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I've got an old Chevy ('75) with a top-opening to the gas tank. The car has been sitting for 3 years without use. I want to clean the stale gas out of the tank. I have drained it as well as I can by disconnecting the fuel pump supply line. However, I'm sure there is a residual amount of gas in the tank. Under normal circumstances, I'd drop the gas tank, however, on this car, the former-owner tack-welded the exhaust pipes together which need to be disassembled to drop the tank. I don't want to have to cut out the exhaust pipes if I can avoid it.

My question is this, is there a safe way (solvent, etc) that could be used to get the stale gas out of the tank without dropping the tank?

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If there is under a gallon of stale gas and you filled your tank up with fresh, I don't think there'd be an issue with the stale gas. It would dilute enough you shouldn't have to worry about it.

If you can't drop (or don't want to) your fuel tank, or cannot put a syphon hose down into it through the filler neck, there's just about no way to get the small amount of gas out which you're talking about, short of punching a hole in the bottom of the tank and then patching it after the fact. To be honest, I'd drop the tank before I'd punch a hole in it.

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  • Not that I'm terribly worried about it with the stale gas, but I'd be afraid to end up with a Darwin Award after trying to punch a hole and patching it up. – N8sBug Mar 31 '19 at 14:33
  • @N8sBug - Yah, Darwin can make a fool out of you if you don't pay attention! :o) – Pᴀᴜʟsᴛᴇʀ2♦ Mar 31 '19 at 17:05
  • Siphon hose= Oklahoma credit card. – Moab Apr 6 '19 at 0:57
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Just dilute it

3 year old gas? Don't worry about it. Drain it as far as possible, then just dilute it by filling the tank with new gas. It'll be fine.

If you're really that, that worried about it, then pull the mixed gas back out into legal cans, and dilute it again with more gas. We're bordering on homeopathy at this point.

Do not mess with fuel lines on a modern car

I notice one advice was to activate a modern car's electric fuel pump. Couple problems with that idea. First, there are typically 2-4 gas lines between engine and tank, which one to unhook? Second, these are high pressure lines running 40 PSI, and use more challenging splice connections - they are not just the ole' rubber and hose clamp (like OP's '75 is). Third, running an in-tank fuel pump dry destroys the pump so you really need to be on-the-ball when the flow stops.

On a modern car, do it old-school, with a rubber hose into the tank. If that doesn't work, some car interiors provide an accessible hatch above the top of the tank so you can access the fuel pump. It may be found under the rear seat.

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If this has an in-tank fuel pump (as most cars do these days, but I wouldn't know about a '75 model), you can use the following method. Basically, you're shorting the fuel pump signal lines to get it to run continuously, and routing the fuel line in the engine bay into an appropriate container.

Disclaimer: be very careful when working with fuel and electricity. Very Bad Things can happen.

  1. Disconnect the battery

  2. Disconnect the fuel line that goes into the engine bay and route it into a fuel-friendly container. If your fuel system has one of those Schrader valves for pressure testing, you can tap into the system there. This will require either removing the valve or valve core.

  3. Remove the fuel pump relay, and short the two signal pins in the fuse box. (Google your relay to find out which pins you need.)

  4. Turn the key to the "on" position.

  5. Reconnect the negative battery cable and watch the fuel make its way into your container. Pay attention as you'll want to disconnect the negative cable again once the fuel stops flowing since you don't want to run the pump dry for too long.

  6. Disconnect the negative cable again

  7. Reinstall your fuel pump relay

  8. Put the Shrader valve back in its original configuration, or reconnect your fuel delivery hose.

  9. Reconnect the negative cable and tighten it down.

Voila.

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  • 2
    '75 is most likely going to have a mechanical pump, but good advice if electrical fuel pump equipped. – Pᴀᴜʟsᴛᴇʀ2♦ Mar 31 '19 at 19:55
  • I have done tis on my car (1971 - but not a chevy) just byre-routing the fuel line to a container, switching the ignition on, but not starting the engine. Fuel pump ticks way, fuel is pumped into container. Is there a need to play with the fuel pump relay etc ? – user2808054 Apr 10 '19 at 15:37
  • @user2808054 My explanation was focused on electronic fuel injection. I couldn't get this to work without bypassing the relay. A carbureted or mechanical injection system is (obviously) different. – 3Dave Apr 24 '19 at 13:48

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