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What damage can be done by leaving gas cap off?

MoparGuy68

MoparGuy68

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Today I put gas in my car at a gas station for only the second time. The gas filler is behind the license plate. Haven't had a car with this configuration in over 30 years. So, what idiotic thing did I do today? Yep, after pouring in some ethanol treatment, then pumping my gas, and trying to be meticulous about the process, I forgot to put the gas cap back on the filler neck.. I left it sitting in the trunk pan, and closed the trunk lid..

I cranked the engine to leave the station, and it backfired through the carburetor (first time I've ever heard it backfire). Cranked a second time and it started up, but sounded weird. Carb was making a loud sucking noise, that I'd never heard it make before. Made one lap around my neighborhood then drove home. Pulled in the garage, opened the hood, and heard this weird sucking noise coming from the carb. Shut the engine off and wondered what the hell was causing this. I walk to the back, open the trunk to retrieve my ethanol treatment bottle, and I see the damn gas cap sitting on the trunk pan.. And figured I just found out why my carb is making strange noises?!?

What do you think leaving the gas cap off did to my carburetor, and or fuel system? I have the Hemi style fuel separator/filter with a stock mechanical pump. It was getting dark when I got home, so I've not had a chance to run the engine, or drive the car, after putting the cap back on. I'm just wondering what terrible things can happen if you stupidly leave your gas cap off on these cars. I think this is the first time I've EVER done this. Never did it on my Challenger. I'm not used to the gas behind the license plate, and frankly I don't like it located there. I'd rather have it on the quarter panel.
 
I've driven off with the gas cap off (gas station attendant's fault) and didn't notice any engine change. The only difference was a long stripe of gas following me down the road. My gas tank is vented so it wouldn't change any internal pressure.
 
They aren't related.... Worst thing leaving the cap off did was cost you a couple gallons of fuel that poured out as you accelerated...

Weird noise under the hood, look for a vacuum hose that popped off when the engine backfired...
 
1 Wild R/T said:
They aren't related.... Worst thing leaving the cap off did was cost you a couple gallons of fuel that poured out as you accelerated...

Weird noise under the hood, look for a vacuum hose that popped off when the engine backfired...
Click to expand...
Was thinking that as well. Or maybe a blown power valve in the carb if it’s an older Holley. Though I don’t know for sure if it would lead to the sounds the op is speaking of?
 
BLK 68 R/T said:
Was thinking that as well. Or maybe a blown power valve in the carb if it’s an older Holley. Though I don’t know for sure if it would lead to the sounds the op is speaking of?
Click to expand...

Doubtful... I've popped a few power valves, it will load up a little but not so much that you hear it.... Plus it would have to be a pretty old Holley, they started adding the check valve to prevent blowing the power valve around 1990...
 
Did your check fuel cap light come on?

In all seriousness, don't worry about the cap; it won't affect anything. What are the timing settings for your engine?
 
BLK 68 R/T said:
Or maybe a blown power valve in the carb if it’s an older Holley.
Click to expand...
:thumbsup: exactly.

Option 2: I have had this happen once - if the backfire was serious enough, you may have blown your 'Turkey pan' - Intake gasket. A 440 can generate a lot of pressure - and it's enough to blow that pan. Tell tale sign will be oil around front and back of the pan - under the intake. If you jack the car up and wait a few minutes, it will run downhill. Or, just push a clean rag under the intake manifold with a flexible rob, then check how oily it is. A decent split in the pan will snag the rag.
 
I have NOT run the engine since I discovered the gas cap in the trunk. It was dark by then and I was ready to close up the garage for the night. It will have to wait till next weekend. Then I’ll find out if the problem still exists. Been too busy with work this week to mess around with the car.

The backfire was not huge, but it made enough noise that I heard it. The loud not normal sucking noise that I heard after the backfire, was at its loudest at idle speed. When I gave it throttle and increased the RPMs the noise seemed to lessen. Maybe it is a vacuum leak I don’t know. I couldn’t tell in the dark.

Something happened, after I pumped the gas and left the cap off. When I pulled into the gas station it sounded normal. Started the engine to leave and then it sounded fucked up. I only put 4 gallons of 91 octane gas in it. After pumping the gas the tank was about 1/3 full.

383 Magnum, Holley Carb 750cfm dual feed 4160 (the one that only has a metering block only the primary side).
 
Mopar Hunter said:
I’d be more worried about the ethanol treatment than leaving the gas off.
Click to expand...
What’s wrong with ethanol treatment? Lucas Ethanol Fuel Conditioner with Stabilizers. I put about one and a half ounces in the tank, before I pumped the 4 gallons of gas. The previous owner was always adding some of this stuff when he got gas and suggested I continue doing it. I’ve had the car for over a year and never had any problem like this until I went to the gas station on Sunday.

He was also putting some Seafoam in the tank, which I have not been doing..
 
It's second nature to me when fetching fuel for Fred, that whole routine of having to pull the tail
of the GTX a little past the pump - then the whole gruntin' and groanin' thing of squatting down
with a live fuel nozzle and heading towards that filler...

I've been doing that boogie for decades now, so the several year gap of time when Fred was in
pieces didn't dull that almost reflex action in me - I fell right back into the routine first time back
to a pump when the car was roadworthy.

It's like riding a bike, I suppose....
I take the filler nozzle and use it as a makeshift prybar, pulling down and holding the license plate so
that my left hand can grapple with the gas cap - then using the gas cap itself to hold the license plate
down/partially open (as a wedge) and off the pump nozzle (and my own mitt) as it pumps.
Reverse the procedure when fueling is over....easy peasy.

Point is, it's not a big deal, certainly nothing to fuss over - in fact, I can recall days way back, when
travelling in a state that mandated a station attendant do the fueling, watching with amusement
as the young lad searched in vain for where to put the gas in the car. :)
They'd have a hell of a time with a mid-50's Chevy, eh?:lol:

Point also is, it has never occurred to me to involve opening the trunk at refueling time - for anything.
The two actions are mutually exclusive usually...

Last point is - there's zero chance of me ever forgetting to replace the gas cap when done.
Given my age, that's a good thing. :thumbsup:
 
MoparGuy68 said:
The backfire was not huge, but it made enough noise that I heard it. The loud not normal sucking noise that I heard after the backfire, was at its loudest at idle speed. When I gave it throttle and increased the RPMs the noise seemed to lessen. Maybe it is a vacuum leak I don’t know. I couldn’t tell in the dark.
Click to expand...
I'll bet you find a vacuum hose has popped off from the backfire.
 
Big Bad Dad said:
Well, how did it sound and run after you put the cap back on? Make any difference?
Click to expand...
I started the engine, for the first time since I began this thread, today. Passenger seat installation and heavy rain kept me from doing it last weekend.

Car had been sitting for two weeks after the gas cap incident. I injected some gas into the primary float bowl first because it had been sitting. But still had to do a lot of cranking before it fired up. Much more cranking than in the past when sitting for only two weeks.. When the engine did fire up there was no backfire and the carburetor sounded completely normal. Just like it did before I ran the engine with the gas cap off. I drove the car today, engine sounded normal. Only thing that was different from two weeks ago was running it with the gas cap in place. I did absolutely nothing to the carburetor or engine since two weeks ago.

I checked the vacuum hoses and plugs. Nothing had blown off, everything was in place like it is supposed to be.

It sure does appear that running the engine without the gas cap had some kind of bad affect. Put gas cap on, engine sounds normal..

Gas tank was replaced in 1999 and the engine has a Hemi fuel pump and Hemi vapor separator/filter. How the tank vent lines are configured I have no idea, haven’t looked into it. All I know is I put the cap in place and it’s running normal again. If the strange sound returns with the gas cap in place, I’ll let you guys know.
 
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