vacuum and pressure problems

[joe_db]

I am going nuts trying to solve a fuel feed problem.
I have 2 vacuum gauges and one pressure gauge. The vacuum gauges are on the inlet and outlet sides of the Racor and the pressure gauge is ont he outlet side of the pump.
Here is what happens:
Engine start, vacuum is 0 both gauges and fuel pressure is 3 PSI or so.
During the first hour, the outlet side gauge rises to 3" vacuum and then the inlet gauge starts to rise as well, but always lagging 3". Eventually the inlet gauge is on 5", the outlet side is 8", and fuel pressure is 0
The engine still runs, so some fuel must be trickling through, but you can tell it would like more. Rapid throttle advance will cause it to bog down and if you run 100% power it will eventually quit and need to sit a few moments.
I have tested the tank vent. No clogs.
I have put on a new filter, replaced the -4 fuel feed with -6, replaced the fill hose that was dropping pieces of rubber into the tank, and taken the tank home and cleaned it. I am going nuts trying to figure this out!

Also note the vacuum reading take a few HOURS to fall back to 0. Right now my plan is to run a hose back to the tank and see if I can just recirculate fuel and duplicate the problem without running the engine.

Another hint: If you jumper the pump to run after the vacuum has built up, it can get the pressure to go up to 2 psi for a second and then it falls back to 0. Even when the vacuum gauges are at 0, the pressure falls to 0 as soon as the pump shuts off. It doesn't seem able to hold the pressure in the line. I have no idea if this is correct or not.

[Dave Neptune]

leaker?

Joe, sounds like the Racor is plugging up. If the first V/G on the inlet is alway 3" lower that is what it takes to draw from the tank. Is the value the same with a full tank versus an almost empty one?

You still could have some KRAP floating around and plugging things up after a while or right away.

Dave Neptune

[joe_db]

Could be, but a clogged filter should have no vacuum on the inlet side I think. Driving me nuts!
Of course the filter AND the tank pickup could both be clogging

[hanleyclifford]

I'm not familiar with the Racor but on the unit I have it is important not to reverse the inlet and outlet connections. Another thought, how about your tank shut off valve?

[ndutton]

I agree that it looks like things are getting clogged and I think the tank is the place to start. Schmutz in the tank would elevate the filter inlet vacuum and as that schmutz makes its way to the filter, the outlet vacuum would increase as well.

Only if the filter were TOTALLY clogged would the inlet vacuum read zero. Also, I would expect the pump outlet pressure would approach zero or nearly so during the brief time the float valve is open.

[hanleyclifford]

When I had a look at my fuel tank shut off I was surprised to see that it was not as "full flow" as I expected. Inasmuch as a previous problem with hose particles was reported and the tank was subsequently cleaned, maybe the fuel shut off valve should be examined as it could be restrictive even in the open position. Is the shut off up or down stream of the filter?

[Kelly]

Joe,

Maybe treating this mystery like an electrical "head-scratcher" would help: couldn't you run a fuel line directly from the tank (fill tube?) to the pump and test knowing there are NO restrictions. Then, add your fuel line elements one by one until you can identify the culprit(s).

I'm no fuel expert but I would think this approach would save you some of the headache of "shooting in the dark". It sounds like your fuel starvation could be coming from a number of sources.

[jhwelch]

I have a hefty bicycle pump with a nozzle attachment that I've used on the ends of suspect gas hoses to blow air back through to help determine if there is a blockage somewhere and to clear it out.

Maybe this will help you?

[joe_db]

I'll try something like that.

I have a hefty bicycle pump with a nozzle attachment that I've used on the ends of suspect gas hoses to blow air back through to help determine if there is a blockage somewhere and to clear it out.

Maybe this will help you?