Fuel filter seals
 

The Volvo truck was running bad the other day, bogging down, puffing white smoke, stinking exhaust, etc. So I decided maybe the fuel filter was plugging up.

Changed both filters and it still ran bad. I'd never changed them before as they get done at the PM's and I've never had a problem.

Next day, the mechanic at the terminal looked at it and told me the seals were installed upside down and it was drawing air in. So he replaced the filters again. He told me the fuel isn't pumped through but sucked through so there's negative pressure in the filter. Apparently it'll draw air when running but it won't leak fuel when it's setting.

Today the truck's running fine. Now I was an auto mechanic for 17 years and I'd never heard of anything like that.

So has anyone heard of this, and what kind of negative pressure are we talking about here?