Gas Cap Question
Thread Starter
Racer
Joined: Jun 2019
Posts: 390
Likes: 126
From: Franklin, TN
Gas Cap Question
I either learned something about my car today or I have an intermittent problem. My 93 convertible was running great on a short road trip to a grandson's baseball game, I stopped for gas and after I opened fuel door and removed gas cap, I noticed the pump was out of order. Leaving the cap off and fuel door open, I jumped in car to move to next pump. The car started and immediately started sputtering. It died after a few feet. I restarted and same thing happened. I got out of car replaced gas cap, just stalling for time. Restarted car, it sputtered but didn't die. I moved to working pump, filled up, replaced cap. The car started fine and ran like a champ for the 50-60 miles home. I had about a half tank of gas when I stopped, so it wasn't lack of fuel or "bottom of the barrel" stuff.
So, my question is, will the car not run properly with the gas cap removed or do I have an intermittent issue about to bite me? I guess I could test this, but I am sure someone with experience knows the answer.
So, my question is, will the car not run properly with the gas cap removed or do I have an intermittent issue about to bite me? I guess I could test this, but I am sure someone with experience knows the answer.
Oil Producer
Joined: Apr 2014
Posts: 9,367
Likes: 2,735
I either learned something about my car today or I have an intermittent problem. My 93 convertible was running great on a short road trip to a grandson's baseball game, I stopped for gas and after I opened fuel door and removed gas cap, I noticed the pump was out of order. Leaving the cap off and fuel door open, I jumped in car to move to next pump. The car started and immediately started sputtering. It died after a few feet. I restarted and same thing happened. I got out of car replaced gas cap, just stalling for time. Restarted car, it sputtered but didn't die. I moved to working pump, filled up, replaced cap. The car started fine and ran like a champ for the 50-60 miles home. I had about a half tank of gas when I stopped, so it wasn't lack of fuel or "bottom of the barrel" stuff.
So, my question is, will the car not run properly with the gas cap removed or do I have an intermittent issue about to bite me? I guess I could test this, but I am sure someone with experience knows the answer.
So, my question is, will the car not run properly with the gas cap removed or do I have an intermittent issue about to bite me? I guess I could test this, but I am sure someone with experience knows the answer.
far as i know, gas cap doesnt need to be installed for car to run properly.
Thread Starter
Racer
Joined: Jun 2019
Posts: 390
Likes: 126
From: Franklin, TN
Well, that is bad news for me. I thought maybe the tank was slightly pressurized or that the pump needed the opening plugged to operate properly. I guess it was wholly coincidental that replacing the cap and the car running occurred.
Heel & Toe
Joined: Sep 2021
Posts: 15
Likes: 1
From: Aurora, IL
I had a similar question with my '86 L98 recently. Car runs great with no codes, but has always been a little "hard starting" (have to crank for 3-5 seconds before it fires up, sometimes have to crank twice). I assumed it was a leaking injector causing a flood condition, and had it marked on the project list for this fall.
I filled up with gas two weeks ago and suddenly the car is starting right up like a champ. Filled up with gas last week (always 93 octane from the same station), and we're back to long cranking.
Buddy of mine thought it might be something to do with pressure in the tank. I've never had the "hiss" when opening the cap. So I bought an aftermarket cap and swapped it out. All of a sudden she's firing right up again. No more long cranking. I'm no expert - but either its a huge coincidence, or the gas cap did have something to do with the car starting cleanly.
I filled up with gas two weeks ago and suddenly the car is starting right up like a champ. Filled up with gas last week (always 93 octane from the same station), and we're back to long cranking.
Buddy of mine thought it might be something to do with pressure in the tank. I've never had the "hiss" when opening the cap. So I bought an aftermarket cap and swapped it out. All of a sudden she's firing right up again. No more long cranking. I'm no expert - but either its a huge coincidence, or the gas cap did have something to do with the car starting cleanly.
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Thread Starter
Racer
Joined: Jun 2019
Posts: 390
Likes: 126
From: Franklin, TN
AuroraILC4, I always get a hugh hissing, pressure release sound when I remove my gas cap on either coupe or vert. That is why I was wondering whether the tank pressure was required for the car to run properly. Every poster who responded says no, so I am not trying to start an "wives' tale" but it was very curious how it recovered. Then again, I get the same pressure release sound when I open a hot 5 gallon gas container to fill my lawnmower. I haven't driven the car since so have nothing else to report. I do plan to check fuel pressure if it happens again, as recommended by Pedricd.
Drifting
Joined: Nov 2015
Posts: 1,709
Likes: 330
Finalist 2020 C4 of the Year - Unmodified
The gas cap must seal properly. My Audi actually has a sensor and warning light for the gas cap leaking air.
The leak will let air in, which has water and will be bad for the gas eventually.
The leak will let air in, which has water and will be bad for the gas eventually.
Drifting
Joined: Jun 2020
Posts: 1,355
Likes: 396
From: Northwest Ohio
I have oversimplified this as there is a vent and purge cycle where there is venting through a charcoal canister (vent solenoid) then “pulling vacuum” to clean out the vapors via the engine (purge solenoid)but hopefully that makes it clear.
If you’ve ever experienced an evap system issue it is very annoying. The solenoids can stick preventing any venting at all during fueling…this will force a very very long and slow fueling time.
This just gave me an idea…if your charcoal canister gets a bunch of fuel in it somehow (maybe faulty solenoids or some other issue? Overstuffing the tank by “topping off”?). It can cause the car to run rich as it attempts to purge the canister it will draw that excess fuel into the throttle body. Or perhaps an “open tank” can act like a vacuum leak?
Last edited by pedricd; Jun 20, 2022 at 12:20 PM.