1999 4.3 mercruiser spark, oil pressure, no fuel?

[pikiewicz]

Working on a friend's 1999 4.3 l MerCruiser. The boat will start if I hotwire the fuel pump. Oil pressure gauge does increase while boat is running. It will not start unless fuel pump is hot wired could it be an oil sensor? Not sure where is located. I found the oil pressure sensor but I understand there is another sensor? Any thoughts?

 

[Chris1956]

You need to look at a wiring diagram. Generally, the fuel pump would be energized by the ign switch in the start position. The fuel pump would then be energized by the oil pressure sensor, when the motor is running.

So the suspect is the wiring from the ign switch start position to the fuel pump. You might look for a wire from the starter solenoid to the fuel pump, perhaps thru a relay.

 

[pikiewicz]

Thanks. I only see two wires feeding fuel pump. I do know that when key is turned to on position, no sound from fuel pump.

 

[pikiewicz]

Any ideas of the color wire I am looking for and where it may go? If there is a relay, could that be the problem? Thanks for any help you can provide

 

[Chris1956]

The ignition switch will only power the fuel pump, when in the start position. The oil pressure switch powers it when engine is running.

I do not know if there is a relay, but if there is, you should see it close to the fuel pump. You can think of the relay as a continuous duty solenoid. When powered by either the solenoid wire or the oil pressure wire, it is energized, feeding voltage to the fuel pump.

 

[Chris1956]

OK, a quick search shows a pur/yel wire from the starter to the fuel pump, runs it when cranking the motor. The oil pressure switch output also is connected to that pur/yel wire. No relay.

See the following for a '96 4.3L

https://forums.iboats.com/forum/eng...ines-outdrives/598304-1996-4-3-wiring-diagram

 

[alldodge]

This is in the OB section and should be in the Merc section GA_Boater

There should be a Purple/Yellow stripe wire coming from the outboard small post on the starter. This is the wire which feeds the pump while the motor is cranking.

Once it starts, power is removed from the Red/yellow until the oil pressure switch closes and supplies power from the purple wire

 

[QBhoy]

It’s the oil pressure switch most likely. Think it’s round the back and port side of the 4.3. Or maybe that’s the sender for the gauge. Can’t remember.

 

[Furbird]

In the image above, item 1 goes to the gauge, item 2 goes to the pump. If item 2 goes out, the pump will not come on. It will be on the port side of the engine at the stern where the oil filter attaches in normal configuration. With a remote mount filter, just follow the hoses and you'll see the piping. They both attach at the same location but 90 degrees to each other.

 

[pikiewicz]

Thanks everyone for the help. Believe it or not there is a strip mounted behind the carberatour that looked like a terminal strip. Turns out there are actually fuses in it. It was a blown fuse. I had no idea mercruiser put fuses in the wiring harness.