I never would have thought....
 

It was beautiful weather in central Texas yesterday and I brought the car out for a hill country drive. Everything was running great, I was blasting through back roads and had to do a small section of I-35. After merging, I hammered past a hapless Mustang put it in overdrive waited for a gap to get around an 18 wheeler and got back on the gas...the motor stumbled wouldn't accelerate, and basically fell on it's face. Traffic speed was fine, but accelerating over 3500 RPM wasn't happening.

I got off the highway, stopped at a light, got on it again and same crap through all the gears at 3500 rpm. WTF.. I pull over, check to make sure my distributor is locked down and my timing isn't AFU...all fuel lines are secured, nothing is messing with throttle linkages.

Luckily, I have a cousin who is a hell of a mechanic and I call him up and describe what's going on. His guess "your gas cap isn't venting". I check the cap, it doesn't hiss or anything. I check my fuel pressure while running, and it's a couple pounds low, but still at 4 which should be enough. I drive home, and the problem is now intermittent. Sometimes it will haul ass all the way to redline, other times it sputters. Pull into the driveway, check the gas cap again, and this time is hisses at me when I unscrew it.

$11 at O'Rilley's and the problem is fixed. I owe my cousin a meal, because I'd have never guessed the gas cap would cause that problem. I'd have had the top end of the motor off looking for an issue before I went to the gas tank..

So, if you ever have something like this happen. It might not be your engine builder that messed up, it might be the decades old plastic cap behind you that has given up the ghost.

Lessons learned cheap are the best ones.

Thanks, Navy, I too would never have thought of t. I associate "bad" gas caps with "check engine" lights, never in a car as old as ours. Thanks for the enlightening post!

Yup, that's a fine cheap lesson that does not ruin your whole day with the car!

No sweat. It had happened to my cousin on his Monte Carlo (with a built 427) a number of years ago. Same kind of symptoms. I figured if it can happen twice, it can happen 3 times and someone else can benefit.

It was kinda gross, but I took the old cap with me to the part store. When they put the new one down, I put it to my mouth to see how much pressure it took to get the vent to open. It wasn't much. I tried the same thing on the old cap (which had dried, but still stunk) and it wouldn't vent at all.

You might want to check your vent/vapor line that runs off your tank to the front of the car to the charcoal canister. It is most responsible for venting your tank. The gas cap is not normally operating in a vented mode. It has a spring loaded valve that will relieve pressure build up inside the tank if it gets hot to prevent expanding of the tank, and it will vent negative pressure (vacuum) if the tank vent line is constricted. If the tank vent line weren't constricted, the gas cap would not have to vent. So, I'd make sure the vent line isn't plugged up by blowing it out.

The test you did by blowing on the cap, is a test of the pressure relief, but not a test of its venting, although it probably wasn't venting, since replacing it made a difference. But depending on the cap to vent should not be necessary if the vent line is working as it should. So, that's something to consider, and you may not be getting as much line venting as you should be.

One of my cars started having the vacuum sound when removing the cap. It turned out that the front of the vent line end where it connects to the rubber hose going to the charcoal canister had formed some rust that plugged the end of the line. I cleaned it out, which solved the problem.

Yes, that's how it is supposed to work. My charcoal canister went away several years ago with all the smog stuff and AC plumbing. That line has long since been cut off and RTV'd closed so that it couldn't leak fuel. The cap had always done the job, right up until yesterday.

Years ago I worked at a Cat Dealer, and we had an old fork truck with a Cat 3208 in it that would run a couple minutes, stumble, then quit. Let it sit awhile, it would do the same thing. Musta been about 8 heavy equipment mechanics huddled around this thing, and someone said "put some fuel in it, it's outta fuel. Dumped a couple gallons of Diesel in it, fire it up, does the same thing. While they're all standing there poking sticks at it, I looked at the cap which was a 2" pipe cap someone had stuck on there! Took the cap, drilled a 1/8" hole in it, and stuck it back on......problem solved!! Made me feel good 'cause I was the new guy!

Aha :) . I'm not using my charcoal canister either, but I keep the tank vent line. The factory vent line came with a loop in it back at the tank, so it wouldn't let fuel get into the line going forward, so it doesn't leak fuel. Up front, I just put a piece of fuel hose on the end of the line and aim it under the frame, where it vents to the atmosphere. That's basically how all old cars were vented prior to the use of charcoal canisters. Your car might even run better with a proper vent :) . Picture of the "loop" below.

https://farm8.staticflickr.com/7813/...61640217_c.jpgIMG_2923 by grand73am, on Flickr

Great info!! I didn't know if there was a loop or anything like that, just that where I cut the line was as low as the bottom of the tank. If I have any more venting issues, I'll restore that factory vent portion to functionality.

Great!...and thanks for sharing your story.