Fueling & Injection - Is this fuel pressure correct? Other problems.
05-25-2012, 05:01 PM
Scenario: New 402ci with stock L92 heads. L-92 truck intake manifold. 42# 8.1L marine injectors. 92mm F.A.S.T. air door. Cam is 228/238 .571/.585 113 LSA +4. Chassis dyno sheet below. Rough idle, slight bog right off idle, intermitant engine die at idle or when decelerating. 2002 Tahoe 4x4, 4:10, 32" dia. tires.
I decided I would check the fuel pressure at the injectors. Here is what I got:
Idle with vacuum hose attached: 45 psi
When accerating engine speed to 2500 rpm: 50 PSI briefly (under load), then 40 psi
Idle with vacuum hose NOT attached: 51 psi
Pressure bleeds to 0 psi after 30 minutes of engine off even though I have an in-line check valve. It is not bleeding from regulator return line. Seems like a back check valve or injector leak?
Are these normal readings? Any suggestions as to the problems?
05-26-2012, 08:48 AM
Being the regulator is vac referenced, the IFR in the tune, has to show the same value at all points.
It appears there's something going on w/ the system, in general.
Those injs are rated to work at 58psi. It's apparent you have the pressure set at some other value. [Looks to be 51??]
Vac on = 45psi, and the line off it's 51...That's an OK change.
What isn't OK, is the drop to 40 as the vac goes down, [MAP up] under load. To work correctly, the pressure should rise back to the line off value of 51. [Even tho it's not correct for that inj.]
As for the psi drop after shut down:
Are the injs fresh, or used and been sitting? [Dirty injs may be your leak].
Where do you have the check valve?
What kind of pump are you using? In tank?
Have you checked for vac leaks?
Idle issues most likely can be identified with a data log[s].
A tune is in order to address the cam, intake system, any injector spec changes.
05-28-2012, 12:32 PM
Thanks for your response Chuck. I understand most of what you mentioned. I decided to do a 'real world' fuel pressure test by wiring the pressure guage to my wipers and take a ride. What I saw was at cruising speed (no load) the pressure was in the 42/43 psi range. Under acceleration w/load, the psi when up to about 51 psi. I didn't see any drops to 40 psi. So it appears the regulator is operating correctly but not at the psi numbers required for my injectors. Unfortunately, my regulator is not adjustable. It is one used and recommended by a local LS airboat motor builder that he uses on his motors. I will now be ordering an adjustable unit and will report back after I get it installed.
Today I was going over the motor trying to figure out why it will die abrubtly. Because I have all new sensors on the motor (TPS, IAC, MAP, Crank, Cam, Knock, etc.) and the way it is behaving, I am now suspicious of the MAF or PCM. I got a new MAF and it ran the same. But then I decided to disconnect the battery cable for 45 minutes and reattached. When I started the motor I really didn't see the rpms going up and down like a normal re-learn, but it did run much smoother except I still have a bit rough idle. As it was idling, I started checking possible wiring problems by wiggling the harness and wiring connectors. Low and behold when I moved the MAP sensor slightly, the motor surged and dropped abrubtly and I could hear a vacuum leak. I pulled the sensor and the seal had a slight tear. Not having another one, I applied some blue RTV and inserted it back in. All-in-all it is doing much better now. The thump in the idle I presume is from my cam specs?
Regarding losing pressure after shut-off, I placed the check valve within 6 inches of the tank pressure out port. After turning off the motor, I disconnected the return line at the regulator to see if it was releasing there, but not a drop after 15 minutes. So I suspect one or more of the injectors are leaking. Sound right?
Thanks for your help.
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