Packard Forum Index » Pre-War (1899-1942)

I have Greta back up and running after her long winter’s nap. Was not running well at first, even quicker choking out than last year but I found the problem, it is old news to most, but something not all would be aware of:
She would run well on high idle, but once she warmed up and the RPMs dropped she had to be carefully babysat with a rough idle. She stalled out a few times and I had to carry a Gatorade bottle of gas to re-prime the carb before she would start. The carb would be bone dry. So I ordered an electric fuel pump thinking that the worn lobe on the camshaft was just not getting enough fuel in at an idle.
I took her out Saturday morning to open up the garage to swap out the snows for summer tires on the Caddy and she was still rough on a warm day. After moving her back into the garage and barely getting her in before stalling I thought I would just leave her til the new pump came in.
I went in to make lunch and had an idea, is there low pressure in the gas tank? I went out and took off the gas cap and sure enough, a big hiss occurred as air rushed in to fill the semi-vacuum!
It was the cap!!!
I had two caps, I had ordered one from Kanter, but while awaiting its delivery I got another one from my jobber here in town and installed it on the car.
The one from the Kanter sat in the parts box downstairs as a spare.
With this new idea I pulled the Kanter cap out of storage and checked it, it has a much lighter release valve in it than the one on the car! I suspect that that one is for a newer engine with higher compression and more powerful fuel pump to allow air in with a pollution control system, whereas the one in my storage from Kanter was low pressure release for the older style, lower compression engine with a lower fuel pressure pump.
Add to that that the gas tank was full now as compared to a quarter tank last year meaning the tank had little air space left in it so the vacuum was stronger and quicker to affect the fuel flow restriction. I also noticed the stalling was more prevalent when coming to a stop after driving fast on a main drag, with the higher engine revs/pump pressure the new style cap would allow the flow but as soon as it dropped to idle approaching the stop the valve would close and increase the very vacuum that caused the stalling, as the new cap’s valve would stay closed at an idle and not allow the air in to displace the fuel used.
So with the proper cap it now runs smooth as glass, I actually have had to turn down the idle! Also runs cooler too.
So if you experience similar problems remember all gas caps are not the same.
Have fun cruising in the warm weather!
Bob J.

Thanks Bob. Good to know. Glad you got her up and running! I hope to follow soon.

Thanks Ben!
I should have included a shot for reference.
As you can see the tops are identical.
On the underside you can see the valve on the left is the one that is too high of a pressure, while the one on the right is from Kanter with the low pressure release.
Still having fun!!
Bob J.

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  Cap comparison 1.JPG (1,152.24 KB)


  Cap comparison 2.JPG (1,274.88 KB)

Modern pressure/vacuum type gas caps are not suited for use on vehicles made prior to about 1972. Earlier vehicles use a vented gas cap in almost all cases. While some modern-type caps may fit, they will likely cause problems similar to what an original cap with a clogged vent will cause.

(It may now be illegal to manufacture or import vented fuel caps here in the Land of the Free)

Older model gas tanks are not designed to hold any significant pressure or vacuum, and old style fuel pumps mounted on the engine will not perform as they should if the gas tank is under any negative pressure. One solution would be to add a vent to an old tank. This would need to be done with care to prevent problems with contaminants entering the tank, and to prevent gasoline from being expelled from the tank under some conditions, and to avoid any chance of allowing gasoline vapor to enter the car body, including the trunk area. Attaching a 3/8 fitting and hose to the upper part of the fill pipe and capping it with a larger style inline filter gas filter and outlet hose might be a good solution, at least on older models. Avoiding dust and road splash at the vent exit would be a must, and the exit would need to be kept as high as practical. Drilling out or otherwise defeating the pressure/vacuum mechanism in a modern style cap might be easier, but that would also need to be done carefully to avoid problems with contaminants or water being able to get into the tank or with gasoline splashing out when making turns with a full gas tank.

I use a cap like the one on the right (low pressure) on a 1972 Ford pickup, and it definitely causes some pressurization. As I recall, it had a label of some sort attached to the flattened area at the center of the chrome cap.
While it requires very little vacuum to open the vent, it requires significant pressure open it, and when it opens, it makes a honking sound. The '72 Ford pickup cap does fit my '37 Packard, and looks similar to the original. Putting a pressure/vacuum cap on a car in storage might help keep condensation out of the tank and prevent decay of ethanol-adulterated gasoline. I'd prefer an original type cap, but the low pressure type cap ought to work, and they do have a good gasket that is not affected by ethanol. ANY restriction that can cause negative pressure in the tank will aggravate issues with fuel delivery, especially in hot weather, and any pressure cycling of an old tank could aggravate any tendency to leak. With a sound tank, some mild over pressure should not cause any problems, and it could help with vapor lock problems in hot weather.

The originals as these two that I found in my stock were very basic and, while I can’t quite make out how they vent, I know they do. These were from my 1939 and 1940 and are very slightly different from one another but the difference is almost imperceptible.

The one on my '37 120 is different. It is rather heavy, and a little larger OD than the ones pictured, and it appears to have a chromed, die-cast cover. It may be original.

It has a pinhole vent and baffle, and inside the baffle there's a tiny coiled brass tube that's prone to clogging. The gasket looks to be some sort of fiber material.

The one that came on my 37 when I got it was much larger than expected. It was for a senior car or an earlier one, not sure. The junior caps are smaller in diameter, even though the neck is the same.