Drove a bit with a loose gas cap and check engine light came on; saw that one reason is that the loose gas cap means car engine system is no longer under “proper pressure” - does it repressurize itself again after I put the cap back on tightly?
No, that’s not it at all.
The gas tank is designed to be a “closed system.” Any vapors that the gasoline in the take generates are supposed to be captured in a charcoal canister, and then burned in the engine.
If the gas cap is loose, these vapors can escape, causing smog. So, the car Evaporative Emissions Control system checks that the gas tank will hold pressure, and if it doesn’t it sets the CEL.
This system has essentially no effect on the operation of the car - it’s all about pollution control.
Assuming everything is working correctly and the loose cap is what actually triggered the code then yes, properly closing it will seal the system and allow the fuel vapor capture system to work as designed. That MAY not automatically clear the trouble code from the computer and shut off the “Check Engine” light.
Close the cap, drive normally for a week or so. If the light still hasn’t shut off, you may need to manually clear the code. CHeck your owner’s manual for details.
Oh OK, so if cap is loose, then just tighten it again, and if light remains on it just needs a reset.
It generally takes a few “cycles” for the system to reset the CEL.
However, it’s also possible that there is some other problem, unrelated to the gas cap. There are lots of hoses that run from the gas tank to the engine and back - if one of these develops a leak, it will cause the same problem, and it can be very difficult to track down…
My car (2012 Honda Civic Hybrid) showed a warning that pretty literally said “Check to make sure the fuel cap is properly tightened”. (The manual indicated it might be as much as a few days before that warning went out after I pulled over and tightened the fuel cap; it was actually just a few miles of driving.)
Then a week or so later a new warning came on, but it wasn’t “check engine”; IIRC it was “check emissions system” (which came on, ironically enough, as I was starting up the car to take her to the mechanic for routine maintenance–oil change, filters, and so on). Turned out to be a defective aftermarket gas cap*. So that was an easy fix.
*Which I had bought not long ago after I drove off from the gas station with the previous gas cap sitting on the trunk of the car. Which had actually also been an aftermarket replacement cap, from the first time I drove off from the gas station with the gas cap sitting on the trunk of the car. :smack: That first aftermarket cap had worked fine for several years.
Yes. Tighten the gas cap and the light/fault should reset after a few engine cycles. You can clear the code if you have access to a code reader/scanner, but if not just let it go for awhile.
I had the CEL on and the code pointed to mass evap leak. I changed out the gas cap for a new one and waited for the CEL to go off. After repeated restarts it did not, so I just disconnected the battery cables. On connecting them again, the CEL disappeared. This was about a year ago and no CEL anymore.
As far as pressure goes, the system runs under a slight negative pressure. That’s kinda the whole point, pull in a slight negative pressure to force the fumes through the charcoal canister, thus reducing pollution.
Most cars (at least in my limited experience, our mechanic dopers may disagree) will throw the same error code for a leaky gas cap as they will for any leak in the system. Sometimes replacing the gas cap won’t make the CEL go away as the actual problem is elsewhere. There is also usually a small pump in the system, and that pump can also go bad.
That’s one way to clear the error codes. 
Just be careful. On some cars, there are procedures you need to go through when you disconnect the battery, otherwise you end up triggering the security lockout on the alarm system or the radio theft prevention system.
Ye ol local auto parts stores will also often read the codes and clear them for you for free, at least around here. They are happy to read off the error codes because they hope you will buy parts to fix whatever the error is from their store.
I personally have a code reader, so I can clear them myself. Mine is a self contained unit, but some of the newer ones are cheap and link to your phone. The phone app has all of the smarts in it and gives you the details about the error codes. I haven’t tried one of those yet. If anyone has experience with one I’d like to know how well they work.
there are 4 likely codes a loose/damaged gas cap could trigger:
P0442 Evaporative Emission Control System leak Detected (small leak)
P0465 Evaporative Emission Control System Tank Detected (gross leak)
P0466 Evaporative Emission System Leak Detected (very small leak)
P0467 Evaporative Emission System Leak Detected (fuel cap loose/off)
my Ranger has an actual “fuel cap” warning light, I think that one will light if the PCM detects an evap leak immediately after an increase in the fuel level in the tank. Or maybe P0467 triggers that light. I believe it will turn into a CEL if the detected leak persists across ignition cycles.
They still make cars with gas caps that aren’t on tethers?
Maybe it’s a Jersey thing (we’re one of the 2 states where people are assumed to be too stupid to pump their own gas, and sadly they may be right), but the gas stations here usually have milk crates or similar containers full of gas caps and are happy to let you rummage around in them if you say they forgot to put your gas cap back on. For all I know, they’re happy to let you rummage around in them even if you’re just shopping for a gas cap.
I was going to say, don’t just replace the cap, get a tether this time.
I will probably go to hell for this but both of my cars (2000 Explorer and 2004 Civic) have the CE light on and both show the same code indicating an evap leak. My mechanic said tracking the leaks down can be expensive and the performance isn’t impacted.I use a code reader to reset the lights for inspection every two years. They come back on after a few days and stay on until the next time. YMMV, mine doesn’t.
The original gas cap had a tether, but after a few years it wore out and broke (it was just a little plastic string). None of the replacement gas caps have had them–maybe I should have splurged and paid whatever extra it costs for the replacement cap with tether, but I don’t recall ever even being asked.
That’s interesting.
You must have a particular lax inspection system. The car will report that the EEC system is “not ready” after a reset, and will not report a pass/fail condition until after several cycles have passed. Out here, they will fail you if the system is reporting “not ready” - to prevent this type of shenanigans,
My Ford Escape doesn’t have a gas cap. It was designed so that a gas cap isn’t needed.
interesting
[quote=“bardos, post:17, topic:845322”]
interesting
[/QUOTE]Scotty Kilmer’s take on it
My '18 Acadia is the same way. The gas lid just snaps shut over the gasoline tank fill hole. What is annoying is you can squeeze in another $15+ of gas once the nozzle fill thing shuts off.
[quote=“dougrb, post:18, topic:845322”]
Scotty Kilmer’s take on it
[/QUOTE]I can’t understand how that guy gained an audience. he’s pretty much considered a joke in the auto enthusiast community.