Some cars seem to get upset if the gas cap isn’t firmly tightened. I mean, why should it care? It should be able to pull gas out of a tank if there is air on top of the gas. But maybe it’s a fire hazard?
It’s an air pollution source. Gas fumes contribute to smog.
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Just to add to @texases reply, gasoline is volatile. It evaporates and creates a lot of fumes. (And yes - those can be a fire hazard, but that’s not the point). The fumes are pollutants. All cars since…IDK - the 80s? 90s? have a system to collect gas fumes, store them, and send them to the engine to be burned off with the liquid gasoline.
So these days cars have systems that check to make sure that the system collecting those fumes aren’t leaking. Loose gas cap = leaking gas fumes. (And yeah, ever see those “no smoking” signs around gas station pumps? KABOOM. If you ever see gasoline ignite it’s the fumes that go KABOOM).
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As do some people.
Some people don’t care so the car has to do it for them.
Among other things…
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OK, but the point is that it’s not about the performance of the car. 'Nuff said. Now, gimme a break. If the cap is on, but isn’t completely secure, there will be VERY little pollution emanating from it. We’re talking about the car complaining if the seal isn’t perfectly tight. That makes little sense.
If it’s leaking, the pressure goes to zero and a warning light indicates that the system isn’t working.
You clearly don’t care one bit about the environment
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The car is not able to distinguish between a small leak that would be okay with you and a larger leak that even you would want repaired. It just flags EVERY leak.
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Don’t tell me. Lemme guess. Your check engine light (or “service engine soon” light) came on. And some nimrod said something uninformed like “oh, that’s just a loose gas cap.” - ??
Why don’t you tell us your “story.” Start with “my vehicle is [year/make/model] with [xxx,xxx] miles on it and the maintenance history is [xxxxx]”
“And then some warning light came up on the dash, and …?”
Fill in the blanks.
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The evaporative emissions controls for fuel require a sealed and PRESSURIZED system. A loose gas cap doesn’t allow the system to pressurize and that sets the flag to light up the “check engine” light. Tightening the gas cap and driving the car normally for a few days will reset it, if you don’t have a scan tool to check the code and clear it.
Sometimes the gas cap gasket gets old and dry, so the seal isn’t good and that also causes issues. A new gas cap usually solves that issue quickly.
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Wrong. A slightly loose cap allows lots of vapors to escape and air to enter as the car heats up and cools off day and night.
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It makes perfect sense . The system is not doing what it is supposed to do that is why there is a warning light.
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Fair point, but as mentioned above the car’s computer is responsible by government mandate to self-test all of its pollution-control equipment, to verify it is all working correctly. The test isn’t sophisticated enough to differentiate between a minor leak that isn’t of practical concern pollution-wise , and a more serious leak that is a pollution concern. It’s like the readiness monitor tests that prevent re-registering the car, could be borderline ridiculous and definitely expensive to fix, but we car owners have to fix it anyway.
All of them are pollution concerns and there are different codes for small (P0456) vs large (P0455) leaks. IDK how that “math” is done. But basically a vacuum is put on the system (as far as I know) - a small leak will probably hold some pressure for some amount of time while slowly bleeding off and the ECU can see that in the sensors. A large one will hold no pressure, or - I’d presume - leak it out within some shorter period of time. The details would be in the math, I suppose.
P0455 is a large leak, P0456 is a small leak (less than 0.020")
A loose gas cap can result in more hydrocarbon emissions from the filler neck than you would find coming out of the tailpipe. Does that make sense?
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