Fuel Erratic Fuel Pressure 1985 GT

DienstXIV · Jan 14, 2026

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 · Jan 14, 2026

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 · Jan 15, 2026

Mcmahst said:
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 · Jan 15, 2026

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 · Jan 15, 2026

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 · Jan 15, 2026

AeroCoupe said:
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 · Jan 15, 2026

So have you replaced the mechanical pump?

DienstXIV · Jan 15, 2026

AeroCoupe said:
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 · Jan 15, 2026

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 · Jan 15, 2026

manicmechanic007 said:
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 · Jan 15, 2026

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 · Jan 15, 2026

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 · Jan 15, 2026

DienstXIV said:
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 · Jan 15, 2026

manicmechanic007 said:
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 · Jan 15, 2026

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

DienstXIV · Jan 15, 2026

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 · Jan 16, 2026

DienstXIV said:
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.

Fuel Erratic Fuel Pressure 1985 GT

DienstXIV

Member

Mcmahst

DienstXIV

Member

AeroCoupe

lube between the nut and the face. I know my lubes

Fuel Thread '1985 GT Fuel Line Diagram'

DienstXIV

Member

DienstXIV

Member

Fuel Thread '1985 GT Fuel Line Diagram'

AeroCoupe

lube between the nut and the face. I know my lubes

DienstXIV

Member

manicmechanic007

DienstXIV

Member

manicmechanic007

manicmechanic007

Blackhawkxx

Advanced Member

DienstXIV

Member

manicmechanic007

DienstXIV

Member

AeroCoupe

lube between the nut and the face. I know my lubes

Similar threads

Fuel Erratic Fuel Pressure 1985 GT

Member

Member

lube between the nut and the face. I know my lubes

Fuel Thread '1985 GT Fuel Line Diagram'

Member

Member

Fuel Thread '1985 GT Fuel Line Diagram'

lube between the nut and the face. I know my lubes

Member

Member

Advanced Member

Member

Member

lube between the nut and the face. I know my lubes

Similar threads

Privacy & Transparency

Privacy & Transparency