Problem with new fuel system

[MileHighDart]

So my fuel system is like this.
Every thing new from the tank to the carb. New 3/8” pickup, 3/8” Holley filter before the pump, Airtex 9psi electric pump mounted below the level of the tank, new 3/8” hardline to the engine bay, then braided steel to the Holley regulator and to the carb.

Just finished my 5.9 swap and up till today I just had it running in the garage. Regulator adjusted to 5.5 psi

So today I have everything finished enough to take my first test drive. Short 10 min drive to the gas station and back. Popped the hood and noticed the gauge on the regulator reading less than one pound. So I adjusted it all the way up and still only reading about 3lbs.
Take it for another 15 minute drive, check it again and it’s barely reading anything on the gauge.
Car ran fine on the drive, no sign of it not getting gas, but I was taking it pretty easy, nothing past 1/4 throttle.

Any ideas what’s going on?

 

[diymirage]

bad tank vent ?

 

[Dubob]

Short 10 min drive to the gas station and back. Popped the hood and noticed the gauge on the regulator reading less than one pound. So I adjusted it all the way up and still only reading about 3lbs.

If the gauge is sealed, heat will cause the pressure in the gauge case to increase and it will cause the gauge to read low. I bought a gauge with an equalization port and when I check the pressure I open the port and it goes to normal.


Aeromotive Fuel Pressure Gauges 15632

 

[MileHighDart]

If the gauge is sealed, heat will cause the pressure in the gauge case to increase and it will cause the gauge to read low. I bought a gauge with an equalization port and when I check the pressure I open the port and it goes to normal.


Aeromotive Fuel Pressure Gauges 15632

Hmmmm, my gauge is a cheap Summit liquid filled gauge. It’ll be interesting to start it back up in the morning after it’s cooled down and see what it reads.

 

[MileHighDart]

bad tank vent ?

This is on a 68 dart. It has a very small maybe 1/8” hardline that comes off the side if the filler tube near the top. It winds around in a circle and then follows the filler tube down and through the rubber gasket that’s screwed to the trunk floor. But I’m sure after 50 years that it could be plugged up. I know back when I was driving the car regularly with the previous engine, when you pulled the cap off to fill it up, there would be a big rush of air, either going in or out. Don’t really know.

 

[dgibby]

This is on a 68 dart. It has a very small maybe 1/8” hardline that comes off the side if the filler tube near the top. It winds around in a circle and then follows the filler tube down and through the rubber gasket that’s screwed to the trunk floor. But I’m sure after 50 years that it could be plugged up. I know back when I was driving the car regularly with the previous engine, when you pulled the cap off to fill it up, there would be a big rush of air, either going in or out. Don’t really know.

Should have a short length of rubber hose where it connects on the filler tube up near the quarter panel in the trunk. Disconnect it and blow air into the vent tube and it should vent out in front of the drivers tire in the sub frame rail. You can also spray some cleaner into the vent tube then hit it with the air.
You can run it with the cap off and see if that helps the pressure too.

 

[MileHighDart]

Ok, I went back to the Summit racing page for the gauge I have and it says right there that as under hood temps go up the reading on the gauge could drop as much as 7-12lbs.
So not to worried about It. For now I guess I’ll just check it every time I fire it up cold, and not worry about what it reads hot. In the meantime I’ll save up for one of those aeromotive gauges that has the pressure equalization pin.

 

[67Dart273]

What is the 3/8 filter you have before the pump? This MUST be an extra large capacity flow, because it's harder to "pull" through it at lower (suction) pressure

Easy to check the tank vent, it ends "open" inside the rear frame rail. You can disconnect the tube in the trunk and check it out

 

[MileHighDart]

Should have a short length of rubber hose where it connects on the filler tube up near the quarter panel in the trunk. Disconnect it and blow air into the vent tube and it should vent out in front of the drivers tire in the sub frame rail. You can also spray some cleaner into the vent tube then hit it with the air.
You can run it with the cap off and see if that helps the pressure too.

I’ll try that too in the morning. I would like to get that opened up.

 

[MileHighDart]

What is the 3/8 filter you have before the pump? This MUST be an extra large capacity flow, because it's harder to "pull" through it at lower (suction) pressure

Easy to check the tank vent, it ends "open" inside the rear frame rail. You can disconnect the tube in the trunk and check it out

View attachment 1715138932

It’s just your basic Holley chrome 3/8 filter. Supposed to be 40 micron, but doesn’t show a gph rating. Do I need one of the 100 gph filters or something like that?
Holley Chrome Inline Fuel Filters 162-523

 

[67Dart273]

I would guess that filter is not adequate. Quick and easy way to find out is bypass it with a chunk of hose and see if the situation changes

Speaking of hose, use the stiffer EFI rated hose for your suction hose(s) which helps prevent collapse

Bear in mind, the suction side is operating at BELOW atmospheric pressure and ANY pressure drop is sort of magnified for lack of better.

 

[yellow rose]

That's why I put my fuel gauge in the cab. It's easier to see and monitor.

 

[famous bob]

Hmmmm, my gauge is a cheap Summit liquid filled gauge. It’ll be interesting to start it back up in the morning after it’s cooled down and see what it reads.

Had this happen on a hot sbc, pull the rubber plug and let the fluid out of that filled gauge. Do this first before chasing it any further. Of course most recommend no filter before the pump, unless very hi flowing. I always, way over due my filter size and run it between the tank and the elec. pump never had a problem, makes the pumps last longer. I have two 1/2" canister in line steel filters on 3/8" pipe outlets on my elec. fuel inj. pump, no problems.

 

[MileHighDart]

Well this morning fired it up cold, after backing of the regulator, which I had cranked up all the way yesterday. It read about 9 and first, adjusted it back down to 5.5. So I think its just the gauge reading low when it gets hot. After a spirited 1/2 hour drive, it's reading about 3lbs. Keep in mind its about 32 degrees right now, so it probably didn't get as warm under the hood as it did yesterday.

I did some full throttle passes from 0 to 80 or so, and there are definitely no fuel delivery problems. No hesitation or stumbling, just hauls ***. I'm pretty happy with the 5.9 swap right now.

 

[famous bob]

Well this morning fired it up cold, after backing of the regulator, which I had cranked up all the way yesterday. It read about 9 and first, adjusted it back down to 5.5. So I think its just the gauge reading low when it gets hot. After a spirited 1/2 hour drive, it's reading about 3lbs. Keep in mind its about 32 degrees right now, so it probably didn't get as warm under the hood as it did yesterday.

I did some full throttle passes from 0 to 80 or so, and there are definitely no fuel delivery problems. No hesitation or stumbling, just hauls ***. I'm pretty happy with the 5.9 swap right now.

The problem w/ filled gauges is most likely, the fluid heating up and expanding /affecting the gauge readings. Try draining it as I suggested, it wil probably work right . I don`t pay the extra for filled gauges any more on anything !

 

[MileHighDart]

The problem w/ filled gauges is most likely, the fluid heating up and expanding /affecting the gauge readings. Try draining it as I suggested, it wil probably work right . I don`t pay the extra for filled gauges any more on anything !

I'm going to give that a try for sure.