What purpose does the vacuum line serve on a '97-'98 C5 fuel rail?
03-16-2002, 06:52 PM
#1
What purpose does the vacuum line serve on a '97-'98 C5 fuel rail? My LS1 uses the '97-'98 C5 dual feed fuel rail. I just read my fuel pressure for the first time today since getting my car running and noted that at idle it runs at 56PSI and when I took the car out for a good WOT run up to about 115MPH that the fuel pressure would drop about 3PSI to ~53PSI at the top of each gear as I shifted. I disconnected and plugged the vacuum line that goes to the fuel rail FPR and it made zero difference.....it still dropped about ~3PSI at WOT. My vacuum line taps into the rubber air inlet hose just in front of the throttle body like the C5's. What's the deal here?
03-16-2002, 07:31 PM
#2
TECH Addict
Join Date: Nov 2001
Location: St Louis, MO
Posts: 2,551
Likes: 0
Received 0 Likes
on
0 Posts
Re: What purpose does the vacuum line serve on a '97-'98 C5 fuel rail? Talking in old carb terms here. You're using ported vacuum instead of manifold vacuum. I dont know which is correct for this application.
On the LT1's the fuel pressure regulator is attached to manifold vacuum so that pressure is reduced at idle and part throttle. When there is no vacuum, in other words wide open throttle, full pressure is applied.
If I was having your problem I would hook up a hand held vacuum pump to the fuel pressure regulator and observe pressure changes corresponding to vacuum with the engine running at idle.
Perhaps you have a bad fuel pressure regulator?
Eric
On the LT1's the fuel pressure regulator is attached to manifold vacuum so that pressure is reduced at idle and part throttle. When there is no vacuum, in other words wide open throttle, full pressure is applied.
If I was having your problem I would hook up a hand held vacuum pump to the fuel pressure regulator and observe pressure changes corresponding to vacuum with the engine running at idle.
Perhaps you have a bad fuel pressure regulator?
Eric
03-17-2002, 10:18 PM
#3
Re: What purpose does the vacuum line serve on a '97-'98 C5 fuel rail? I agree Eric, that's what doesn't make sense about this set-up. Because it uses venturi vacuum, the vacuum signal will actually get stronger as the RPM climbs because more air is rushing in through the throttle body. Just the opposite of manifold vacuum.
03-18-2002, 12:43 AM
#4
Re: What purpose does the vacuum line serve on a '97-'98 C5 fuel rail? Thanks for the reply Eric. So with venturi vacuum (which is what this is set up for as opposed to manifold vacuum), should my FP actually go down or up if the FPR is working correctly?
03-18-2002, 12:48 AM
#5
TECH Addict
Join Date: Nov 2001
Location: St Louis, MO
Posts: 2,551
Likes: 0
Received 0 Likes
on
0 Posts
Re: What purpose does the vacuum line serve on a '97-'98 C5 fuel rail? FP should rise as vacuum rises. If it drops as vacuum rises, then maybe it needs to be hooked up to manifold vacuum??
Just a guess.
Eric
Just a guess.
Eric
03-25-2002, 04:20 PM
#6
TECH Resident
Join Date: Jan 2002
Location: dallas,tx.
Posts: 828
Likes: 0
Received 0 Likes
on
0 Posts
Re: What purpose does the vacuum line serve on a '97-'98 C5 fuel rail? the vacuum line hooked to your fuel pressure regulator reduces your fuel pressure at idle and lower rpm's as the engine doesn't need the extra fuel. at wot, you need the extra fuel pressure to feed the injectors and that's what happens when the vacuum pressure drops at the regulator. if the line is not hooked up, you are just creating unnecessary pressure at your injectors, and probably wasting fuel. someone correct me if I got that wrong.
