Fuel pressure regulator vacuum.

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
If I'm reading this right, there should be a vacuum hose from the FPR to the air resonator on 2002 thru 2004.

If you cap it you won't get full fuel pressure when accelerating.
 

Simon01

Member
Dec 5, 2011
116
Ace1875 said:
So it's variable vacuum line?

Yes, manifold vacuum varies based on the engine rpms. It is highest at idle and gradually drops as the throttle opens.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Ace1875 said:
No not the manifold vacuum the fuel pressure regulator vacuum.

I'm :confused: as to what you're asking.

The FPR is controlled by the (variable) vacuum provided from the intake manifold.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Are you missing your hose or something? Why even ask the question? :confused: In my experience, if the hose falls off, your vacuum is messed up and idle RPM rises to 900 and things aren't right.
 

Ace1875

Original poster
Member
Mar 29, 2012
276
I think I have a bad hose cause it's very very loose. And after replacing everything I can think of, the rpm between 1000-2000rpms are not so smooth while driving.

I capped the regulator and on my autel ds708 I noticed that my regulator was at max PSI so I reinstalled that little hose and slight tape around it.

I noticed that the K&N CAI replaces the resonator. What do they do to that regulator hose?
 

Ace1875

Original poster
Member
Mar 29, 2012
276
Items replaced.
New plugs
New coils
New CPAS
New water pump
New fuel filter
New thermostat
New temp sensor.
New exhaust manifold.
New O2 sensors.
New gas tank.
New fuel pump.
eFans installed.
Cleaned throttle body.
New throttle body gasket
Cleaned intake manifold.
Seafoam'd engine twice prior to replacing plugs and coils.

Every part was either AC Delco or GM. no off brand parts where used.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Ace1875 said:
I think I have a bad hose cause it's very very loose. And after replacing everything I can think of, the rpm between 1000-2000rpms are not so smooth while driving.

I capped the regulator and on my autel ds708 I noticed that my regulator was at max PSI so I reinstalled that little hose and slight tape around it.

I noticed that the K&N CAI replaces the resonator. What do they do to that regulator hose?

There should be a connection in the CAI tube for FPR hose.

There should also be a connection for the PCV hose.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Ace1875 said:
yeah i saw them. are they hollow or closed at the intake tube? anyone have one?

77-3036KP.jpg


They wouldn't put them on there just for looks.:wink: They are hollow.
The big one is for the PCV.
The little one is for the FPR.
 

matthewcc17

Member
Jan 2, 2012
10
the roadie said:
Are you missing your hose or something? Why even ask the question? :confused: In my experience, if the hose falls off, your vacuum is messed up and idle RPM rises to 900 and things aren't right.

So i hooked up a fuel pressure guage, and ran the engine. The pressure sat at 56psi at idle (600rpm) and while I rev it. I pulled the vacuum hose off with no change in pressure or rpm. Still 56 psi. I blew in the hose to the fpr with no change. I sucked on the hose to the fpr and the fuel pressure dropped down to 48psi.

From what I have read, the fuel pressure should increase when there is more vacuum. Does this mean that my fpr needs to be replaced?
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
matthewcc17 said:
So i hooked up a fuel pressure guage, and ran the engine. The pressure sat at 56psi at idle (600rpm) and while I rev it. I pulled the vacuum hose off with no change in pressure or rpm. Still 56 psi. I blew in the hose to the fpr with no change. I sucked on the hose to the fpr and the fuel pressure dropped down to 48psi.

From what I have read, the fuel pressure should increase when there is more vacuum. Does this mean that my fpr needs to be replaced?


I believe it is just the opposite.
Vacuum is highest at idle so the FPR lowers the fuel pressure.
When you accelerate the vacuum is lower and the FPR allows full fuel pressure.

ADDED BY THE ROADIE: Backwards. Vacuum is lowest at idle, and the way the FPR works is to return the most fuel back to the fuel tank when vacuum is lowest.
 

matthewcc17

Member
Jan 2, 2012
10
Wooluf1952 said:
I believe it is just the opposite.
Vacuum is highest at idle so the FPR lowers the fuel pressure.
When you accelerate the vacuum is lower and the FPR allows full fuel pressure.

Ok. This vacuum line is connected to the intake tube.

The air filter provides restriction to air movement.

At idle only a small amount of air volume is needed for the engine to run. So with less flow through the air filter, there would be minimal to almost no vacuum in the intake tube (only slightly lower than atmospheric pressure).

Under WOT a very high volume of air is needed for the engine to produce power.
With a this great demand for air, the air filter is now creating more restriction to this high volume demand, increasing the amount of vacuum (lower pressure) in the intake tube.

A simple test of removing the air filter and holding your hand over the end of the intake hose to simulate the air filter (just leave a small hole) while idling then have a friend or significant other rev the engine and the difference can be felt.

How could it possibly be any other way?
 

tblazerdude

Member
Dec 4, 2011
321
So maximum suction would be at idle? I feel little to no suction on the air hose running from my CAI tube to my FPR, at idle.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
matthewcc17 said:
Ok. This vacuum line is connected to the intake tube.

The air filter provides restriction to air movement.

At idle only a small amount of air volume is needed for the engine to run. So with less flow through the air filter, there would be minimal to almost no vacuum in the intake tube (only slightly lower than atmospheric pressure).

Under WOT a very high volume of air is needed for the engine to produce power.
With a this great demand for air, the air filter is now creating more restriction to this high volume demand, increasing the amount of vacuum (lower pressure) in the intake tube.

A simple test of removing the air filter and holding your hand over the end of the intake hose to simulate the air filter (just leave a small hole) while idling then have a friend or significant other rev the engine and the difference can be felt.

How could it possibly be any other way?

You are probably right. I was thinking of engine vacuum, and that's measured after the throttle body.
 

tblazerdude

Member
Dec 4, 2011
321
And the result is, FPR does not need true engine (post throttle body) vacuum to function properly. It will function correctly running from the intake tube.
 

matthewcc17

Member
Jan 2, 2012
10
Wooluf1952 said:
You are probably right. I was thinking of engine vacuum, and that's measured after the throttle body.

Ah. Yes, I understand why you said that now. It makes sence seeing as the throttle plate is closed during idle.


So does this mean my fpr is working improperly?
Does anyone else get more than 56 psi at any time? (Specifically at WOT)
 
Aug 29, 2017
4
Florida
I get the same problem. I get little to no vacuum out of that little hose. Could the resonator, were the vacuum comes from be dirty or clocked.

There should be a connection in the CAI tube for FPR hose.

T
I get the same problem. I get little to no vacuum out of that little hose. Could the resonator, were the vacuum comes from be dirty or clocked.
I think I figured out what the problem is I had the same problem. I'll show you the little hose was connected to the wrong port there is a port down to the right .I will upload the pictureof for you.

See the bottom tube. That's we're the vacuum is. Connect ur little hose to the bottom metal tube. And plug up the top port. Then all you need to know is whether the regulator is working or not. Now you will have vacuum
 

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Mooseman

Moderator
Dec 4, 2011
25,664
Ottawa, ON
1. You are responding to a 5 year old dead thread. The OP has probably resolved his issue by now or has moved on.
2. Please do not post successively within 10 minutes. Your three posts have been merged.
 
Aug 29, 2017
4
Florida
Yes I just figured that out what I wanted to know is that little hose connects to the resonator Black Box and there is little to no vacuum wants to know if that's correct
 

Mooseman

Moderator
Dec 4, 2011
25,664
Ottawa, ON
Yes, that is correct.
 
Aug 29, 2017
4
Florida
Thank you. So there is two ports. That I see. The top one that is plastic and has little to no vacuum to it.Does the little hose from fuel regulator connect to the top black port. I will attach a picture

The one with the plug I think is connected to

But the shiny port . one at the bottom has vacuum. So to which one am I supposed to connect it
 

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