air in fuel filter

[local 1068]

:confused:I have a problem with air getting in my fuel filter. If you crack the bleeder valve,leave it open long enough with the engine running, it will bleed the air out and get a steady flow of fuel. Drive the truck under normal conditions and crack the air bleeder again and , you have air again. let it run and again it will bleed itself. This just started about a week ago as it died one day and there was no fuel in filter. I bled the filter and it started right up. The lift pump works at start up and there is lots of oil pressure when running. There seems to be vacum in the tank when I open the fuel cap so I don't think i have rust holes in the tank or supply line. Could a lift pump operate and still be defective that it would cause a situation like this? Scratching my head on this one. The vehicle is a 1998 chevrolet 2500 silverado, owned since new, rust checked every year,walt',s mods for pmd replacement 175000 km . Any thoughts? thanks

[grancito]

Perhaps too much vacuum in the tank. Run it with the fuel cap loose.

[local 1068]

Thanks for the reply and your thoughts, I tried that Saturday and it didn't seem a whole lot better. I will try it that way for a couple of days and give it a fair test and see what happens. Another thought crossed my mind was possibly the screen on the end of the fuel pick up gone south and creating a partial blockage in the line.

[JD_countryboy]

How can you be sure no pin hole air leaks in suction line between lift pump and tank? If your lines or tank sending unit have rust on them, you could have a pin hole in the line. I know of a couple of air in fuel issues as you describe and it was due to rusted fuel lines. No signs of fuel escaping the line, but since its a suction line, it isnt under pressure to push fuel outta. Remeber, air moves more freely, and thru smaller holes then liquids will. Pull the pressure line off of your lift pump and hot wire pump on and watch for air bubbles in fuel strem commin outta pump. If you see any air bubbles, you know ya got a hole in suction line someplace. Also could be a bad fuel strainer assembly, not sure if our diesels have a plastic hose in the tank connecting the strainer to the steel line suction line or not, but if this little hose gets brittle and gets a pin hole in it, it will cause fuel issues when fuel level gets below the hole.

[local 1068]

Thanks for the reply, I am basing the no pin hole theory on the fact is that there is vacum in the tank when I remove the fuel cap. I would have thought that if there was pin holes I would lose that pressure in the tank through the pinhloes, leaving some fuel on the driveway.but having said that, I will check that out.

[JD_countryboy]

My bad, the vacume in your tank sentance got lost between my eyes and brain. However, does it do this all the time? or does it not do it on a full tank? If the trouble is fuel level related, then the trouble is in your tank, either the sending unit, or pickup strainer. Since you do have a vacume in the tank tho, you do need a new fuel cap. And make sure you get the vented cap for diesel pickup. Gas caps are not vented. fuel system vents thru fuel emmisions system on a gasser. Diesel don't have the same system, as diesel doesn't evaporate like gas.

[local 1068]

Thanks to grancito and Jd countryboy. With respect to my problem this is where I stand. I ran the truck for a few days with the cap loose and i have the same results, air in the filter. I Cleaned the fuel cap off real well and tested by blowing through it and you cannot do which I believe is correct. If you put a vacumn on it or suck on it, the valve opens allowing air into the tank so I believe the cap is fine Although the lift pump is working mechaniclly and flowing fuel , I better do a pressure test to see if the pump pressure is in its range. There is a possibility the vacumn required to open the valve in the fuel cap could be greater than the lift pump can provide but having run the truck for a few days with it off, I don't think that's causing my original problem.:) I have no problem with pulling the tank and replacing lines, and i may have to do that. I will keep you informed. I appreciate your time. Thanks guys.

[grancito]

If you are getting a good fuel flow out the bleader after driving a while, then the sock in the tank is not blocking. Try pressurizing the tank, a vacuum cleaner on blow will not cause damage, and look for fuel leaks between the tank and the lift pump.

[JD_countryboy]

Fuel pressure test will help you diagnose how fast your suckin air into system. When you do the test though, run the engine at idle check pressure, and have someone crank the throttle open, and at wot cheeck then. it shouldnt drop. Friends truck was showin no signs of fuel leaks, but was losin fuel pressure over about 1/4 throttle, only had 1 psi at wot. He pulled pressure line off lift pump, and ran clear hose into jug, and watched all the air bubbles in the fuel line. He then sprayed soapy water on steel line and took airhose and pressurized suction line (from the lift pump back to tank) and found a rust spot bubbling air and found the pin hole in the fuel line. He replaced steel suction line, and tank sending unit. Solved the trouble. Good luck.

[local 1068]

Well I finally got some time to work on the truck and the problem was an air leak in the supply line on the sending unit. It is surprising what a little soap and water will show when you apply a little air pressure inside the lines. I rereplaced the unit with a genuine GM unit. " ouch" big canadian bucks for that part, but it's done now, Having the truck oiled every year made the job alot easier as the nuts and lines came apart easier. I was a little disapointed as the one spot the rust proofing didn't get and that was a spot on the sending unit , which rusted and allowed the pinhole. Everything else was like new. Once again thanks to grancito and JD countryboy for their thoughts and input.

[JD_countryboy]

Glad your up n running good again.