Troubleshooting Mercruiser 4.3L Fuel Pump Problems

Posting has been a bit slow on DestinSharks due to our continued development of our EarthNC Marine Charts for Google Earth project and some down time on our Regal 2760. Our Regal is powered by twin Mercruiser 4.3L engines which have the distinction of being the only engines in the Mercruiser line with electric fuel pumps.

Anyone who has owned boats can tell you that mechanical problems are a fact of life (unless your boat is a canoe or kayak) and our Regal is no different. This time, it was the starboard engine which quit and would not restart during our last outing in early July. Fortunately, we were only a 1/2 mile from our slip (rather than the 20 miles away we had been earlier in the day) so we didn’t have far to go before the troubleshooting began.

The Symptoms

Obviously, the motor wouldn’t run. I quickly ruled out hydraulic lock (very bad) both because the engine quit while running and because the starter would turn over the engine. Given that the engine was warm and gave no obvious signs of distress, it was likely a fuel system problem.

Understanding the Merc 4.3 Fuel System

The Mercruiser 4.3L engines use an electric fuel pump. These pumps are fairly reliable and problems are usually with other elements in the wiring and/or fuel delivery chain. Elements of the fuel delivery chain include:

  • Fuel Tank Anti-Siphon Valve
  • Water Separating Fuel Filter
  • Fuel Pump
  • Carb Inlet Screen

Elements of the wiring chain include:

  • The Ignition Switch (key)
  • Ignition Circuit Breaker
  • Oil Pressure Switch
  • Alternator Field Circuit

The fuel pump is energized via 2 methods- the starter circuit and the ‘on’ circuit. When the key is turned all the way to start, the pump is energized even if there is no oil pressure. After the starter is released, the key reverts to the ‘on’ position. In this circuit, the fuel pump is energized if the oil pressure switch (not to be confused with the oil pressure sending unit for your gauge) senses at least 4 psi. Oil pressure switches tend to fail every 2-4 years and will shut the engine down when they do. They are inexpensive (about $30) and can be changed on most installations without undue aggravation.
Initial Trouble Shooting

Given that I have 2 engines and the port engine was running fine, I doubted that I had a blockage / fuel quality problem. A secondary indicator was that my ignition circuit breaker popped as the engine quit. It’s common that this occurs when the oil pressure switch begins to fail and usually resetting the breaker will allow the engine to restart and run a while before the next shutdown. Resetting the breaker didn’t help in this case.

My next step was to bypass the oil pressure switch with a jumper wire. The newer switches have a cannon plug that you can disconnect which makes running a jumper easy. With the jumper in place, I once again attempted to start the engine with no success. A note of caution, be sure to check your oil level prior to restarting in case you did have an actual oil pressure problem. If the engine does restart using the jumper, watch your oil pressure gauge as it’s your only indicator of safe oil pressure with the switch by-passed.

This left the problem to be one of power or the pump itself. Next I disconnected the cannon plug leading to the pump itself (located on the top of the engine just left of the thermostat housing). With the pressure switch by-passed, you should be able to measure 12V on a multi-meter at this plug with the ignition switch on. Take care when doing this to not accidentally turn the engine over as you will be working near the serpentine belt. If you get 12 volts at the plug, then either your pump is bad or you have a fuel block (or it’s a different problem – like bad coil).

You can test the pump by applying 12V to the pump leads. Take care to connect the leads properly. The pump will make an audible hum with the engines off if it’s working. In my case, I only could get 1V at the plug and the pump ran fine on direct battery power. This led me to troubleshoot the wiring.

I started around the oil pressure switch with no obvious problems. Most of the wire (purple is what you’re looking for) is grouped with other wires in a protective sheath -making these areas unlikely for a problem. I pulled my ignition switch panel at the helm and with the batteries off, checked the resistance of the ignition circuit breaker – which checked out a 0 Ohms.

Going back to the engine, I had the impression to check around the alternator. It turns out, the purple 12V wire is connected to the alternator. This connection looked pretty tight to the front of the engine so I undid the plug and pulled the wires free for inspection. I immediately found that the wire had been rubbing against an edge of the engine manifold which had chaffed off the insulation about 2 inches down from the connector.

Problem Found

The result was predictable and matched my symptoms exactly. Under heat and vibration, the wire made a solid enough contact with the manifold to trip the ignition breaker – shutting down the engine by fuel starvation. Although I had reset the breaker, enough contact remained to drain the line of it’s voltage, but not enough to trip the breaker. After checking that the main conductive wires weren’t compromised, I wrapped the wire in heavy electrical tape and the problem was solved.

Hopefully you’ll find this helpful if you’re suffering from similar Merc 4.3L fuel problems. Please leave us your own experiences in the comments section.

6 thoughts on “Troubleshooting Mercruiser 4.3L Fuel Pump Problems

  1. Pingback: » Blog Archive » Blue Angels Beach Airshow - A Great Weekend for Boating

  2. Hi, I have a similar problem on my 454 – 7.4MPI engine. Last Sunday whilst on my way back the engine suddenly stopped. It turns but it will not start. Normally when I turn the ignition on position one, I hear 2 beeps and could hear the hissing sound of the petrol pump. I tried about 6 times to start the engine. Sometimes the beeps were heard and sometimes not. but the hissing noise of the engine was never heard. I disconnected the wire to the oil pressure switch and put it in again and tried to start – no luck. I isolated the wires of the pump and gave it 12v direct from the battery. after a couple of sparks in the holder (cause I was trying to hit the right contacts inside the holder) the pump started. I reconnected the cable and tried to start the engine the normal way. This time the engine started and was running without problems. I shut down and restarted about 6 times and all times the engine started. So my question is: Shall I change the Oil pressure switch? Shall I change the pump? Do petrol pumps on my particulat engine get stuck (seeze)? Do I need to add some flued to the gas to clean the petrol pump? Thanks in advance for your help. Jesmond

  3. I was misleading myself….I used a OMC schematic for the 4.3 that refers to a 6-12 Amp Circuit Breaker and Relay in the Fuel pump circuit—-Apparently there is a different design on the M/C 4.3——-I see what appears to be the Switch on the lower, rear. left hand of the motor with the purple wire going to it —-tomorrow will tell…………….

  4. I have a MC 4.3 Alpha One carbed engine. I have been having trouble with it for a year. Last fall it began to stall and surge repeatedly. Never died. Have spent nearly $1500 since then and the problem still is not fixed, although it isn’t as bad. Was told timing was off 23 degees. Re=timed. Carb has been rebuilt. Plugs, wires, fuel pump have been checked. This boat (97 Glastron 185 FS) used to JUMP out of the hole with the 5 bladed ss 15X21 prop that is on it. I could pull two slaloms out without any trouble. Now I have to go hammer all the way down to pull out one skier. Uses 25% more gas. It acts like I have about 5 3/4 cylinders working. Runs noiser. Every once in a while will perk up and go well for a few seconds, but then falls back. At wits and pocketbook end.

  5. Sorry to hear about your trouble. Have you checked your anti-siphon valve in the top of the fuel tank for debris? This can cause an overall loss of power and surges (it did in my previous boat). Check your distributor (and the internal rotor) for wear or corrosion.


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