Fuel Cap Hiss?

[jspringator]

Should the fuel cap hiss upon loosening? I am worried about the defective fuel cap thing talked about on this board.

[whatnot]

It depends on it it is pressure or vacuum. If pressure, it shouldn't hurt anything. Last time I got fuel, the cap flew right out of my hand from the pressure.
If you have pressure, it is probably caused by temperature changes. When it is cold outside, it will cause a vacuum and the cap should let in air. Then when it warms up, the fuel expands and you get pressure.

[gmctd]

In warm weather the fuel tank(s) have slight pressure stored, as result of Diesel fuel volatility and heated fuel-return from the engine. You'll notice more pressure-release volume when fuel volume in the tank is low, as when you're re-fueling. You can also note less pressure-relief volume if the cap is opened soon after filling the tank, as when you arrive back home.

The Diesel cap has an excess pressure relief, but maintains a low pressure to assist the lift pump, and prevents drain-back, which could cause air bubbles in the fuel supply line to the injection pump.

The cap also has vacuum relief for cold weather conditions. The vacuum relief serves when warm fuel is pumped out of underground tanks into the vehicle tank, which, at ambiently cold temps, results in fuel volume contraction. The cooling fuel can contract quicker than the fuel-return system can compensate for, resulting in increasing vacuum.
The engine-heated return fuel will also be contracting as cooled by ambient temps. Even slight vacuum in the fuel supply line is definitely not good for the injection pump.
You probably will never notice the vacuum relief unless the cap is defective or plugged from environmental conditions such as rust, mud, salt, ice, etc.
In that case, tank vacuum could make the fuel cap difficult to remove, not to mention difficult to diagnose starting\running problems.

Some guys here have defeated the Diesel-service reliefs designed into the cap, which results in the tank and contents being more exposed to environmental conditions. Not a good idea, I would think.

jd

[triggerman]

Soon after I bought my 97 a couple of years ago , the truck would not start in the mall parking lot. I had run it fast befor I shut it down with no probs. Had it towed to the shop, replaced the lift pump, all was ok again . When the truck was dead in the lot I happend to loosen the fuel cap and found one hell of a suction VAC under the cap. later I took the stock cap apart and found that it was built to only let pressure out of the tank with a one way valve. I went to Auto Zone and bought a replacment for a 97 gas job. This cap has a yellow two way valve built into it. I found it took considerable vac to open the relief valve so I poped the valve apart, cliped a coil off the valve spring, snaped it back together and now I feel better ! Some here will tell you just drill a small hole in it. but you will find foamed fuel WILL make its way out on to the side of the truck if you like to fill ALL the way up like I do.

[gmctd]

The cap for GM Diesel service will be marked as such. Older caps where the paint has been worn off can still have the 'Diesel' imprint from the hot-stamp.
It is very important to ensure the cap is not a standard gasoline cap, which is identical in appearance, but not function.
The single valve can also relieve vacuum, much as the radiator cap vents at 16psi, but also reverse vents as the coolant contracts when the engine is off.
As in the radiator cap, rust\trash may prevent vacuum venting, but the pressure relief can still function.
Some manuals indicate regular intervals for fuel cap replacement.
Triggerman's experience would also be valid indication for replacement. smile.gif

jd

[triggerman]

Oh, I forgot to mention the guy that posted here about the imploded fuel tank he experianced. I guess his one way diesel cap should have been replaced. Look, I know what a one way check valve looks like. And I firmly beleive the cap on a 97 will vent pressure ONLY. Its really very simple, take one apart and see for your self ! As for the "gas cap", I'm very happy with a very light pressure/vac relief when I loosen the autozone cap. It sure beats the whooosh of that OEM one wayer! And I'm sure my lift pump likes it too. :cool:

[gmctd]

Cool...got my Stant matched locking pair from Autozone, also.
And, I had a collapsed tank experience.

Pristine piece, no rust, no stains inside to indicate water had ever been there. Clean.
Sealed it off with those little rubber caps used for vacuum ports on carburetors and such - guaranteed not to leak. Locking non-vented fuel cap....oh yeah!
No way was Mother Nature gonna ruin that unit before I had a chance to install it.
Yeah, right! Who knew the temperature was gonna drop 40deg in thirty minutes - mid seventies to mid thirties.
Sure was a shock to Houstonians, leaving the house dressed for 75deg weather. Texas winters are very mild, don't you know.
The tank? Looked like it had been worked over by Mike Tyson.
But, I'll warrant one thing - bet it did not have one flake of rust on it when the guy pushed the START button on the crusher.
Hmph! It don't pay to mess with Mother Nature.

jd

[norm]

I got a cap (a stant I think) from the W.A. store. Generic listed diesel the same as gas. I have never had pressure (remember diesel is nowhere near as volitile as gas) but have had vac. Am I doomed? Is there any definitive answer on what fuel cap to use?

[gmctd]

Dunno about 'doomed', but vacuum is bad, pressure is better, from a fuel dynamics standpoint,
The mechanical lift pump lowers its inlet pressure, and atmospheric (or tank) pressure pushes fuel into the inlet.
The electric lift pump is an impulse type, similar in function to the mechanical pump. At some point in the fuel supply system, atmospheric pressure is gonna rear it's ugly head, and try to push the fuel back into the vacuumed tank, causing minimal or no flow.
Should result in a condition similar to vapor-lock - no flow.

Was a running argument on here a few years back, where about half claimed vacuum, half claimed pressure. Seems like some of each group just punched holes in their caps, and the thing faded away.
My brother (several million miles, and counting)sez his eighteen wheeler tanks never show vacuum or pressure at cap off.
Mine always show pressure - not much volume when adding conditioner to full tank, more volume when tank is low. Appears to be same pressure level, just more or less volume.

Texas Gulf coast winters are very mild - seems like the vacuum group were mainly in Northern climes, where deep cold is the winter norm.

(Hmmmmm! Fleeting thought - maybe my caps are bad, and I'm doomed....... Nah! Perish that thought!)

What's the answer? Who's to say.

My caps appear to function correctly in this locale, and experience is the forge for tempering opinion, so I'm open to a definitive answer.

jd

[ 12-05-2003, 03:17 AM: Message edited by: gmctd ]

[StephenA]

Read a post in the last 6 months where one member had collapsed his tank and fuel lines flat by driving with a clogged fuel cap. Makes sense - the IP has more that enough strength to do it. He paid big bucks to replace everything, not to mention the wear & tear on the IP. The vent in the cap is not just to offset hot fuel cooling down or altitude changes. Air's gotta go in if fuel goes out. Nature abhors a vacuum... well, at least thanks what my bank says. :D
Pressure is ok- the vent holds it in. Vacuum = clogged vent. Clean off the cap, put your lips on the inside threads & blow. Should seal. Now suck air in: should allow flow in. If not, repair or replace pronto. Leave it off until fixed.

[gmctd]

Here's a possibly almost definitive
answer :

:http://www.stant.com/brochure.cfm?brochure=1252&location_id=170

It appears to be a very definite maybe. smile.gif

jd

[gmctd]

Not to flog a dead horse, but I always do an oral vacuum check on a new vented fuel cap before putting it into service. I know what can happen to a good tank with a non-vented cap, and at 25 bucks a pair, I'm not about to take any chances.
The results are not scientific, but workable.
And, new doesn't leave a bad taste in the mouth.

Just got back in the house from doing a scientific check on all five diesel gas caps.

I used a hand vacuum pump intended for testing various and sundry items in the fuel supply and emissions schemes on today's modern vehicles.
It's of oriental manufacture, small and precision in appearance, precise in function.
(To all us shade-tree type guys, that means accurate, but I like to use those 'professional' terms in case some engineer reads the post) ;)

Each cap, all Stant brand for Diesel service, would vent at 2-3" initial pump-up, but would not hold greater than 1" on succeeding pumps.
Stop pumping, and the vacuum would slowly bleed to zero.
27" = 1 psi

A question was raised, where the tank always(?) shows vacuum when the cap is removed, with concerns about being doomed.
Good question, by the way.

With a fully functional fuel cap, 1" vacuum should not be enough to collapse even an empty fuel tank.

So, a quick check can be accomplished by taking a glass of milk and some paper towels out to the truck, removing the fuel cap, wiping the Diesel fuel off the threaded portion with the towels. (save one - you'll need it later, for the milk procedure)
Put the end of the threaded portion in your mouth, and draw thru like it was a really big straw in a thick milk shake.
If it doesn't vent, replace it. Not necessary to turn purple in your efforts. The cap makes a distinct venting noise, new or used.

The milk? You'll probably want to rinse the oily taste from your mouth - milk works for me, but brew or mouthwash is also an option.

Enjoy. smile.gif

jd

Guess I should edit this to include the prerequisite delimiter.

PRE-REQUISITE DELIMITERS:
Check you owner's manual - should inform you if your truck requires vented or non-vented fuel cap.

Note: the cell-phone has become the greatest boost to replacement fuel cap sales (ever notice how often you'll see an orphaned fuel cap placed on top of the pump at the station?), and sticker shock (what??? 25 bucks a pair for a simple fuel cap?????) is why you'll often find the cheapest fuel cap available on the used truck you just purchased.

[ 10-14-2003, 03:26 AM: Message edited by: gmctd ]

[StephenA]

Nice article on link posted above! (-Gives history of fuel caps). Thanks!