fuel filler cap

[c_hasbeen]

Are the filler caps supposed to be vented?

I am asking because my bus came with out the filler cap, I have it covered with a rag and tape and it is inside the door.
I was wondering if a regular pipe cap would work or if I need to go to a truck dealer to get a replacement.

thanks

[crazycal]

Mine are brass. You might be able to pick one up at a truck parts place.

[Alan N]

Originally Posted by c_hasbeen

Are the filler caps supposed to be vented?

I am asking because my bus came with out the filler cap, I have it covered with a rag and tape and it is inside the door.
I was wondering if a regular pipe cap would work or if I need to go to a truck dealer to get a replacement.

thanks

Your tank is most likely vented.
A pipe cap will work fine.

[superdave]

my cap has real fine threads, not npt. be carefull not to ruin the threads with the wrong thread.

[c_hasbeen]

yea these do actually look fine threads. I will go talk to them at kenworth on Tuesday

[nat_ster]

I wish you lived closer. We have a ton of scrap buses. I could have given you a fuel filler cap.

Nat

[c_hasbeen]

Originally Posted by nat_ster

I wish you lived closer. We have a ton of scrap buses. I could have given you a fuel filler cap.

Nat

The funny thing is I called the guy who I bought this from because he said he had a bunch of old scrap buses but he would not go look for one.

I will find one close by, I am sure

[superdave]

i stand corrected, my 96 thomas gas cap is 2" npt, no vent stamped on it.

[cowlitzcoach]

Short answer, some are and some are not vented.

Putting a vented cap on a tank that is vented can cause some issues.

Putting a non-vented cap on a tank that is not vented can cause other issues.

Just make sure you put the correct one on as it does make a difference.

Also be aware, some are male thread and some are female thread.

[bansil]

crawl under bus and look at tank, see if there is a rubber line coming from top of tank and draped over the side, I have actually pressurized my tank to a couple pounds using a tapered plastic fitting for inner tubes and air mattresses, stuck in that vent hose when filling new filters and bleeding injector lines (do not over pressurize!!!)

[HolyBus]

Originally Posted by bansil

crawl under bus and look at tank, see if there is a rubber line coming from top of tank and draped over the side, I have actually pressurized my tank to a couple pounds using a tapered plastic fitting for inner tubes and air mattresses, stuck in that vent hose when filling new filters and bleeding injector lines (do not over pressurize!!!)

As I read the first portion, I thought you were talking about filling inner tubes and air mattresses from the gas fumes in the tank. I can picture that. Not good.:icon_e_surprised: Glad you clarified.

[bansil]

:icon_e_ugeek:

what is this emoticon?

A geeky, colored glasses goatee wearing...HippY , or maybe a techy guy ????


still kind of cool, so I will use it....Random-ness interwebz.....rowdy-ness

[c_hasbeen]

I was not able to find a tube hanging over the side of the tank, i called a couple of truck places here in town but they had no reference book.
I put a regular pipe cap on it.

If it is supposed to be vented cap will I notice a vacuum if it is running and I hold my hand over the tube?
maybe a piece of plastic bag?
thanks
Cliff

[Redbear]

Originally Posted by c_hasbeen

. . . I put a regular pipe cap on it.

If it is supposed to be vented cap will I notice a vacuum if it is running and I hold my hand over the tube?
maybe a piece of plastic bag?
thanks
Cliff

If it is supposed to be vented but is not, I don't think the fuel draw will be fast enough to feel suction on your hand, but I may be wrong. But once the engine pumps enough fuel out, there will usually be a vacuum that will prevent drawing any more and seem like you ran out of fuel.

As a result, you open the cap and see there is indeed fuel, put the cap back on and voila!, the engine starts and runs again until it doesn't. My snowblower lost a vent spacer in its cap this winter, and with the cap on tight it was pulling this trick until I figured it out.

On the sheet metal passenger car tanks, I have heard that with a strong enough fuel pump the vacuum can collapse the tank instead of starving the engine. But the heavy tank on a bus should be strong enough to resist this.

If the engine will run for an hour with the un-vented pipe cap on the tank, you should not need to add a vent and be good to go.

If you have any doubts, a pinhole in the pipe cap would relieve the vacuum. I'm guessing that 1/8 inch or even smaller might do the trick. I would maybe even start with 1/16 inch. If the bus idles and cruises fine, but seems to starve going up long hills unless you take your foot off the throttle, you might gradually enlarge the vent until you have continuous power at all loads.

[bansil]

Sounds like great advice Redbear

[Redbear]

I forgot to add that the effect is greater when the tank is full of fuel, and not nearing empty. If a vacuum is formed, a near-empty tank would try to expand a lot of air just a little bit, and may not cause fuel starvation.

A vacuum caused in a near-full tank would need to expand the little bit of air a lot, and cause a multiplication of the pressure drop and its effects.