Fuel Cap

Posted by Bob J On 2024/4/9 18:57:58

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.