Classic Mustangs List Archive

This read-only message was archived from a public mail list.
Mail From: jsanti2 (jsanti2)

Hi All (and Mark esp.),

I was interested in running down some of the diagnosis/repiar of the problem
you ran into with the gas cap. My `72 has/had the same symptoms (only the
vacuum seemed to build a bit quicker and release less, so the stalling became
progressively quicker). I would have never suspected the gas cap, but after
your response I think that may be my culprit--I have an aftermarket cap (one
of the billet ones...now we know why they call them "trick" eh!), and after
reading your post I eased it off and sure enough a long slow (and loud) hiss
of air resulted. After starting her up with the cap off, she ran well at first
(as usual), but I immediately found a fuel line leak--my guess is that the
systems pressure from the vacuum was keeping a tight seal on the line until it
was released. I haven't gotten around to fixing the leak.

Anyway, I'm wondering if there is something I can do to keep using the new cap
(if that's the real source of my problem). Does anyone know how the stock caps
are vented, and can it be reproduced on an aftermarket cap? I was wondering if
there was somewhere I could drill a small hole, or maybe dimple the filler
tube mouth? Any thoughts? Thanks in advance.

Best, -john

>----------------------------------------------------------------------
>Date: Fri, 30 Jul 2004 02:20:12 +0000
>From: "mark thispot" <(email redacted)>
>Subject: [CM] RE: Fuel starving (was RE: Coil failures)
>To: (email redacted)
>
>I had a hair-pulling stalling problem a few years ago that drove me nuts for
>about a year. My car would run perfectly until I had it on the road for at
>least 45 minutes. Around that point it would slow and stall. It could not
>be started for about 10 minutes at least. The in-tank pump was the first
>thing I replaced but eventually I'd worked on the entire fuel system & most
>of the electrical. Finally I happened to mention this weirdness to the
>right guy who once had a similar problem with a motorcycle. A kinked fuel
>tank vent would not allow any air in to replace the gas that was consumed
>when driving. The fuel pump would struggle against the growing vacuum until
>it just couldn't cope any longer & the car would die. The 10 minute wait
>would allow enough air to seep in to let the car start. The tip-off was
>noticing that when I would sometimes stop on the way to work to gas up there
>was a big whoosh of air into the tank when I unscrewed the cap. On short
>triips there was never enough vacuum created to cause a problem. My commute
>to work is about 50 minutes so this happened many times to me & destroyed my
>trust in the car.
>
>The gas cap on old Mustangs is vented to allow air in (except certain
>California-emissions models) but if it's been replaced with some universal
>aftermarket unit it might not work as needed.

This read-only message was archived from a public mail list.
Mail From: w427 (David)

The gas caps in rear mount positions are special "Surge" caps that don't allow fuel spillage under hard acceleration, but do allow venting in both directions. My cap takes a surprising amount of suction to vent towards the tank side and extremely slowly to the outside. I suppose something could be rigged with either one-way vacuum units or cannibalizing a surge cap. Let us know what you end-up with!
HTH,
David

*********** REPLY SEPARATOR ***********

On 8/16/2004 at 11:28 AM jsanti2 wrote:

>Hi All (and Mark esp.),
>
>I was interested in running down some of the diagnosis/repiar of the
>problem
>you ran into with the gas cap. My `72 has/had the same symptoms (only the
>vacuum seemed to build a bit quicker and release less, so the stalling
>became
>progressively quicker). I would have never suspected the gas cap, but
>after
>your response I think that may be my culprit--I have an aftermarket cap
>(one
>of the billet ones...now we know why they call them "trick" eh!), and
>after
>reading your post I eased it off and sure enough a long slow (and loud)
>hiss
>of air resulted. After starting her up with the cap off, she ran well at
>first
>(as usual), but I immediately found a fuel line leak--my guess is that the
>systems pressure from the vacuum was keeping a tight seal on the line
>until it
>was released. I haven't gotten around to fixing the leak.
>
>Anyway, I'm wondering if there is something I can do to keep using the new
>cap
>(if that's the real source of my problem). Does anyone know how the stock
>caps
>are vented, and can it be reproduced on an aftermarket cap? I was
>wondering if
>there was somewhere I could drill a small hole, or maybe dimple the filler
>tube mouth? Any thoughts? Thanks in advance.
>
>Best, -john

This read-only message was archived from a public mail list.
Mail From: mach70 (mark thispot)

John,

Have you tried contacting the aftermarket supplier? Hopefully they're doing
more than making pretty parts. I guess as a last resort I would consider
drilling a *very* small hole in the cap to vent to the atmosphere. It's not
environmentally responsible (gag) but there aren't many choices for these
old cars. Hell I've seen cars running down the road with a rag stuck in the
filler tube. How do you spell "Molotov"? I had a buddy with a California
DSO 70 Grande that had a cluster of vent fittings on the top rear of his
tank. I'm sure his tank breathed just fine but I'll bet you weren't
thinking of replacing the tank just to keep that billet cap, right?

Mark
70 Mach1

>Date: Mon, 16 Aug 2004 11:28:46 -0500
>From: jsanti2 <(email redacted)>
>Subject: [CM] RE: Fuel Starving -Gas Cap
>To: (email redacted)
>Message-ID: <(email redacted)>
>Content-Type: text/plain; charset="US-ASCII"
>
>Hi All (and Mark esp.),
>
>I was interested in running down some of the diagnosis/repiar of the
>problem
>you ran into with the gas cap. My `72 has/had the same symptoms (only the
>vacuum seemed to build a bit quicker and release less, so the stalling
>became
>progressively quicker). I would have never suspected the gas cap, but after
>your response I think that may be my culprit--I have an aftermarket cap
>(one
>of the billet ones...now we know why they call them "trick" eh!), and after
>reading your post I eased it off and sure enough a long slow (and loud)
>hiss
>of air resulted. After starting her up with the cap off, she ran well at
>first
>(as usual), but I immediately found a fuel line leak--my guess is that the
>systems pressure from the vacuum was keeping a tight seal on the line until
>it
>was released. I haven't gotten around to fixing the leak.
>
>Anyway, I'm wondering if there is something I can do to keep using the new
>cap
>(if that's the real source of my problem). Does anyone know how the stock
>caps
>are vented, and can it be reproduced on an aftermarket cap? I was wondering
>if
>there was somewhere I could drill a small hole, or maybe dimple the filler
>tube mouth? Any thoughts? Thanks in advance.
>
>Best, -john
>
> >----------------------------------------------------------------------
> >Date: Fri, 30 Jul 2004 02:20:12 +0000
> >From: "mark thispot" <(email redacted)>
> >Subject: [CM] RE: Fuel starving (was RE: Coil failures)
> >To: (email redacted)
> >
> >I had a hair-pulling stalling problem a few years ago that drove me nuts
>for
> >about a year. My car would run perfectly until I had it on the road for
>at
> >least 45 minutes. Around that point it would slow and stall. It could
>not
> >be started for about 10 minutes at least. The in-tank pump was the first
> >thing I replaced but eventually I'd worked on the entire fuel system &
>most
> >of the electrical. Finally I happened to mention this weirdness to the
> >right guy who once had a similar problem with a motorcycle. A kinked
>fuel
> >tank vent would not allow any air in to replace the gas that was consumed
> >when driving. The fuel pump would struggle against the growing vacuum
>until
> >it just couldn't cope any longer & the car would die. The 10 minute wait
> >would allow enough air to seep in to let the car start. The tip-off was
> >noticing that when I would sometimes stop on the way to work to gas up
>there
> >was a big whoosh of air into the tank when I unscrewed the cap. On short
> >triips there was never enough vacuum created to cause a problem. My
>commute
> >to work is about 50 minutes so this happened many times to me & destroyed
>my
> >trust in the car.
> >
> >The gas cap on old Mustangs is vented to allow air in (except certain
> >California-emissions models) but if it's been replaced with some
>universal
> >aftermarket unit it might not work as needed.
>
>