: 4.6 northstar fuel pressure?



d'elegance#1
11-11-07, 02:36 PM
1 st post here and hope you guys/gals can answer? I have been testing the fuelrail pressure on my co-workers cadillac Deville D'Elegance's 4.6 northstar engine. And I am only getting 24lbs. with the key on engine off and 36-7lbs running. Now shouldn't the pressure be 40-50lbs either way? :hmm: This all started with a very hard starting engine. Meaning you have to continuiously crank it for a couple of seconds for it to start. Worse when the motor is cold better when it's hot! Any help would be appreciated! Thanks in advance! Think the pump is going doing a filter change and checking the pressure regulator monday. Some said there is a filter in the regulator. Found him a pump on-line but don't know if I can get the pump out without destroying the sending unit? First caddy I have worked on. But changing the plugs is a BLESSING nice and easy, 20 minutes MAX! :cool:

Ranger
11-11-07, 07:17 PM
Yes, fuel pressure should be about 45 psi. 24 psi! No wonder it won't start. FPR would be the first thing I would check. No filter in it. You should have no trouble replacing the pump without damaging the sending unit if necessary. Don't know what year you are working on, but about '99 they started putting an access panel in the trunk so as to avoid dropping the tank. Have a look first if it comes to that.

d'elegance#1
11-11-07, 07:57 PM
Thanks Ranger it's a 1997 D'Elegance 4.6. I figured the rail pressure was way out ! The chiltons he got for me really leaves alot to be desired, I have several Haynes and they seem more indepth for information. The only test the chiltons has is remove the vaccum line from the regulator and see if there is fuel in the vaccum port, and if the pressure jumps up when you remove the vaccum line? It says it has a filter screen? Thanks I'm praying right now it has a access somewere in the trunk! Cause I really don't want to drop the tank if I don't have too! :ill: Does anyone know if the sending unit housing is plastic or steel. Hope I can change just the pump the assembly all together is like $242 bucks. Trying to save him a few bucks by just changing the pump itself.

Ranger
11-11-07, 08:57 PM
Chiltons & Haynes manuals are better left in the bottom of the bird cage. Get a FSM by Helm Inc. Pull the vacuum line off the FPR with the engine idling. If there is any fuel present at the nipple, replace it.

I just saw a post the other day regarding changing a fuel pump and it had a link to a Bonneville site with pictures. I think it said that they were the same. I am pretty sure the '97 does not have an access panel.

d'elegance#1
11-11-07, 11:01 PM
LOL!! I'll agree on the chilton/Haynes! Going to have to look for a FSM hard to beat that! I have a dodge ram and download a FSM, no comparison to the store boughts! Man that's what I was afraid of ! I just been searching and found many FPR test threads, thanks ranger. I'll look up fuel pump change next. Pics would be a bonus, don't like just going in blind! Thanks for the help, looks like I have to elimanate the FPR, but I got a BAD feeling he'll be buying the pump! :(

Ranger
11-11-07, 11:27 PM
Check Ebay for the FSM.

d'elegance#1
11-11-07, 11:45 PM
Thanks will do! By the way is there any kind of procedure you have to do after removing the TB on the 97 4.6? Like holding the gas pedal down or something like that for the ISC to re-learn?

Ranger
11-12-07, 12:30 AM
I don't think so. Why are you going to remove the TB?

97Concours1
11-14-07, 12:45 PM
Keep in mind that the fuel pressure will vary based on the manifold vacuum. The 45 psi would be with no manifold vacuum. At idle, the manifold vacuum is the highest and the FPR will reduce the fuel pressure by the amount of the vacuum (that's the FPR's job). 20 inches of vacuum equals about (-10 psi) (vacuum). Subtract this -10psi from the 45 psi and you have roughly 35-37psi.

Ranger
11-14-07, 06:02 PM
I don't think that is correct. The FPR's job it to maintain a relatively constant fuel pressure across the injectors. It should have the spec'd pressure all the time. Granted it WILL vary some with manifold vacuum. If what you are saying is true, 45 psi would be the WOT spec. FSM does not give that. It just lists 45 psi as the fuel pressure and that would be at idle.

97Concours1
11-14-07, 08:47 PM
I don't think that is correct. The FPR's job it to maintain a relatively constant fuel pressure across the injectors. It should have the spec'd pressure all the time. Granted it WILL vary some with manifold vacuum. If what you are saying is true, 45 psi would be the WOT spec. FSM does not give that. It just lists 45 psi as the fuel pressure and that would be at idle.

Ranger,

I think we are almost saying the same thing. Here is my understanding of it:

I'm no expert on Cadillacs, but I'm sure all fuel injection systems are similar. Actually, the FPR's job is to maintain an EXACT pressure drop of 45 psi (or some other constant design pressure) ACROSS the injectors. Across the injectors means from the injector's input (the fuel rail), to its output (the intake manifold runner), which is under vacuum. The injector's output is NOT to ambient pressure. It is to manifold vacuum. When you measure the fuel rail pressure, your gauge is measuring the fuel pressure relative to ambient pressure, not relative to manifold vacuum. If you could stand inside the manifold and read the gauge, it would read a constant 45 psi.

One thing that I don't think is understood by many is the operation of the "injector". They don't actually pump (inject) fuel into the intake themselves. They are just a valve. They open up briefly and allow the fuel to flow through them. Their fuel output is based on the DURATION that they are open (pulse width). They DON'T pump a given amount each time they are pulsed. The ECM sends a certain pulse DURATION to the injector to get a certain amount of fuel to the cylinder. The ECM depends on the fuel pressure ACROSS the injector to be exactly 45 psi. The ECM doesn't account for this pressure changing. It depends on it to be exactly 45 psi at all times. All the injector pulse duration tables in the ECM program are developed based on this pressure being 45psi. If the FPR fails, then everything gets screwed up and the engine will not get the correct air fuel ratio.

If anyone has a different understanding of how this works, please feel free to correct me.

Ranger
11-14-07, 10:05 PM
Agreed. I do not agree with "The 45 psi would be with no manifold vacuum". That would be at WOT. I am sure the spec's are for idle, where a tech would be testing it. Not sure that I agree with the amount of pressure drop you listed, though I have to admit, I never really checked to see. I'll have to hook up a gauge some day and watch it to satisfy my curiosity now. I know it will drop. Just didn't think it was that much.

97Concours1
11-15-07, 07:04 AM
Agreed. I do not agree with "The 45 psi would be with no manifold vacuum". That would be at WOT. I am sure the spec's are for idle, where a tech would be testing it. Not sure that I agree with the amount of pressure drop you listed, though I have to admit, I never really checked to see. I'll have to hook up a gauge some day and watch it to satisfy my curiosity now. I know it will drop. Just didn't think it was that much.

I'm not sure the design pressure is 45 psi either. I messed around with some other GM products (i.e. Tuneport Corvette and Turbo Syclone). Seems like 42.5 psi was the pressure that GM used most of the time. All this gets really complicated when you add a turbo into the equation. If you run 20 lbs of boost, your maximum fuel rail pressure will be 42.5+20=62.5psi. The fuel pump needs to deliver extra fuel flow and pressure at the same time. These applications need very powerful fuel pumps.

Back to the Northstar, keep in mind that the manifold vacuum is usually measured in in-hg, and the fuel pressure in psi. If the engine is pulling say 20 in-hg at idle, this converts to approximately negative -10psi. The fuel rail pressure should be reduced by 10 psi. I wonder if the FSM says to pull the FPR vacuum line off briefly and check the fuel rail pressure (while the engine is running). This should give the constant design pressure we are talking about.

97Concours1
11-15-07, 07:46 AM
I guess I should've done a little homework before all this. I just read the FSM procedure. It looks like the pressures are higher. They are saying 48-55 psi with the FPR vacuum line disconnected. Looks like they are shooting for about 52 psi instead of 45 psi.

Ranger
11-15-07, 12:05 PM
Yeah, I forget the exact figure. I always say ABOUT 45 psi. Don't think I have ever read the procedure either as I have not had to trouble shoot it yet. Thanks for doing the research.