Most of the time my fuel line sight glass has bubbles in it [like a newly poured Pepsi!] when the car is running. It is intermittent, and my mechanic suggested it occurs only when the fuel pump is active. He's an expert on high end classic mechanical restorations and loves and knows old Cadillacs. The engine runs well despite the bubbles, although subject to vapor lock in warm weather. We don't think the bubbles should be there, but remain stumped as the the cause. Any one had and solved this problem?? Thanks. Bubbles don't show well in the photos.
One possibility is is slight air leak somewhere in the fuel lines or connections.
I had the same problem. The metal filter housing was slightly warped and the glass bowl didn't sit flat on the gasket. With the filter gasket removed, put the glass bowl on and see if you can rock it back and forth. If so, it's not far against the metal. You can gently bend the filter housing. And there should also be a gasket on the inside of the housing where the filter sits. The replacement doesn't usually include that gasket.
Thank you both, Dave and Walter, for your replies. Walter, I'll try your suggestion It sounds very logical. And, I''l let you both know the outcome.
Does it do this when the engine is totally cool? Or only once the engine is warmed up fully? If only when the engine is hot, see my post at the related forum question: http://forums.cadillaclasalleclub.org/index.php?topic=132328.0
If it does it all the time, even when the engine is cool, it sounds more like it is drawing in air and you have a leak somewhere on the inlet side of the pump. Could be as simple as the screws are loose that hold down the upper housing of the pump or a leaky diaphragm or a leak at the inlet side of the fuel pump (where the line comes into the pump). If it is this latter, you probably won't see a fuel leak, as the pump is creating low pressure on the inlet side and drawing fuel (and perhaps air) into the pump. So on the inlet side, you could have a slight air leak and no visible fuel leak.