The car that idled way too slow

March 17, 2012 1:37 PM

The car had run out of gas. When it got gas and finally started, it ran very strangely. How come?

The '95 Ford Thunderbird ran out of gas, completely, and was parked on the shoulder when a gallon of gas arrived and was poured into the tank. The starter was cranked, cranked, cranked to pump the gas to the engine. Finally it started, kinda, but sounded very strange. At idle, the sound from the tailpipe was a quiet pum... pum...pum pum....pum... like individual cylinders firing.

Any ideas what was going on? Does running out of gas do something to a a fuel-injected engine? (I don't know what happened thereafter, as I left in my own car.)

posted by exphysicist345 to travel & transportation (15 answers total)

Fuel injected cars have been designed for a long time to keep the fuel lines from completely starving - they shut off if the fuel pump, which is usually in the tank itself, runs dry. Running them down to nothing is not great for the fuel pump, because the gas surrounding it is supposed to keep it cool. Or so I've heard.

The hard starting and the rough engine may have been due to the fact that it was a '95 car owned by someone who treats their car so well they let it run out of gas.
posted by randomkeystrike at 1:47 PM on March 17, 2012 [1 favorite]

It's why they say you should never run completely out of gas. "Gunk" collects at the very bottom of the tank, and when you run out completely, the gunk get's into your engine.
posted by Rash at 1:48 PM on March 17, 2012 [6 favorites]

As Rash said above. With luck, the gunk has clogged your fuel filter and not your injectors.
posted by buggzzee23 at 1:50 PM on March 17, 2012

There are cleaning products you can add to the gas tank. Check with your local auto supply store worker.
posted by jeffamaphone at 2:18 PM on March 17, 2012

Also air in the fuel system. You can prime the system by turning the key to the 'Run' position from the 'Off' position a few times. It would have decreased the cranking time, too.
posted by narcoleptic at 2:46 PM on March 17, 2012 [1 favorite]

I've always heard gunk in the tank blamed for this, too.

The problem I have with this is that, since the position of the fuel intake in the tank does not change, the only way more gunk would enter the lines when the car ran out of fuel would be if the gunk had been floating on top of the fuel, which given that the density of gas is less than 0.8 that of water, isn't a lot of the categories of gunk you might find in a gas tank. The more vicious and hard to burn fractions of the gas tend to be the heaviest, too, so they wouldn't be up there. Even the surface tension of gas is low, so things would not tend to float that way.

However, the lines would have aspirated air, as I see from additional comments narcoleptic has already said, and I think that's more likely to be the culprit.
posted by jamjam at 2:53 PM on March 17, 2012 [1 favorite]

They're viscous as well as mean, too.
posted by jamjam at 2:55 PM on March 17, 2012 [3 favorites]

This is most likely from either aeration in the lines (air bubbles causing line pressure issues) and crap out the bottom of the tank. Revving the engine up will clear the air and small bits of crap, injector cleaner will solve the rest if it is minor.

If it doesn't clear after those two you may need more detailed injector or pressure regulator cleaning which is a mechanic job. But chances are it will clear with some revving (like 3-4000rpm or so) and a bottle of injector cleaner in the tank.
posted by Brockles at 3:29 PM on March 17, 2012

At idle, the sound from the tailpipe was a quiet pum... pum...pum pum....pum... like individual cylinders firing.

Forgot this bit - that's exactly what it was, incidentally. The car was misfiring, which is when only some of the cylinders are firing properly or at all. If the injector has air in it or is partially blocked, the fuel cannot generate enough pressure to force open the end of the injector or to spray properly (atomise) when it is opened. This will cause crappy combustion and a less then even power delivery as a result, even at idle.
posted by Brockles at 3:35 PM on March 17, 2012

Thanks, all. I guess I'm inclined to believe it was some residual air in the fuel line, so fuel wasn't reaching all the cylinders. Gunk from the bottom of the tank trapped in the fuel filter could have contributed too, by lowering the fuel pressure at the injectors.
posted by exphysicist345 at 5:10 PM on March 17, 2012

It's why they say you should never run completely out of gas. "Gunk" collects at the very bottom of the tank, and when you run out completely, the gunk get's into your engine.

That is a myth; the fuel pump always pulls fuel from the bottom of the tank. It doesn't move.

I would blame it on air bubbles in the lines, or possibly a partially dead battery.
posted by gjc at 7:40 PM on March 17, 2012

It's not a myth - the pump always pulls from the same place, but the gunk in the tank remains in suspension in the volume of fuel. So there can be lots of tiny bits of crap all over the bottom of the tank and even floating in the fuel. When the car moves around it shakes it all up and keeps it spread out. With less fuel, the gunk is obviously more concentrated and when running out the chances of the crap getting pulled into the pump approach 100% as the gunk can't spread from the pump intake.
posted by Brockles at 8:07 AM on March 18, 2012

This would be caused by air in the fuel lines, as others have mentioned. It should have smoothed out pretty quickly once the engine was running. The gunk at the bottom of the fuel tank thing is a bit of a myth (same with water at the bottom of the tank) - if there truly was that much stuff floating around in the fuel tank, it would get sucked into the intake every time the pump started, because it would settle at the lowest point of the tank (where the intake is) every time the car sat still for a while.
posted by dg at 4:14 PM on March 18, 2012

Seriously, it is not myth. I've no idea why the assumption is that the problematic detritus is heavier than fuel and sinks to the bottom. It's not bits of metal any more as most tanks are plastic (perhaps where the idea of it being myth comes from). I've opened up huge numbers of tanks (road and race cars) and there is always bits of shite floating in the fuel left in the bottom of the tank. The same stuff is found in the filters if you cut them open. It is a genuine, established and known problem if you run the tank dry.
posted by Brockles at 5:30 PM on March 18, 2012 [1 favorite]

Well, there's no doubt that there would be foreign matter in pretty much any fuel tank, because it comes with the fuel as a free added bonus. That's the stuff you find in fuel filters and that you see in the tank if you are able to look into it. But there's no difference in the chances of it being sucked into the intake if the tank is close to empty, because it's already being sucked into the intake all the time (which is why you find it in the filters) The fuel/crap proportion would be pretty constant, because there's a constant flow of crap in and out - it gets added when you add fuel, it gets sucked into the fuel intake and picked up by the filter constantly when the engine is running.

When you start up an engine that has been run out of fuel and where the fuel system has not been fully bled, it will run rough for a period from a few seconds at least while the air bubbles are worked through. A carburettor-fed petrol engine much much less than a fuel injected engine and that less than a diesel, because each system has a different level of tolerance to varying fuel pressures. I'm not saying that crap in fuel tanks is a myth - I've also seen plenty of that. I'm saying that running the car low on fuel doesn't increase the chances of that foreign matter being sucked into the intake. In any case, the fule filter should deal with it.
posted by dg at 10:37 PM on March 18, 2012