Power drain overnight

If you are sure you heard the fuel pump, leave the battery connected tonight and pull out the fuel pump relay and see if you have a strong battery in the morning. Fuel pump should not be running with key off.

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Looks like the heavenly bodies have smiled on you and provided a clue. As @eddo says leave the battery connected and make sure you pull the right pump fuse. Then just a question of relay, pump, pressure switch or short. Prolly go out in the middle of the night and the pump would be running. I had that with my hvac fan. Would continuously run due to a bad hvac controller. But it wasn’t intermittent.

The rotating selection-dial of your meter appears to be set to the 2 Amp scale, so the displayed units are amps. 0.0815 A is 81.5 mA. 1000 mA = 1 A. Since it quickly drops to 50 mA, then to 3 mA, that looks normal, presumng the meter was in series with the battery negative, and battery was otherwise connected to the truck normally, I see no significant parasitic drain, at least at the time you measured it.

Don’t know if it applies to your vehicle design, but fuel pumps (radiator fans too) on some vehicles will sometimes run with everything else off and vehicle parked, normal, fuel pump is just priming the fuel rail. It’s possible of course something (maybe fuel pump) is turning on in the middle of the night. Some more $$$ meters have a min/max mode, so with those you can leave the meter connected overnight , next am it will show the minimum and maximum currents during the night. That way you can catch something in the truck turning on in the middle of the night. You auto parts store might lend you one of their meters, one having the min/max mode.

It might be possible to configure a cell phone camera to take a photo of the meter every 30 seconds automatically. Then if something turned on you could see it upon reviewing the photos the next AM. How to do that? No idea.

If you can figure a way to disable fuel pump but leave everything else connected, and no power drain that way, pretty good clue problem is the fuel pump turning on. Could be caused by a fuel-pump anti-drain valve. That’s a one-way valve that prevents fuel in the fuel rail from going backwards back into the tank. That will drain the fuel rail over time, and on some vehicle designs could cause the fuel pump to turn on automatically. The fix for that problem is usually a new fuel pump.

Note, other problems can prevent engine from cranking even if battery fully charged. Neutral safety switch, bad battery connections, faulty starter motor, etc. A vehicle that won’t start when you want it to is very frustrating, ask me how I know? … lol … Just have to keep on with keeping on, scientific method, make a guess for the cause and test if it is or not, if not, repeat, repeat …

Thank you guys for such invaluable information! Well, what used to be a random act of the fuel pump being on while the truck is off is now an all-permanent situation. (At least, I believe it is the fuel pump since it is a high-pitched sound coming from the gas tank. All these few days, I have been disconnecting the positive pole of the battery, and as soon as I reconnect the battery, the truck starts right up. So today, I started the truck and removed the fuel pump relay, and nothing happened. Great! But then, I turned the truck off, got busy doing something, and then, a while later, restarted the truck; then, I realized that I had forgotten to reinstall the fuel pump relay! I then repeated that with no problem; somehow, the truck started without the need for the relay. I then chased the fuel pump fuse, and the noise stopped as soon as I pulled it out. Try to start the truck, and the engine will just turn over. I then reinstalled the fuse, the noise comeback and the truck started immediately. So it seems the relay has been bypassed somehow, and now the fuel pump is always on with truck on and off? Any suggestions?

My prior car, late 70’s VW Rabbit, developed a problem with the part the fuel pump relay plugged into, VW called it the 'fuse box". Inside the fuse box is a bunch of heavy current capable bus bars configured in a designed connection network. On the Rabbit some of the bus bars disconnected inside the fuse box (presumably b/c of heat), and even if the relay was “on”, no power could get to the fuel pump. In that case the shop fix was the bypass the faulty bus bar connections with wires configured outside the fuse box. In your case what might have happened, internal bus bars in the fuse box which shouldn’t be connected have become connected, and are now bypassing the fuel pump relay. It’s possible the faulty current path is beyond the fuse box, but a fuse box problem seems the most likely explanation.

Ask your shop to probe the point in the circuit where the fuel pump relay’s fuse box connects to the wire going to the fuel pump.

In any event, it sounds like you’ve got to the bottom of the power drain. Suggest to be very cautious whenever working w/fuel system.

Disconnect the oil pressure switch, there should be no power passing through that switch to the fuel pump when the engine is off.

This was before my time, but before EFI low oil pressure would disable the fuel pump the carburetor bowl would not be supplied with any more fuel. Engaging the starter would force the fuel pump to run regardless.

Did vehicles not get destroyed by running out of oil back then so much? Come to thin of it, I don’t remember hearing of people running vehicles out of oil. Maybe it wasn’t just because people were smarter in the past?

The point is, if the low oil pressure switch is stuck closed, it can make the fuel pump run. That’s what kept it running once the key is moved away from the start position. This circuit configuration has remained even after end of caruretors, so then engine will stay running if the fuel pump relay fails. But it no longer offers protection from low oil pressure.

I think my 50 year old Ford truck engine would still pump gas to carb & run if oil pressure warning light was on. I have heard of some engines w/that engine protection scheme though.

Only on a very, very few cars. Although my back up generator has a low oil pressure cut off.