• Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • 1994 - 1995 Specific Tech

vacuum reading and fuel pressure

  • Thread starter Thread starter 95mustang_gt
  • Start date Start date Jun 7, 2006
9

95mustang_gt

New Member
Mar 30, 2005
364
1
0
Wisconsin
Jun 7, 2006
#1
  • Jun 7, 2006
  • #1
I took vaccum reading and check fuel pressure.The reading for the vacuum was 13-16.The fuel pressure was 35 w/vacuum and 39 w/o vacuum.Are those good numbers.If my valvetrain was the problem for my rough idle,will the needle on the vacuum be going crazy or just bounce around?

thanks
Jesse
 

Swarzkopf

Member
Sep 23, 2004
973
0
16
Pittsburgh
Jun 7, 2006
#2
  • Jun 7, 2006
  • #2
Someone correct me if I'm wrong here:

You checked this at idle? Your vacuum reading should NOT be moving by 3lbs at idle! That is a dead on indication of a fairly large vacuum leak.

Furthermore, with stock heads and a stock cam, you should be seeing 17-22lbs of vacuum at idle.

So...you probably have a significant vacuum leak causing idle problems.
 
F

fordtech28

Member
May 12, 2006
312
0
17
Riverside Cali.
Jun 7, 2006
#3
  • Jun 7, 2006
  • #3
those numbers are normal your rough idle can be caused by misajusted valvetrain or a sensor like the tps, maf, iac these three are the big players when it comes to idle
 
S

slocomo

New Member
Mar 19, 2006
48
0
0
Jun 7, 2006
#4
  • Jun 7, 2006
  • #4
vacuum and fuel pressure

I just added a vacuum can from comp cams. I created about four extra inches of vacuum at idle and about 10 when reved to about 2 thousand rpm. before my idle was about 6 inches now it's about 9 to 11. I have the aeromotive adjustable fuel pressure regulator and i have I think your almost right on with your fuel pressure everytime muscle mustangs and fast fords or 5.0 and super fords magazines publish dyno results they usually end up with about 40 to 42 psi fuel pressure at idle. I set my fuel psi to 40 and it seemed to drive better. It drops about 4 or 5 psi when under load. I got the little fuel pressure 1 1/2" auto meter fuel psi gauge. it's pretty cheap from summit like 30 bucks with the fitting I think. It works great. don't get a liquid gauge I hear If the liquid heats up causing the reading to be off. I hear you need a minimum of like 15" of vacuum for your brakes to work a 100 % but I'm not sure about that. I know my peddle is definitely firmer now with the extra vacuum. If the gage is bouncing then you have a leak somewhere. your stock vacuum cannister is near your ac blower fan take that stuff apart and check the hoses and connections. check your charcoal cannister hoses for deterioration. I know my hose is about to crumble away I'm sure I could my vacuum to stabilize a bit more if I replaced mine. I plan on doing it soon. If you removed your egr pump or any of the egr stuff like me then check all those hoses. make sure your valve cover breather seal is good and mak sure you have the hose in place that goes from the passenger valve cover breather to the cold air intake pipe connected or your vacuum will be off. I would just check everything you can before you do anything with your engine. once your do everything else then get a leak down tester if you don't have one and then you can see how well the seals in the engine are. Oh i forgot to say that the vacuum can actually stabillized my vacuum a bit too. so if all else fails you can get one of those they aren't as big as they look. I mounted mine right above my brake booster. I have a double cowl hood and therefore the clearance but it's screwed into the plastic in front of the winshield wiper motor.
 
S

slocomo

New Member
Mar 19, 2006
48
0
0
Jun 7, 2006
#5
  • Jun 7, 2006
  • #5
fordtech28 said:
those numbers are normal your rough idle can be caused by misajusted valvetrain or a sensor like the tps, maf, iac these three are the big players when it comes to idle
Click to expand...

yeah I had to play with the idle air control screw some to get a better vacuum and idle.
 

95Cobra302

Founding Member
Apr 9, 2002
394
3
19
Daytona Beach, FL
Jun 7, 2006
#6
  • Jun 7, 2006
  • #6
I just checked my vacuum and i'm getting a steady 19. That's with a pretty much stock engine. What else are you all getting??
 

Swarzkopf

Member
Sep 23, 2004
973
0
16
Pittsburgh
Jun 7, 2006
#7
  • Jun 7, 2006
  • #7
Yeah....a bouncing vacuum reading is NOT normal or okay for a stock cammed car....according to Haynes, it is indicative of a vacuum leak.
 
9

95mustang_gt

New Member
Mar 30, 2005
364
1
0
Wisconsin
Jun 8, 2006
#8
  • Jun 8, 2006
  • #8
slocomo said:
I just added a vacuum can from comp cams. I created about four extra inches of vacuum at idle and about 10 when reved to about 2 thousand rpm. before my idle was about 6 inches now it's about 9 to 11. I have the aeromotive adjustable fuel pressure regulator and i have I think your almost right on with your fuel pressure everytime muscle mustangs and fast fords or 5.0 and super fords magazines publish dyno results they usually end up with about 40 to 42 psi fuel pressure at idle. I set my fuel psi to 40 and it seemed to drive better. It drops about 4 or 5 psi when under load. I got the little fuel pressure 1 1/2" auto meter fuel psi gauge. it's pretty cheap from summit like 30 bucks with the fitting I think. It works great. don't get a liquid gauge I hear If the liquid heats up causing the reading to be off. I hear you need a minimum of like 15" of vacuum for your brakes to work a 100 % but I'm not sure about that. I know my peddle is definitely firmer now with the extra vacuum. If the gage is bouncing then you have a leak somewhere. your stock vacuum cannister is near your ac blower fan take that stuff apart and check the hoses and connections. check your charcoal cannister hoses for deterioration. I know my hose is about to crumble away I'm sure I could my vacuum to stabilize a bit more if I replaced mine. I plan on doing it soon. If you removed your egr pump or any of the egr stuff like me then check all those hoses. make sure your valve cover breather seal is good and mak sure you have the hose in place that goes from the passenger valve cover breather to the cold air intake pipe connected or your vacuum will be off. I would just check everything you can before you do anything with your engine. once your do everything else then get a leak down tester if you don't have one and then you can see how well the seals in the engine are. Oh i forgot to say that the vacuum can actually stabillized my vacuum a bit too. so if all else fails you can get one of those they aren't as big as they look. I mounted mine right above my brake booster. I have a double cowl hood and therefore the clearance but it's screwed into the plastic in front of the winshield wiper motor.
Click to expand...


Well right not my idle is not perfect so when i was doing the vacuum test the rpms were moving.I reset the iac and got it to idle at 900 but its still a little boucing going on and cleaned the maf.I still have to check the tps.Right now for the valve covers i had to plug all the holes with grommets untill my oil filler from trickflow comes in,so there is no hose going into the valve covers.

thanks
Jesse
 
You must log in or register to reply here.

Similar threads

M
01 mustang 3.8 fuel pressure issue
  • Mhaley2006
  • May 29, 2026
  • 1996 - 2004 SN95 Mustang -General/Talk-
Replies
0
Views
133
1996 - 2004 SN95 Mustang -General/Talk- May 29, 2026
Mhaley2006
M
K
Engine 2000 Mustang GT - Fuel issue post-blown spark plug repair
  • kenster1092
  • Feb 26, 2026
  • SN95 4.6L Mustang Tech
Replies
4
Views
340
SN95 4.6L Mustang Tech Mar 10, 2026
kenster1092
K
SURGING IDLE / NO IDLE CHECKLIST FOR 1996–1998 MUSTANG GT (4.6L 2V) - Work In Progress
  • Noobz347
  • Aug 30, 2025
  • 1996 - 2004 SN95 Mustang -General/Talk-
Replies
0
Views
727
1996 - 2004 SN95 Mustang -General/Talk- Aug 30, 2025
Noobz347
C
Fuel pressure help
  • clayton seiders
  • Aug 31, 2024
  • 1994 - 1995 Specific Tech
Replies
12
Views
1K
1994 - 1995 Specific Tech Sep 1, 2024
clayton seiders
C
B
'03 GT Fuel Pump/Pressure/Delivery Issue
  • Brodach
  • Oct 21, 2025
  • SN95 4.6L Mustang Tech
Replies
8
Views
884
SN95 4.6L Mustang Tech Oct 31, 2025
gkomo
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • 1994 - 1995 Specific Tech
Menu
Log in

Register

  • Forums
  • What's new
  • Media
  • Resources
  • Contact
  • Sponsor
X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?

X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?