the connectors were clean and greased and reseated it securely the boat has been performing flawlessly. If the issue returns, I'll pick this back up again, but as for right now it seems to have been the cannon plug.

Thanks for the help.

Erik