Fuel Pressure - Does not change (w/ or w/o vac line)

[onebad7up]

My 87 GN is not building boost so I am going through it to check the basics. Haven't seen it going over 1-2psi with my analog boost gauge.

Fuel pressure at idle (w/ vac line attached) : 39-40psi
Fuel pressure at idle (w/0 vac line) : same

Should the pressure drop whenever the vacuum is attached? Vacuum at idle is 18.

I replaced regulator with a replacement stock unit and it does the same thing. What should I look for?

 

[dynoman]

Check all of your vacuum lines for cracks & zip tie all connections first .

Should the pressure drop whenever the vacuum is attached

Yes , you should be able to feel the suction with your thumb over the open end .

 

[onebad7up]

Okay, I will check lines again tomorrow. The end that plugs into the regulator has strong vac.

I even hooked the mitey vac to pull vacuum on the regulator. It did not affect fuel pressure.

 

[dynoman]

Watch fuel pressure while pressurizing the regulator with the mityvac and see if it rises 1 for 1 .

 

[dynoman]

If fuel pressure does not rise with a boost pressure , DO NOT get into boost if you have to drive it .

 

[dank GN]

What kind of rig do u have ? What kind of fuel pump?

 

[rag231]

Also with vacuum removed your idle should go up a little bit and as soon as you put the line back on it should go down to normal. This is a stange issue. You are normally running an aftermarket fuel pressure regulator, it also looks like you are running about 3.5 -4 lbs low on fuel pressure if your fuel pressure gauge is correct also. Has the gauge been swapped out for accuracy as well? Is the car bone stock and how long have you been driving it seeing low boost (how long have you owned it)?

 

[Nasty Wendy]

#1 Either the gauge isn't responding or the fuel pressure regulator is bad.

#2 I see where you put a mighty vac on the regulator. Doing that should lower the fuel pressure with the car running. You should see the gauge increase when the mighty vac is disconnected and decrease with the mighty vac pulling vacuum on the regulator. If this is not happening then see #1.

 

[onebad7up]

What kind of rig do u have ? What kind of fuel pump?

I can't remember the pump number, but it was new when installed the hot wire kit. I also replace the filter today just in case.

 

[onebad7up]

I applied air pressure and the fuel pressure increased.

#1 Either the gauge isn't responding or the fuel pressure regulator is bad.

#2 I see where you put a mighty vac on the regulator. Doing that should lower the fuel pressure with the car running. You should see the gauge increase when the mighty vac is disconnected and decrease with the mighty vac pulling vacuum on the regulator. If this is not happening then see #1.

The fuel pressure gauge is brand new and I have it screwed into the shrader valve. Whenever the vacuum was applied with the mighty vac, I was expected the pressure to decrease but it didn't.

 

[Nasty Wendy]

Does any fuel come out of the regulator's vacuum port when you run the mighty vac there? Is this a stock regulator?

 

[Chuck Leeper]

"Haven't seen it going over 1-2psi with my analog boost gauge".
A clue to possible vac line issues: The dash ga is not working, either.
Have you been working on the vac lines? If so, are the vac check valves in the right place, and in the right direction?
Where are you getting the vac for testing? [18"]

 

[dynoman]

The fuel pressure gauge is brand new and I have it screwed into the shrader valve

Is the core removed from the shrader valve ??

 

[onebad7up]

No the core is still in place.


I decided to pressurize intake system to see if there were any leaks. The pcv system is leaking badly and all of the pressure was blowing out the driver side valve cover oil fill hole.

I blocked off the pcv that goes into lower rear of intake manifold and blocked off the tube that goes to passenger side valve cover. Car makes boost now!!!!!

I identified a couple other smaller leaks that need to be addressed, but it looks like I am going in the right direction.

Fuel pressure still doesn't want to drop below 49 but it might still be vac peaks. Need to rule that out first.

 

[Chuck Leeper]

No the core is still in place

You remove the core to check the psi, unless you have a Snapon psi tool that depresses the stem...

 

[87-WE2]

^^^^^ this. Been there and done that once. Pull your schrader valve out.


Sent from my iPhone using Tapatalk

 

[TurboDave]

Have you tried the simplest of tests yet? pull the vacuum line off the regulator and put your finger over the end of the hose. Can you feel any vacuum pulling on your finger???

 

[Pronto]

Your pcv valve is fubar. Put a us plastics check valve in the line and replace the pcv valve. With the line off the regulator, the fuel pressure should drop below 38. If it's not then you have a bad regulator or the return line is restricted.

 

[onebad7up]

Have you tried the simplest of tests yet? pull the vacuum line off the regulator and put your finger over the end of the hose. Can you feel any vacuum pulling on your finger???

Yes and it has vac. 18 at idle and around 20 when driving.

 

[Nasty]

I had this problem with my GN. In my instance the return line was restricted. To test for it, you will have to rig a rubber hose from the return at the rail back to the tank. If it corrects the problem then replace the return line at the very least. It is likely rust in the line. I replaced all of them with new S/S.