Fuel Erratic Fuel Pressure 1985 GT

[DienstXIV]

Hi Everyone,

I have a 1985 Mustang GT with a brand new Holley 600 CFM Street Warrior carburetor. When I installed the new carb I noticed that the stock mechanical fuel pump had a return line that was capped. I installed a fuel pressure gauge and found that the fuel pressure was over 11 psi and the needle was bouncy. I decided to reinstall the fuel return line and checked the fuel pressure again and it's now super high and bouncy. It also doesn't run very well. How could this be? Please check out the video.

What should I do to lower the pressure to 7psi? I was thinking of installing a dead head fuel regulator or one with a return and just connecting that return it to the return from the pump, but I am worried about the return line pressure getting to high.

Any help is much appreciated!

Cheers!
Alex

 

[Mcmahst]

You might just need a new fuel pump. They do fail, and it’s either a blown diaphragm or a failed pressure regulator. This one on Rock Auto sounds like the right specs you need. I’m sure there are others that would work as well:

More Information for SPECTRA PREMIUM SP1157MP

Good luck.

 

[DienstXIV]

You might just need a new fuel pump. They do fail, and it’s either a blown diaphragm or a failed pressure regulator. This one on Rock Auto sounds like the right specs you need. I’m sure there are others that would work as well:

More Information for SPECTRA PREMIUM SP1157MP

Good luck.

My car doesn't have a regulator, I thought Carbureted cars didn't have a regulator, but I honestly have no idea. If my car does need a regulator, what kind do I need?

Thanks,
Alex

 

[AeroCoupe]

This thread may help you.

D

Fuel Thread '1985 GT Fuel Line Diagram'

Apr 17, 2025

Hi Everyone,

I have a 1985 Mustang GT with a 670 CFM Street Avenger Holly carb. Does anyone have a fuel line diagram for a 1985?

With the warm weather I have been smelling fuel pretty strongly. The whole car just smells like gasoline. I checked for leaks but couldn't find any. I noticed that near the front drivers side of the engine where I believe the fuel pump is there is a plugged line coming out of the pump. I am wondering if this is for some sort of fuel return line going back to the tank that I need to reconnect. Also when I remove the fuel cap there is a lot of pressure that is...

• DienstXIV
• Replies: 8
• Forum: Fox 5.0 Mustang Tech

 

[DienstXIV]

AeroCoupe, that is a thread I posted last year about the same issue, been wrestling with this messed up fuel system for too long. 1985 was a transitional year and I have been just replacing parts hoping to fix the issues. Right now the car runs worse than it did back in April.

 

[DienstXIV]

This thread may help you.

D

Fuel Thread '1985 GT Fuel Line Diagram'

Apr 17, 2025

Hi Everyone,

I have a 1985 Mustang GT with a 670 CFM Street Avenger Holly carb. Does anyone have a fuel line diagram for a 1985?

With the warm weather I have been smelling fuel pretty strongly. The whole car just smells like gasoline. I checked for leaks but couldn't find any. I noticed that near the front drivers side of the engine where I believe the fuel pump is there is a plugged line coming out of the pump. I am wondering if this is for some sort of fuel return line going back to the tank that I need to reconnect. Also when I remove the fuel cap there is a lot of pressure that is...

• DienstXIV
• Replies: 8
• Forum: Fox 5.0 Mustang Tech

AeroCoupe, that is a thread I posted last year about the same issue, been wrestling with this messed up fuel system for too long. 1985 was a transitional year and I have been just replacing parts hoping to fix the issues. Right now the car runs worse than it did back in April.

 

[AeroCoupe]

So have you replaced the mechanical pump?

 

[DienstXIV]

So have you replaced the dual pump?

There is only one mechanical pump on the side of the engine block. Carbureted cars only had one pump. Just bought a new pump and am going to install it tonight or tomorrow

 

[manicmechanic007]

That car should have a fuel return / tee / regulator
Near the carb where the return line takes off
Those T's were hard to get back then, 84-85 and I am sure they are hard to get now
We used to have problems with cars and trucks that were starving for fuel, that regulator valve return port was the problem

 

[DienstXIV]

That car should have a fuel return / tee / regulator
Near the carb where the return line takes off
Those T's were hard to get back then, 84-85 and I am sure they are hard to get now
We used to have problems with cars and trucks that were starving for fuel, that regulator valve return port was the problem

The way the car is set up now, there is a 5/16" line going from the tank to pump. The pump has a 1/4" line going from the pump back to the tank as a return line. Then there is a line going from the pump to the carb and there is an inline fuel filter and my fuel pressure gauge after the filter.

I bought a new pump and plan to install it this weekend and add a holly dead head (no return) regulator after the pump. The holly part number is 12-803 you think this set-up will fix my erratic fuel pressure?

 

[manicmechanic007]

Yes, that low pressure one should work
Car really needs a 3/8 fuel line
Was a 3/8 line from the factory IIRR
5/16 will work until you try to do 140mph

 

[manicmechanic007]

Some trivia
I used to drive, align the front ends and balance the tires on Cop car Mustang interceptors back in 82-89 or whenever they quit driving Mustangs for the Highway Patrol. The carbureted versions would do 85mph in second gear. The very next year, same car with fuel injection would only do 72mph in second. The EFI was limiting top speed and the troopers hated it

 

[Blackhawkxx]

I bought a new pump and plan to install it this weekend and add a holly dead head (no return) regulator after the pump. The holly part number is 12-803 you think this set-up will fix my erratic fuel pressure?

Yes.

 

[DienstXIV]

Some trivia
I used to drive, align the front ends and balance the tires on Cop car Mustang interceptors back in 82-89 or whenever they quit driving Mustangs for the Highway Patrol. The carbureted versions would do 85mph in second gear. The very next year, same car with fuel injection would only do 72mph in second. The EFI was limiting top speed and the troopers hated it

Wow! You definitely can't do that with 4.10 gears!

 

[manicmechanic007]

Right, I think those cop cars had 3.55's or something like that

 

[DienstXIV]

I plan on sticking with my 2.73, I drive my Mustang 34 miles to work everyday (when it's not broken down or in pieces). I need those extra mpgs more than a few tenths of a second on my 0-60 time

 

[AeroCoupe]

There is only one mechanical pump on the side of the engine block. Carbureted cars only had one pump. Just bought a new pump and am going to install it tonight or tomorrow

My iphone auto corrected mechanical to dual most likely due to me misspelling it. I am pretty aware as to what types of pumps came on these cars but looked like an idiot there.