question for c4 mechanics
Thread Starter
Instructor
Joined: Apr 2012
Posts: 179
Likes: 3
question for c4 mechanics
I converted my 81 to an aftermarket TPI system with an adjustable fuel pressure regulator stand alone unit that allows the pressure to drop completely off possibly with a fair amount of the fuel above the injectors returning to the tank making for extended cranking until the pressure builds back up, so my question is: on stock TPI units does the fuel pressure remain in the fuel rails after shutdown instead of dropping off and also does the fuel pressure regulator BODY have ports on the side that I can use to adapt just the regulator in my system as I can’t use the rails on my F.I.R.S.T after market system.
Thread Starter
Instructor
Joined: Apr 2012
Posts: 179
Likes: 3
Thanks, so eventually the fuel pressure will drop down to zero? Also I am hoping that there a fuel ports on both side of the body so that I can incorporate this into my system with barbed connectors or something similar. Thanks
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Eventually. It shouldn't really matter. The pump should re-charge the lines w/in about 2-3 seconds. If the stand alone has the same feature of running the pump for ~2seconds at key on, then it should be ready to go when you crank it.
Thread Starter
Instructor
Joined: Apr 2012
Posts: 179
Likes: 3
Thanks, my concern is that the stand is allowing too much fuel to return back to the tank while the car is sitting requiring more like 5 seconds of cranking before 40 psi at the injectors is achieved, seems like an intermittent thing like the fuel is being siphoned or pulled back into the tank, I was told to cycle key on off several times before cranking to build pressure not my style though it needs to be right
Drifting
Joined: May 2008
Posts: 1,692
Likes: 489
From: Denver, NC
The fuel pump has a check valve that prevents fuel from draining back to the tank at shutoff. This prevents air in the rail that requires extended cranking or keying multiple times to fire the pump.
The fuel pressure regulator shouldn't relieve pressure until the spring is overcome - otherwise it shouldn't let fuel return to the tank. At key off, it doesn't allow the rail to drain.
With that said, if your regulator is allowing fuel to return to the tank to the point of draining the rail, that does not appear to be correct and would appear to be a problem.
Adjustable regulators just add or relieve pressure to the relief spring.
The fuel pressure regulator shouldn't relieve pressure until the spring is overcome - otherwise it shouldn't let fuel return to the tank. At key off, it doesn't allow the rail to drain.
With that said, if your regulator is allowing fuel to return to the tank to the point of draining the rail, that does not appear to be correct and would appear to be a problem.
Adjustable regulators just add or relieve pressure to the relief spring.
Last edited by Ed Ramberger; Feb 8, 2021 at 07:17 PM.
Thread Starter
Instructor
Joined: Apr 2012
Posts: 179
Likes: 3
Thanks but the adjustable pressure regulator that I have let's the pressure drop to zero as soon as the car is turned off and I can hear fuel returning to the tank which I believe is leaving a void that has to be filled, maybe there are stand alone fp regulators that maintain the pressure when the car is turned off
Team Owner
Joined: Sep 2003
Posts: 25,348
Likes: 2,728
Put a fuel pressure gauge on it and test it a few times. I'd think a very simple/straight forward check of the 'actual situation'. I don't believe all pumps actually have efficient check-valving in the 'supply' line. You've not mentioned much about the system specifics!
Last edited by WVZR-1; Feb 8, 2021 at 07:32 PM.
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Safety Car
Joined: Jun 2010
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Thanks but the adjustable pressure regulator that I have let's the pressure drop to zero as soon as the car is turned off and I can hear fuel returning to the tank which I believe is leaving a void that has to be filled, maybe there are stand alone fp regulators that maintain the pressure when the car is turned off
Can you post a picture of how you have plumbed the regulator?
Thread Starter
Instructor
Joined: Apr 2012
Posts: 179
Likes: 3
Thanks for the replies I just Google the adjustable FPR that I have aeromotive 13301 and it holds zero pressure even after the prime, poor design to me, so I will be replacing it with a compact adjustable FPR that holds pressure for at least after the prime glad to have figured this out this should put and end to the extended crank time while pressure builds
Drifting
Joined: Mar 2018
Posts: 1,360
Likes: 400
From: DFW TX
Thanks for the replies I just Google the adjustable FPR that I have aeromotive 13301 and it holds zero pressure even after the prime, poor design to me, so I will be replacing it with a compact adjustable FPR that holds pressure for at least after the prime glad to have figured this out this should put and end to the extended crank time while pressure builds