Fuel Pressure dropping

[2Blew2B]

Can anyone tell me what could be the cause of my fuel pressure dropping?

History:
Fresh rebuilt engine
Electronic fuel pump (Airtec) 5-9 psi spec.
Flushed all fuel hoses
New fuel filter.

What’s going on: I start it up and the fuel pressure jumps to 5 psi after running for about 5 min or so it starts dropping all the way until 1 psi. Then the car starts having problems running (fuel starvation)

I checked pump by feel, its running. No hoses are collapsed.

Anyone have any ideas?

 

['68falconohio]

Does it happen at all fuel levels? How old's the tank, could it be picking up crud there?

 

[FALCONAROUND]

junk fuel pump....

 

[69.5Mav]

Check the fuel filter. If its got junk in it then check the tank. My car did the same, just enough run time for you to get out of the neighborhood then dead as a dornail and you push.

My 72 Mav had chunk 3/8 inch thick coming out of it when I took it off. Took it to a radiator shop. They boiled it out sealed it and rust proofed it for around two hundred. Thats the way to go if you can't find a repro tank.

 

[FalconSedanDelivery]

Either its not Venting ( the Tank ) , or the sock , ( pickup on end of sender is clogged with rust etc . ) doubt its a bad fuel pump

 

[2Blew2B]

I washed out the tank with 5 gal of keriosen and ran a couple of gal of fuel out. Then ran 2 fuel filters to weed out any remaining junk.

 

[addo]

Might be worth checking that the pump is still getting 12-13V as its output pressure falls.

Also, if not fitted already, a return style regulator is a good idea.

 

[FalconSedanDelivery]

But did you check the tank sending unit ?, if not , its a start, My brothers car had the same problem

 

[2Blew2B]

All,
what about the sending unit could cause it to start out ok but get worse?

 

[FalconSedanDelivery]

Ok when you start it , rust and debris get sucked in slowly pluging it up , you turn it off and they fall away , start it up and it repeats , its an easy job , you dont even have to remove the tank , plus there should be a drain plug in the lower right corner, if when you pull it apart , you find tons of rust a new tank will probably be needed, ttyl Faron

 

[2Blew2B]

Ok, I purchased a new fuel sending unit and hope to drain the tank over the next few days. I'll let you know what i come up with. Also, I notice a comment on a return system but not sure how that all woks. the sending unit i saw in a eriler post had 2 connections. Does any one know which one that is? I could use some advice on the return system.

Steve

 

[69.5Mav]

Your sending unit has a tube that picks up gas from the tank. This tube has a filter on the tank end of it that looks like a sock slipped over the gas tube.
When debris builds up in the tank it reachs the level of that sock filter and begins to clog it.
Over time you reack a point as you drive the car the sock gets covered in nonsticking debris and you engine starves out.
The debris falls away after awhile and you can again start the car but it will run for a shorter and shorter time.

You can get some releif by blow high presure air down the line but you will most likely break the sock filter open.
This is both good and bad.
Bad because you no longer have a filter in the gas tank.
Good because the clog point will now move to a filter that can be a lot easier to change.
I have seen people use big oil filters to do this and get a lot of miles out of them.
I do not recomend this however. The real fix is to clean the tank out.

Car the have return lines from their fuel pressure regulator are mostly fuel injected. The sending unit will have two gas tubes on it, a short one and a longer one. The short one is used to return gas to the tank. the longer one is the tank gas pickup tube. A radiator shop can add a return tube to your seder unit.