With Limitless Fuel, How Long Could My Car Idle?

Factual Questions
1

So I sat in a parking lot today for about three hours, waiting for a client to come out of the dentist’s office. Since it’s kinda cold, I had my SUV running to keep the heater operational. This got me to thinking…

If I had an infinitely large gas tank, how long could my SUV (or any passenger car/truck/suv/van/etc.) idle, and what would cause it to finally stop? I assume that at some point it would run out of coolant and the engine would overheat. How long would that take (assuming that the weather isn’t exceptionally hot)?

Does the vehicle have to move through the air a certain amount time during operation to keep the engine cooling system working? With infinite coolant, could it idle forever (or at least until the engine oil broke down)?

2

The radiator has a fan that will kick on when your not moving enough (or when it’s really hot). The coolent shouldn’t run out. If it’s a closed system, it really shouldn’t, unless there’s a leak. Even if it’s not closed, it should dump whatever it needs to into the overflow tank, but then it should stay consistent. The problem I would think, is that eventually the oil would get dirty/break down and the engine would cease, but I’d imagine you’d have to idle a VERY long time. Like weeks maybe longer.

3

I suppose there could also be a problem with a clogged fuel filter or injectors (or carburator). But now that I think about it , I think that before any of the things I mentioned, the air filter would get clogged. But it would still take quite a while.

4

How long before the gas tank runs out (I’m taking away the infinite fuel conditions)? Days? Weeks?

5

Depends on the size of the gas tank. According to this page:

If we split the difference and call it .75 gal/h, a vehicle with a 15 gallon tank will run out in 20 hours.

6

In a gas car, probably that day. But I don’t have a cite.
Based on instrumented research, however, if you had a diesel car, probably a matter of a week or two. A 1980 Mercedes sedan with a diesel engine idles comfortably on a gallon of diesel fuel oil per day.
I’d like to point out that another potential culprit in your experiment would be your crankcase filling up with gasoline. Most cars run ever so slightly rich at idle, and I suspect that if you idled with an “infinite” gasoline reserve, your crank case would get more and more contaminated with gas.
Gas is not good for motor oil. It promotes oxidization, and lowers the heat tolerance of the contaminated motor oil/gas mixture.
The question is, would your oil stop lubricating properly first, or would your crankcase fill up to the brim with gasoline/oil, which would then get beaten into a frothy mixture by the motor (which, after all, is never supposed to be filled TO THE BRIM with motor oil).

7

What happens to all that gas normally? Does it just not fill up fast enough that regular oil changes get rid of it? Does the PCV vent it out? Or is it a small enough amount that it just burns off on the cylinder walls (and then I assume it would be vented out through the PCV?

8

Just going to remark that two months ago I saw a 1989 Ford Ranger with 118,000 miles on it have its ORIGINAL air filter get swapped out. The air filter was ugly, but there was no obvious difference between the performance of the truck before and after the vehicle’s fitler was changed.
Assuming 35 MPH average speed, that was 3,371 hours of operation. Unless that Ranger was atypical, we can assume that air filter clogging would not become a problem in less than 140 days of operation.
Man, if I was rich I swear I would buy a new Honda Civic and a 1,000-gallon tank of gas (to be refilled on a regular basis) and do this experiment.

9

I suspect it would. Gasoline has a fairly low boiling point of around 100 [sup]o[/sup]F, while the oil temperature in the crankcase runs around 200-250 [sup]o[/sup]F, depending on a variety of factors. The idle oil temperature is probably somewhat lower, but still hot enough to boil off any gasoline that finds its way in. Also, any significant gasoline leakage into the crankcase is likely due to a problem, such as a perforated fuel pump diaphragm. A normally-operating engine won’t accumulate any measurable amounts of gas in the oil.

10

Any gasoline in the oil will burn off fairly quickly. There are even some engines that are equipped with an oil dilution system for cold weather use. You inject gasoline directly into the motor oil to thin it for starting, and the gasoline burns off later when the engine warms up.

11

I would also vote oil break-down to be the biggest issue.

Idle keeps the engine cooler than normal. Without the higher heat created when the engine is under load, the oil system will get dirty as varnish develops inside the engine (still not very quickly but it will). Eventually the oil will get very thick and black. Eventually the engine will seize.

How long though? I’ve seen cars go 10,000 KM without an oil change (I used to work at a local oil change place) and the engine kept running.

I would probably say at least a few months before the engine is in trouble. With a by-pass filter (changed monthly) I would assume an engine might run until metal fatigue snaps something (maybe decades).

12

Several years ago, I worked for a Texas company constructing oil/gas projects in North Dakota and Minnesota. None of our vehicles were particularly suited for the climate, ie engine block warmers, etc., and we had trouble getting them started in the mornings.

We rented a Ford LTD, and essentially left it running from November until April using it to jump start as necessary and for general running around in. We did shut it down on Fridays for a weekly oil change, but it ran constantly aside from that. We had to fill the gas tank daily. There was no discernable wear or problems that we noticed.

13

As Otto from “The Simpsons” would say… COOOOOOOOOOOL! :smiley:

14

Depends on where you parked it. I know neighborhoods in my city where it would be, as the movie said, gone in 30 seconds.

15

I’m pretty sure an infinitely large gas tank would collapse under its own gravity into a black hole. You and your SUV would be crushed out of existence. This would happen pretty quickly.

16

Motor oil can and will build up a substantial deposit of gasoline. Values above 2% have been noted.
Used Oil Analysis Cite with .5 % Fuel Dilution: http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=3;t=001311

I certainly wish the gas in the oil did burn off, but it appears to stick around long enough to do damage.

17

Found a cite I was looking for. It basically notes the following two data points:
A) “Normal” cars can and will build up around 2% gas in the crankcase oil sometimes. Our hypothetical “infinite fuel” vehicle will probably pass that at some point.
B) In oil analysis, values are usually flagged for you to change out the oil when 3% of your motor oil has become gasoline.
Cite: http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=3;t=001080

Finally, if you don’t idle much, you may have lower dilution numbers than 2%, even with higher mileage. Here’s some oil with 17,400 mostly highway miles on it, and under 2% fuel.
Cite: http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=get_topic;f=3;t=000558

18

I think you’re going to have electrical problems before the oil breaks down. I’m not convinced that a typical alternator puts out much current at idle. Seems to me that the battery would slowly discharge until there wasn’t enough voltage to run the electronic ignition or fuel pump.

19

Yes, but the point your cites make is that significant fuel present in crankcase oil is indicative of a problem–most likely with the PCV system. None of them explicitly claim that a significant percentage of fuel is normal. Even the first cite that shows .5% gasoline in an analysis of crankcase oil said, “We again found a little gas in the oil from your Passat, but we found even less this time than last. It will probably disappear entirely in the next sample.”

20

I think the OP is asking more along the lines of how long the engine might last if just idling, rather than the car itself- hence the assumption of an unlimited supply of fuel.

Now, if you simply parked a brand-new, modern electronically fuel-injected car next to a massive storage tank of gasoline, connected a small hose to keep the car’s tank full, and just left it to idle, that car could very likely idle for decades.

It’s first failure would likely be external to the engine- as noted, perhaps the alternator or AC compressor fails, or one of the sensors. It’s first internal failure would likely be a relatively minor ancillary part, like the water pump. (Either of which might last the equivalent of 100,000 to 150,000 miles.)

A modern EFI car with an oxygen sensor and such, would greatly minimize oil fouling (since the A/F ratio would be kept near-perfect) and deposit buildup.

Up on the North Slope, the oil companies have fleets of diesel trucks that, in some cases, have been running constantly save for an occasional oil change, for years. And that’s running and being used, so the car, if just idling, would last… well, probably a lot longer than you want to wait around and watch.

Now, if you did a sort of labratory experiment, and put the same new car engine on a test stand, and connected it to external coolant and fuel, removed as many external accessories as possible (smog pump, water pump, etc) could swap out any sensor that fails, as it’s running, and used an external oil sump so the oil could be carefully filtered and changed, also while the engine is still running, that engine could, concievably, run essentially forever.

With a pressurized oiling system, the only true metal-to-metal contact is the occasional piston-ring-to-cylinder-wall contact, and even then usually only at peak cylinder pressure. The rest of the time, nearly everything is running in an oil-bath, and there’s basically no actual contact between moving parts.

That’s not to say parts won’t fatigue eventually, but assuming the OP’s “just idling”, that minimizes the strain- parts designed to withstand sustained 8,000 RPM doing a mere 700 rpm, etc. Basically you’ll be waiting for a small part to fail first- a clogged injector, a valve spring breaking, etc.

To answer the OP: Given unlimited fuel and good conditions, yes, that SUV could easily idle for several years.