# Fuel pressure dropping



## 69-JUDGE (Jun 27, 2009)

Well here is another strange problem. I am running dual Edelbrock 500 cfm carburetors on a Pontiac 400 c.i. engine that has a ram air 4 type cam, aluminum heads, and heat wrapped headers. The fuel is pumped from the fuel tank to the carbs by a Carter high volume mechanical fuel pump. The fuel lines run along the driver’s side frame and appear to be wrapped with some sort of cloth similar to the header heat wrapping. The fuel line is exposed (bare metal) from the rear mount of the upper A arm until the front mount, where a braided steel and rubber line connects, then it connects to the fuel pump inlet.

When the engine is first started the fuel pressure is between 5 and 6 psi. As the engine continues to warm up and reaches the 200 degree water temperature mark the electric fans turn on and the water temperature lowers and stays in the 190 degree range. At this time the fuel pressure lowers to around 3 psi and if the engine is allowed to continue to run it will drop to 0. If I increase the rpms the fuel pressure may or may not go back up. (The engine is idling right now at 950 rpms.)

When I shut down the engine for a few minutes and re start, the process repeats itself. 

So what do you think, vapor lock, fuel percolation, messed up fuel pump, or what?

By the way the GTO’s engine is running while the car is parked in the garage. 

All suggestions and comments welcome. Thanks.


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## Eric Animal (Oct 28, 2007)

Could be: a bad fuel pump. A clogged fuel filter. A bad tank sender (sock)....maybe a clogged tank vent or improper gas cap.


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## AlaGreyGoat (Jul 6, 2006)

Sounds like a stopping up sock filter in the gas tank.
To test, pull the tank line from the fuel pump and 
replace with a line to an external tank of gas. This will see if
the problem is before or after the fuel pump.

Larry


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## 69-JUDGE (Jun 27, 2009)

Eric Animal said:


> Could be: a bad fuel pump. A clogged fuel filter. A bad tank sender (sock)....maybe a clogged tank vent or improper gas cap.





AlaGreyGoat said:


> Sounds like a stopping up sock filter in the gas tank.
> To test, pull the tank line from the fuel pump and
> replace with a line to an external tank of gas. This will see if
> the problem is before or after the fuel pump.
> ...


Thanks guys, The fuel pump is fairly new, as is the sending unit, the fuel filter is a brand new high flow unit, and the gas cap is the correct one. The fuel pump and sending unit were installed by the previous owner and since I have already found and corrected other items that the previous owner had done, I guess I can't rule anything out.

It sounds like AlaGreyGoat's suggestion is the best way to find out if the problem is before or after the fuel pump.

Eric Animal, the 69 GTO uses a non vented fuel cap and I believe it vents using a vent pipe that loops through the trunk and then back out to the atmosphere. Is there another tank vent? Also if it is the vent or vents causing the problem would taking the fuel cap off solve the problem as a test?


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## jetstang (Nov 5, 2008)

69-JUDGE said:


> ? Also if it is the vent or vents causing the problem would taking the fuel cap off solve the problem as a test?


Yes, taking the cap off will tell you if maybe a dirt dawber made a nest in your vent tube. If it's not vented, then the tank should suck in and the gas cap will suck big air when you take it off.
It may be a vapor lock issue, does the fuel pump have a return line to the tank to keep fresh gas at the pump? Returns have 3 lines hooked to the pump. I would swap fuel pumps, there cheap, just to see if yours may have a weak diaphram before I dropped the tank for the fuel sock.
Is it a new tank in the car? A dirty sock should be clogged all the time.


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## Eric Animal (Oct 28, 2007)

yes, removing the gas cap will trouble shoot a vent issue. Also, as JETSTANG suggests,check the fuel pump lines and make sure they are hooked up correctly!!


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## 05GTO (Oct 6, 2004)

69-JUDGE said:


> The fuel lines run along the driver’s side frame and appear to be wrapped with some sort of cloth similar to the header heat wrapping. The fuel line is exposed (bare metal) from the rear mount of the upper A arm until the front mount, where a braided steel and rubber line connects, then it connects to the fuel pump inlet.


I would check the connection where all of the rubber tubes connect to the metal fuel line, you may be sucking air thru a loose connection. Also, run the fuel line inside the boxed frame from the floor board up to the cross member, there should be openings in the frame. This will also correct the problem if indeed it is vapor lock.


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## 69-JUDGE (Jun 27, 2009)

jetstang said:


> Yes, taking the cap off will tell you if maybe a dirt dawber made a nest in your vent tube. If it's not vented, then the tank should suck in and the gas cap will suck big air when you take it off.
> It may be a vapor lock issue, does the fuel pump have a return line to the tank to keep fresh gas at the pump? Returns have 3 lines hooked to the pump. I would swap fuel pumps, there cheap, just to see if yours may have a weak diaphram before I dropped the tank for the fuel sock.
> Is it a new tank in the car? A dirty sock should be clogged all the time.


This fuel pump only has one line in and one line out. I was told at the time of purchase that the fuel tank was new but after closer inspection I do not think that it is. The fuel lines appear to be new, at least what I can see of them. There are two lines running along the frame until the braided line connects from it to the fuel pump. The smaller line is capped at this point.

This car has been one giant pain in the *** since I purchased it two and a half years ago. 

I am learning a very expensive leason, Never ever buy a car sight unseen no matter how much documentation there is, how many pictures are sent, or how well you know the invidual. I started out just wanting to install a dual quad set up and for 22 months all I have done is correct screw up after screw up from the previous owner.

OK rant over for the moment. Thanks guys for the help.:cheers


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## 69-JUDGE (Jun 27, 2009)

Eric Animal said:


> yes, removing the gas cap will trouble shoot a vent issue. Also, as JETSTANG suggests,check the fuel pump lines and make sure they are hooked up correctly!!





05GTO said:


> I would check the connection where all of the rubber tubes connect to the metal fuel line, you may be sucking air thru a loose connection. Also, run the fuel line inside the boxed frame from the floor board up to the cross member, there should be openings in the frame. This will also correct the problem if indeed it is vapor lock.


Thanks guys for the suggestions.


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## jetstang (Nov 5, 2008)

OK, replace the fuel pump with a correct fuel pump with a return line and see if that fixes the problem. If they put that much work in the car, they bought an expensive fuel pump, then realized it didn't have a return, and now the car is vaporlocking. That's my bet.. Or what I would do. It's odd they insulated the headers if THEY weren't having the same problem...


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## 69-JUDGE (Jun 27, 2009)

jetstang said:


> OK, replace the fuel pump with a correct fuel pump with a return line and see if that fixes the problem. If they put that much work in the car, they bought an expensive fuel pump, then realized it didn't have a return, and now the car is vaporlocking. That's my bet.. Or what I would do. It's odd they insulated the headers if THEY weren't having the same problem...


Does any one make a high volume mechanical fuel pump for this car that uses a return line?

I also found out something interesting off of another forum, and that is that the liquid filled fuel pressure gauges do not work very well under the hood because of the high heat. It seems that they do just what I have been stating mine is doing. Any one else heard about this?

Well it does seem that I have several items to check on. Thanks Jetstang and all of the rest that replied.:cheers


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## Rukee (Feb 8, 2007)

69-JUDGE said:


> Does any one make a high volume mechanical fuel pump for this car that uses a return line?
> 
> I also found out something interesting off of another forum, and that is that the liquid filled fuel pressure gauges do not work very well under the hood because of the high heat. It seems that they do just what I have been stating mine is doing. Any one else heard about this?
> 
> Well it does seem that I have several items to check on. Thanks Jetstang and all of the rest that replied.:cheers


What happens when you just drive the car?? Is it really running out of fuel??
And you could run a fuel hose to relocate the fuel gauge inside the car temporarily to see if the pressure still drops.


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## 69-JUDGE (Jun 27, 2009)

Rukee said:


> What happens when you just drive the car?? Is it really running out of fuel??
> And you could run a fuel hose to relocate the fuel gauge inside the car temporarily to see if the pressure still drops.


I have not driven the car as yet, so I don't know. So far it is not shuting down in the garage, although it did start ideling roughly a couple of times. I guess I should say rougher than usual.


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## 69-JUDGE (Jun 27, 2009)

Well you learn something new every day. I drained the glycerin out of the gauge and it worked correctly. Took the GTO out for a little test spin Sunday. After 22 months of no GTO It felt very very good to be out in it again.

One thing that I did notice by watching the gauge is that I have 5 to 6 psi at start up but after the engine is warmed up and running for a while, the pressure drops to 3 1/2 to 4 psi. Do you think that I still may have a problem? The pump is feeding 2 four barrel carbs. Also I noticed that a few minutes after engine shut down the fuel pressure drops to zero. Is this normal?

The car ran well and did not act like it wanted to stall, but I did not drive it hard as I wanted all the fluids to circulate, after sitting so long.


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## Eric Animal (Oct 28, 2007)

Score....a good call Rukee! 4 psi seems a little low. If it is running good, don't mess with it.


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## Rukee (Feb 8, 2007)

4 lbs is fine, as long as it can pump the volume needed at that pressure. As long as it doesn't puke out when at WOT you're golden.


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## Eric Animal (Oct 28, 2007)

:agree


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## jetstang (Nov 5, 2008)

On holley carbs, 5 psi was almost too much and would push through needle and seats. If the low pressure is bothering you, just remove the gauge, ignorance is bliss!


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## 69-JUDGE (Jun 27, 2009)

Eric Animal said:


> Score....a good call Rukee! 4 psi seems a little low. If it is running good, don't mess with it.





Rukee said:


> 4 lbs is fine, as long as it can pump the volume needed at that pressure. As long as it doesn't puke out when at WOT you're golden.





jetstang said:


> On holley carbs, 5 psi was almost too much and would push through needle and seats. If the low pressure is bothering you, just remove the gauge, ignorance is bliss!


I guess I'm getting a little gun shy because of the many things that have gone wrong with this GTO in the past. After reading what you guys and others have stated, as well as my own experience, I'm going to run with it the way that it is and keep an eye on it. The main thing is that the car is running! I won't be going full throttle for awhile because of the brake booster problem anyway and I still need to fine tune everything. Thanks guys for all the help.


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