Yesterday I ended up running out of gas on the road and I had a gallon of antifreeze on me thinking it was gas and I filled the full gallon of anti-freeze in my gas tank and later realized it was actually anti-freeze and not gas. I then used a five gallon gas tank in my trunk to fill it up but now my car won’t start. How can I fix this?

  • Holy smoke! I hope you don't have a habit of carrying gasoline in non-approved containers! That's a good way to spend years in excruciating pain and disability, going from skin graft to skin graft until you finally look like the Phantom of the Opera. Aug 2, 2022 at 22:48
  • Gasoline fumes are dangerous when working with raw fuel like emptying it from a tank into another container. As warned, do not power any electrical device with gasoline fumes floating in the air - preferably all work done outdoors to allow fresh air and winds to blow fumes away.
    – F Dryer
    Aug 2, 2022 at 23:55

3 Answers 3


No shortcuts unfortunately. You need to:

  1. Drain the fuel tank. This will likely involve removing it from the vehicle and clearing out all the antifreeze.

  2. You started/tried to start the car so antifreeze is now in your fuel lines and your fuel injectors. You'll need to open up the fuel lines and clean them out. Usually people use compressed air and blow out the antifreeze. As noted in the comment, you will want to change the fuel filter.

  3. You will want to ensure that you don't have any liquid antifreeze in your cylinders. If so you will need to clean that out which may require you to pull the cylinder head(s). You may be able to "blow" any liquid out of the cylinders by removing the spark plugs and cranking the engine. This assumes you didn't end up with a hydrolock condition and have damaged your connecting rods or pistons.

Yes, this is a BIG problem and a lot of work to fix.

  • It's not possible for a cylinder to fill with antifreeze that way. If it was, we'd have cylinders frequently hydro-locking with fuel. The fuel tank is below the injectors and the fuel pump only runs when the engine is (expected to be) turning. On the other hand, pulling the injector rails and injectors is prudent (more than I would do, but I can't argue with the advice). Aug 2, 2022 at 23:36
  • I believe the OP said that he ran, or at least tried to run, the engine. (Note that he said: "Now it won't start.") So it's quite possible that some AF ended up in the cylinders.
    – jwh20
    Aug 3, 2022 at 0:48
  • 1
    Not in any quantity, and air is moving with it too. Very little fuel is injected during normal startup. Fuel contains a lot of energy - the volume of a cylinder is enough to drive the car a mile or two. Modern cars have to make a good showing on full-cycle smog tests, so they can't afford to inject excess fuel, which would blow their HC numbers. Aug 3, 2022 at 18:54

Safety is everything here. Fuel is unbelievably dangerous stuff when it's outside of an approved tank. The young often dismiss this as "YOLO, if I die, I die" - but in fact you probably won't die, but will survive in extreme pain after a long stint in a burn ward. Fear this and consider bringing in a pro.

Cleaning, the hard way.

I did this last year myself. First, the fuel tank will need to completely cleaned out. You start typically inside the car at a hidden hatch which gives you access to the top of the tank and the fuel pump. Fuel is highly flammable so the utmost care must be used. The consequences for failure are terrible. Gas is also stinky as hell, so you don't want it in your car's interior.

When disconnecting fuel lines, take care to plug them immediately. Talk to an auto parts about fuel line plugs. Have every size on-hand in case you are wrong about fuel line size. You can make plugs by taking bolts long enough to have an unthreaded shank, and cutting off the thready parts.

Once the fuel lines are disconnected from the fuel pump, you can run some low pressure fuel line to an approved gas can, and run the fuel pump and let the fuel pump empty the tank for you. I do that by disconnecting the starter solenoid wire, and then switching the car into "run". Sometimes it requires putting it into "start" - that's why you disconnect the starter solenoid.

One line is pressure and the other is return. If in doubt just run them both to the gas can lol.

Do not run the fuel pump dry - once you hear it wheezing shut off. It needs the fuel to stay cool and lubricated.

Antifreeze is heavier than fuel, so this should have taken out nearly all the fuel leaving antifreeze. The fuel pump cannot empty the last gallon or two. Now you need alternate methods via reaching in through the access hole and get all remaining liquid out of a tank. DO NOT DO NOT DO NOT use any sort of motorized device to do that, like a Shop Vac, unless you enjoy skin grafts. (hint: you don't). The most "power" thing I've used is an all-plastic hand vacuum pump. Yes, this is dirty miserable hand work, if that's not your cuppa, hire a pro.

Then reinstall the fuel pump and refill the tank with a few gallons of gas. Plumb up the pressure line to the tank, but re-route the return line to a gas can. This is to purge all the crud that is in the supply line, filter, injection rail and return line. When you get a half gallon you should be fine.

Then replace the fuel filter and do a similar purge just in case.

Now you can reconnect, and make efforts to start the engine. It will start quite hard since there will still be a few CC's of antifreeze in the spurs to the fuel injectors. You're dumping that through the engine and catalytic converter (be nice to temporarily remove that if able) because the alternative is costly. But finally it should catch.

Once it's running decently, run the wheels off of it to move a lot of fuel through the system. If you are an anti-ethanol purist, drop that for now - run E10, or if your car is rated for it E85. Ethanol helps dissolve and carry away water trapped in gas tanks.


Talk to your community household hazardous waste about disposal of contaminated gas. I'm sure they'd much rather have you give it to them than dump it God knows where. Gas has nasty and poisonous additives that will go into the soil.

If you leave the gas/antifreeze emulsion alone for a long time, it will separate into gas (above) and antifreeze (below). If you have an appropriate container with a bottom drain valve, you may be able to drain off the antifreeze leaving gas fit for lawnmowers and whatnot (things that are easy to fix if you're wrong).


Many EFI engines only run the fuel pump when the engine turns over during starting and when running. At ignition on time (engine not yet started) the fuel pump may run for a few seconds then cycle off. Disconnecting the main fuel feed line between fuel fill pipe and tank may have a rubber fuel line where manually pumping out the fuel tank can be performed with a large plastic vacuum pump made to move large volume of fluid. The rear end will have to be jacked up for access to the fuel line between filler pipe and tank. The fuel rail should be blown out by removing the fuel test valve core and disconnecting the fuel filter so low pressure air can blow thru the fuel rail and empty the lines at the fuel filter. Injectors may need removal for cleaning or once the rail is blown/flushed, injector cleaner may be used on the fuel test valve to run the engine on injector cleaner with everything reassembled, fresh gasoline in the tank and a new fuel filter in place. EFI engines should have a fuel pump 87a relay that can be bypassed with a small wire jumper across relay socket terminals 30 and 87 to send power immediately to power the pump. Be extremely careful when manually powering the fuel pump. Spark plugs should be removed and cleaned of antifreeze using solvent for reuse. Contaminated fuel mixed with antifreeze may be accepted by most gas stations required to accept used motor oil. Ask in advance before bringing contaminated fuel.

  • In my experience the "few seconds" at startup is NOT timer-based. It is stopped when the fuel rails reach target pressure. If that never happens, the fuel pump runs indefinitely. I've drained a tank exactly this way. Aug 3, 2022 at 18:57
  • I'm familiar with GM EFI systems and base fuel pump operation from service manuals and practical experience. At ignition on time (engine not started) the ecm has a program to operate pump for 2-3 secs then shuts off. No engine starting, no pump as a safety measure. When starting, the crank sensor generates the precise timing signals allowing the ecm to operate; fuel pump, ignition system for spark and pulse injectors. If the engine stops for any reason, the crank sensor stops outputting timing signals to the ecm, effectively halting pump running, kill spark and stop injector operation.
    – F Dryer
    Aug 4, 2022 at 15:57

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