OK Guys I know this has been discussed MANY times before. But here we go again. I am dealing with dropping fuel line pressure. On cold starts I get a nice steady 6 PSI for about 5 - 10 mins then the slow decent to zero. I am amazed the car still idles and runs. All fuel lines replaced, pick-up in tank cleaned, nice healthy stream to pump. I do have headers that are wrapped. Plan of attack to is incorporate heat protection on fuel lines that run parallel to the headers and a heat shield to the on the carb. Regarding the fuel pump, it is an OEM style mechanical pump with a fuel tank return line. Prob some knock off I forget. I was planning to replace with a Carter mechanical pump with the return line. That pump advertises a max 35 GHP and 7 to 9.5 PSI. It will be feeding a 454 HO create engine with a 770 Holley St Avenger. This is a relatively mild-set up advertising 450HP; plenty good for me. I live in LA so no matter how hard I try I spend most of my cruising time idling in traffic, so I wanted to maintain the return line.

So as I work thru this my question is, is the gas flow volume on this fuel pump sufficient for my set-up and low-key driving habits. OR go performance pump and incorporate a return line thru an inline filter or fuel press reg.

Thanks In Advance
Din

 

Have you tried a different brand fuel pressure gauge?
Hard to believe it runs at zero.
Assume the carburetor is a Quadrajet?

 

He does say the Carburetor is a Holley 770 street avenger. Basically a vacuum secondary standard 4 barrel Holley.
As per pump volume. Considering your power levels and use. Yes I think that should be enough pump.

 

Missed it was a Holley.
So when the gauge reads zero, did you check the float levels with the engine running?

 

Might this be a sealed vs. vented tank problem? If the car once had a charcoal canister that was removed while keeping the sealed gas cap, maybe it's creating a vacuum in the tank. Just a WAG, but maybe?

 

I would agree this could be simply a non venting fuel tank issue.
OP,
Can you post a couple pictures of your carb, fuel line and regulator set up?
The OP mentioned that he wasn’t currently set up with a fuel return line but plans to set one back up in the future.
If he is running a fuel pressure regulator set up for a return line but isn’t, what sort of issues if any could that cause?
Without a return line, I would think he needs to run a dead head regulator.
Also check to see if there is a vent on the regulator itself and if so that it isn’t capped off or plugged.

That small brass fitting on my fuel pressure regulator is a vent that cannot be plugged or capped off.

 

Yes the float maintained.

 

I re-read the first post am a little bit confused.
Are you currently running a pressure regulator with a return line or no pressure regulator and no return line?

How are you venting the tank?
Simple test of a venting issue would be to run the engine without the gas cap installed and see if the problem goes away.

Then the next thing to check would be to see if you are actually getting fuel and pressure at the carb.
You stated you have good flow at the pump but did you check it at the carb.
Did you just check the flow coming into the pump or did you check the flow at the outlet port as well?
If you are running rubber fuel lines off the pump, could one be pinched or collapsing when under pressure?

Some carbs have a filter or screen internal at the inlet port.
If yours does did you check the filter or screen?

Is it possible you have a pin hole leak in a float that’s filling with fuel and isn’t actually floating in the bowl?

 

Is that vent a return port. That is nice Press Reg!

 

So how I got here. I was dealing with cold start issues. I originally plumbed my EVAP canister inline with the PCV to Full Manifold Vac. Subsequent researched revealed that could cause my Problems. I kept the two port, no purge valve, canister, purchased new at time of the rebuild, and plumbed it to Ported Vac. I maintained the sealed gas cap. That cleaned up my starting and idling issues. At the same time I incorporated an Aeromotive, dead-end style, fuel pressure reg and gage and observed the drop in pressure as the engine heats up. Thinking now that the tank only vents at open throttle, the first thing I did was allowed the car to cool down completely and than ran it with the gas cap off and same symptoms. I was using an OEM style fuel pump with a return line back to the tank.

So I will verify with a second pressure gage. I will address possible heat issues; the metal fuel tubes run along the frame a few inches from the headers. I will see if I can plumb the pressure side to the carb further from any heat sources. The current pump could be failing. It's possible it's a cheap knock off. I read that the BBC OEM style Carters, with a return line are more reliable. I will also replace the suction side fuel hose to the pump and the return side from the pump, as Old Car Bum replied, there could be a pin hole, pinched or collapsing.

None of the performance pumps have a return fuel port; which lead me to the second biggest online forum controversy, next to Full Man Vac vs Ported Vac for Vac Adv., that is, does a return line keep fuel cool by circulating heated fuel back to the tank or does it heat up the fuel in the tank and....

I have 110 GPH Holley I can mount and rig up a return line; I am still pro return line! But do I want to push three times the volume of fuel and the extra plumbing if an OEM style Carter at 35 GPH, already set up with a return port will be enough carb.

I hope I addressed all the great replies. I will upload before and after photos with a length of clear tubing into the carb for bubbling or vapor lock issue.

Thanks For The Responses
Happy Easter All
Bob

 

If you determine your fuel pump is the problem there is a performance pump with a return line but it ain't cheap. it's the Robmc pump. I have been running one on my 496 for the last 5 years with no problems. the pressure stays between 6-8 psi.
happy Easter
Pat

 

Thank You. I have never heard of them I will investigate directly

 

Most zero psi problems are the pump pulling a vacuum on the warm gasoline, and causing vapor. Which it can't pump well.
You absolutely need a vapor return line, somewhere, especially on a BB.
You can run a return line a lot of places, off the pump, off the front mounted fuel filter, or after the carb.
You can use a .060" orifice or a pressure regulator.
They will all help get rid of the vapor.
Ethanol boils at 178*F
Modern gas is not 1970s gas.
And 27* timing, at idle, with 15* initial, and 12* on manifold vacuum can, will help the engine run cooler.
That's not a myth.

You have a carb heat shield, good.

The fuel pumps thru the corvette vendors are better than the ones thru Rock Auto, etc.
A stock 72-74 454 fuel pump has a big bowl, vapor return, and enough fuel flow for ~500HP, unless you lose psi in third gear on a drag strip after a long WOT pull. If you are not doing that, I wouldn't worry about it.
If you are, check your fuel psi in the traps.

 

Ok guys to close out this thread, I am happy to report that my dropping fuel pressure issue has been resolved thanks to all the great replies. I replaced the rear fuel lines and filter at the tank. They did look to be in pretty good shape. However the molded fuel line on the suction side of the pump was cracked and deteriorated. I had no evidence of gas leaking from the line. But, when I squeezed the fuel line with a hose remover pliers gas oozed out This fuel line is only 5 years-old; I am sure motor heat is the cause for the quick deterioration, see the photos. I replaced that hose with an added heat shield sleeve. I replaced the fuel pump with a Cater OEM style with a Vapor return. I added heat shield sleeves over the metal fuel lines that run along the frame inches from the headers and a heat shield sleeve over the pressure side fuel line and filter going to the carb.

My least favorite thing is working around dripping gas. So I chose to address the three potential causes, fuel lines, heat and a failing fuel pump, all at the same time; It could have been a combination of the three, who knows, but I only had to yank the fuel lines off once and the problem is solved.

Thanks for all the replies

Unprotected fuel lines inches from the headers

Cracked molded fuel line

Beautiful 6 PSI

Heat shield sleeves on fuel lines

Heat shield sleeves on fuel lines

 

Looks like you definitely needed to replace the fuel line. one thing after seeing your pictures and rereading your first post about your car still running when showing no fuel pressure. you have a liquid filled fuel pressure gauge mounted in the engine compartment. I have heard that those are not a good idea because when they heat up they can tend to show no fuel pressure - something to do with the liquid inside. a new fuel pump doesn't hurt but now I wonder if it was just your gauge.
Pat

 

Per Blueprint:

• Liquid filled pressure gauge readings will change as ambient (under hood) temperature changes. For most accurate reading, check when engine is cool. A change in ambient temperature can cause case pressure to change as much as 1-3 psi.

With a BB, cruising in Traffic, you will have to do everything correct to keep her running cool, and avoid vapor lock. Suggestions: Use all, or as many as needed:

• Lip spoiler in place (#1)
• All foam in place around and above radiator. Check with a light, and add more to seal any light gaps.
• Elec fans running at idle (or) mech fan running at idle. New, known good, fan clutch. Fan close to shroud. H.D. fan clutch.
• (#2) Ignition timing on a performance tune. 25-28* at idle (15* static and 12* vacuum, on manifold vac)
• (#3) Fuel return line, any style, off mech fuel pump, gas filter, or carb input line (best)
• (#4) Carb heat shield, especially for a Holley (it's accelerator pumps are low) (for radiant heat) (and)
• 1/4 - 1/2" carb insulator (for conductive heat)
• 160* thermostat.
• Insulated fuel lines

Modern gas has very low vapor pressure, and ethanol containing gas begins to boil at 178*F. Calif state gas is unique anyway.

 

As always, Leigh brings up excellent points for keeping things cooler under the hood.
It is especially more difficult with a big block.
I’ll add one additional thing and that is to find a way to allow or force the heat out of the engine compartment, like the addition of a hood vent so hot air can escape and can help pull ambient air to enter at slow speeds.

 

pspicci the molded fuel line was cracked throughout and very deteriorated so that had to be replaced and while I was down there it was just two bolts and $50 to replace the pump. I know I swapped out an old cheap one for a new cheap one. I researched that pump you mentioned in your post; $275, but that looks like a great option. I just scored a Quickjack lift that made life a lot easier but dealing with flowing gas is one of my least favorite things.

I am happy to see that I have already incorporated Leigh1322's engine cooling recommendations except running the Vac Adv to full manifold vac. On that note, my Street Avenger has a smaller Full Man Vac port next to the larger PCV port. I am guessing that that smaller port is what I would connect my Vac Adv to. However, when I checked the vacuum the smaller port read 5" and the PCV read 14" I will recheck that to make sure I got that right. What would be another way to tap into full Man Vac? Plumb with the PCV or tap in another way.

Thanks Again
Happy Motoring