Adding a Pushbutton to Activate a Fuel Pump

by Rod Stewart

The Motivation
From what I have read, there are many owners of carburetted 4.3L engines that find these engines very hard to start after sitting idle for a few days. My 1996 4.3LX had the same problem since day one. It was extremely aggravating.

It seems the problem is likely that the gas somehow drains back from the carb bowl into the fuel tank after the engine has been sitting idle for a few days. (As long as it is run every day or two, there is no problem. It starts and runs just fine with no fuss.)

Unless the engine is running the electric fuel pump does not run, even with the key in the “ON” position. This is because the pump circuit is equipped with a low oil pressure cutout switch that is intended to shut off the fuel pump and kill the engine in the event that oil pressure is lost. The pump is only energized when the key is in the “START” position. The starting procedure can be long and difficult because the gas must then be pumped all the way from the tank through all the lines and fill the carb bowl, before the engine will fire. All the while the voltage to the pump is likely less than 8 volts (due to the huge draw of the starter), and the pump runs painfully slowly. It can take a full 30 seconds or so of starter engagement before the engine will start. Also causing problems with engines equipped with automatic chokes is that the choke motor is energized while you are cranking the engine. If the engine fails to start, the choke ends up fully open. Then you are trying to start a cold engine with no choke.

The Cure
The cure is to “prime” the carb bowl with fresh gas before attempting a start. This is what I did to fix the problem with mine. I wired up an automotive/marine rated momentary contact pushbutton switch to run the electric fuel pump just before starting. I am pleased to say it works perfectly. When the switch is pushed, I can hear the fuel pump working for about 4-6 seconds, then it cuts out, presumably when the carb bowl is full. Then following my normal cold start procedure I give her one small throttle shot in neutral lockout, and hit the start key. Bingo, she starts right up on about the second or third firing stroke and runs perfectly.

The Procedure
The pushbutton switch is rated at 15 amps, and I installed it in a 5/8" hole just beside my key switch. I took a +12 volt feed from the "I" terminal of the key switch. (This terminal is hot only with the key "ON". This means the pump cannot be activated without the key on.) Then I ran a single 14 gauge wire from the other terminal on the switch back to the engine and connected it to the existing purple/yellow wire running from the starter solenoid to the electric fuel pump. This effectively bypasses the oil pressure cutout switch and feeds +12 volts to the pump when the switch is pushed. Pretty simple job, it took about an hour to do once I got to it."

Further Info
added 8/20/2008

It turns out that these older style fuel pumps have a spring loaded ball-check valve in them that can stick in the closed position blocking fuel flow to the carb. Fuel additives are thought to be the cause. The newer revision of this pump has no check valve to stick, and also eliminates the L-shaped connector that is also prone to disconnect intermittently. If you are going to replace an old style pump with the newer version, you should order PN 861155-A6. which will include the newer connector wiring harness, mounting bracket (NEEDED) and related hardware. Installation is not hard, but the new so will have to alter the bends in the fuel lines to make things line up correctly. Anyone looking for advise on this can email me at progg@frontiernet.net