SUPER Complicated problem! Need Master Mechanic!

R

RGBody87

Apprentice
Jan 7, 2018
92
19
8
#1
Okay, lets start off with a brief explanation of the problem, segway into what is in the car, and then all the things I've thought or checked. If you want a real mind-numbing problem... this is the right post for you.

Problem.
The car has an electric fuel pump and has carburetor (Not fuel injected). At initial start up the regulator reads 4.5lbs of pressure. After running 5 minutes it reads 2.5-3 and after 10 minutes to 15 minutes it reads 0-1Lbs and runs great at an idle, but breaks up if you put your foot in it.

What's in it?
1987 Monte Carlo SS - Hardtop.
Mystery 454 Engine with Demon 625 CFM Carb ( I have a built Carb for the car sitting on a shelf 4150 style with more CFM but I know that this car used to run perfectly fine with this one, so I've left it for testing reasons)
Brand new 3/8 stainless pre-bent fuel line for feed, braided 3/8 line for a return to the tank,
New Grand National fuel tank with new sending unit and brand new Aeromotive Stealth fuel pump.
Holley fuel pressure regulator with 3/8 in/out ports and 3/8 return line with a Russel Performance pressure gauge on the regulator. Pre carb (obviously) and up on the plastic fender well away from the direct heat of the exhaust manifold.
Hot Wire fuel pump wire harness
Factory (not touched) vent line and canister in front of vehicle.

Ideas people have had/suggestions that haven't worked.
The first thing I tried was turning the pressure up on the fuel pressure regulator the first time it happened and I was able to get 4.5 lbs again by adjusting it while running. What I didn't check was it from a cold start. The next day I went to start it and check it and it quickly pressurized the carb with 9lbs of pressure. After running for 5 minutes its at 5, after running for 10 minutes its at 3lbs, and after 20 minutes it is back around 0-1 lbs of pressure.
The second thing I thought was maybe the gas cap isn't allowing any pressure out because I hear an absurd amount of air movement every time I open the "vented" cap that is definitely old/seen better days. The change is air pressure in the tank did not make a difference.
The third thing I checked was electric going to the pump/hotwire kit. The wire is run from the tank down the frame with the fuel lines to the alternator directly and the ignition wire is still putting out the 12v signal to trip the relay and power the fuel pump with the full power from the alt which has been about 13-14 volts since I switched to serpentine and a new alt. The electric and grounds all seem to be in check.
The FOURTH thing someone suggested was that maybe the fuel lines are getting vapor-lock from the heat of the engine or exhaust. --- ALL factory shields are still in place and the lines seem to be pretty protected from heat naturally from being out of the way. The lines are run through the frame up front and come out in front of the manifold itself. I have a hard time believing that this could be an option just because I do not believe that the lines could be getting that hot.

THE ONLY CONCLUSION IVE BEEN ABLE TO DRAW.
The power source for the pump itself seems to be a 14-16 gauge wire, that connects to the factory style GN sending unit, and inside the sending unit it appears to step down to a 18 gauge wire (something Smaller), and then the plug from the sending unit to the pump itself jumps the wire back up to a larger gauge wire. I was thinking maybe that small 3" section of wire is heating up and building resistance? But realistically, it was made to power a factory fuel pump in a GN. It is IMPOSSIBLE to change the wires from the plastic clip that does not come off the sending unit prongs to the plug for the pump.. I'd literally have to replace the sending unit or find another way to run wires to the pump directly.
ONLY OTHER CONCLUSIONS....
1. Faulty Regulator
2. Faulty Aeromotive Stealth kit.

PLEASE HELP. I'm losing my mind.
 
Last edited:
pontiacgp

pontiacgp

Canadian Prime Minister
Mar 31, 2006
23,522
4,238
113
Kitchener, Ontario
#2
You have a carbonated fuel pump?....:mrgreen:

where is the fuel filter and is it possible that is clogged?
 
Turbo Zach

Turbo Zach

Royal Smart Person
Sep 8, 2015
1,099
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50130
#3
First it looks like you have a pretty big pump for a carb. I have the same basic fuel system running a Holley Sniper at 60psi. At that low of a psi you are circulating a lot of fuel not to mention heating the fuel up. As for your problem. Do you have electric fans? If so I think your alternator is not big enough to keep up with everything. I am betting you are having a voltage or amp drop to the pump. I would find some one with a amp meeter and see what the draw is. Good luck! Now that I think about it, how is your return line ran? You are not deadheading the pump are you?
 
R

RGBody87

Apprentice
Jan 7, 2018
92
19
8
#4
You have a carbonated fuel pump?....:mrgreen:

where is the fuel filter and is it possible that is clogged?
Thank you for the heads up! I edited the post lol. Thank you for reading it carefully. I have a fuel filter right in between regulator and the carb. One of the cheap glass ones... again, I haven't replaced that filter or the carb because I can see through this filter and the other one is a russel racing black iodized one that you cannot see through.
 
pontiacgp

pontiacgp

Canadian Prime Minister
Mar 31, 2006
23,522
4,238
113
Kitchener, Ontario
#5
How do you have the return line routed?
 
R

RGBody87

Apprentice
Jan 7, 2018
92
19
8
#6
First it looks like you have a pretty big pump for a carb. I have the same basic fuel system running a Holley Sniper at 60psi. At that low of a psi you are circulating a lot of fuel not to mention heating the fuel up. As for your problem. Do you have electric fans? If so I think your alternator is not big enough to keep up with everything. I am betting you are having a voltage or amp drop to the pump. I would find some one with a amp meeter and see what the draw is. Good luck! Now that I think about it, how is your return line ran? You are not deadheading the pump are you?
GREAT point. I do have a set of 3000 CFM dual fans that I just installed recently prior to driving the car on the street again. I am running a stock 100 AMP alt from a caprice or S10. I have an Ampmeter I can hook up this week and check it.
My return line is ran pretty unique. I had to add another 1/4 tap to drop more fuel into the tank because the factory sending unit only has a 1/4 drop that puts the fuel right back under the pump. I'll post pictures, but I had to add a Y connector to split from 3/8 to two 1/4" return lines into the sending unit. The tank has only been FULL the times that I've run it. It says 26.60 AMP draw... but it doesn't tell me if that is both fans or one.
 
R

RGBody87

Apprentice
Jan 7, 2018
92
19
8
#7
First it looks like you have a pretty big pump for a carb. I have the same basic fuel system running a Holley Sniper at 60psi. At that low of a psi you are circulating a lot of fuel not to mention heating the fuel up. As for your problem. Do you have electric fans? If so I think your alternator is not big enough to keep up with everything. I am betting you are having a voltage or amp drop to the pump. I would find some one with a amp meeter and see what the draw is. Good luck! Now that I think about it, how is your return line ran? You are not deadheading the pump are you?
Turbo Zach,

It seems that each fan pulls 13.3 Amps and the fuel pump draws 5 amps. should be totaling up to around 33 AMPS total of the 100 amp alt.
 
Turbo Zach

Turbo Zach

Royal Smart Person
Sep 8, 2015
1,099
1,136
113
50130
#8
GREAT point. I do have a set of 3000 CFM dual fans that I just installed recently prior to driving the car on the street again. I am running a stock 100 AMP alt from a caprice or S10. I have an Ampmeter I can hook up this week and check it.
My return line is ran pretty unique. I had to add another 1/4 tap to drop more fuel into the tank because the factory sending unit only has a 1/4 drop that puts the fuel right back under the pump. I'll post pictures, but I had to add a Y connector to split from 3/8 to two 1/4" return lines into the sending unit. The tank has only been FULL the times that I've run it. It says 26.60 AMP draw... but it doesn't tell me if that is both fans or one. View attachment 96300 View attachment 96301 View attachment 96302
I know with a factory one wire alternator on my car (I think 90amp) it could not keep up with the demand of the fans and fuel pump. The fuel pressure was dropping with the fans on. I ended up changing mine to 140 amp alternator and it seems to have fixed the problem. A test you can try is. Unhook the fans and see if the pressure comes back up.
 
Turbo Zach

Turbo Zach

Royal Smart Person
Sep 8, 2015
1,099
1,136
113
50130
#9
Turbo Zach,

It seems that each fan pulls 13.3 Amps and the fuel pump draws 5 amps. should be totaling up to around 33 AMPS total of the 100 amp alt.
Is that actual draw or estimated? What is the hole car amp draw?
 
R

RGBody87

Apprentice
Jan 7, 2018
92
19
8
#10
I know with a factory one wire alternator on my car (I think 90amp) it could not keep up with the demand of the fans and fuel pump. The fuel pressure was dropping with the fans on. I ended up changing mine to 140 amp alternator and it seems to have fixed the problem. A test you can try is. Unhook the fans and see if the pressure comes back up.
I will do exactly that! Great Idea. Thank you.
 

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