TPI Vette Eng Help PLEASE

[Tow Guy]

Hey People I've Got A Stumper For Myself:
I Have A Tuned Port Motor In A Street Rod And It Loses Fuel Pressure With The Key Off......The Plugs Look Ok Not Rich But Not Too Lean I Think???
When I First Turn The Key On-Fuel Pump On Press Goes To Approx 30psi
After About 30 Seconds The Pressure Drops To 20 psi
After About 1 min The Pressure Is 10 psi
After 2 min The Pressure is 0
While Running The Pressure Seems To Be Around 15 psi
WTF :( :(
I Guess This Is Probably Why It Is Hard To Start After Sitting A While
When It Finally Starts It Seems To Run Like The Timing Is A Little Off But When I Hit The Pedal It Goes Like Hell!!!!
Any Suggestions Would Be Appreciated.......Thank's In Advance...

[FryJet]

Hmmmmm........seems like it could be a bad regulator or a bad injector causing the presure to bleed off. Im no expert but thats what I would go for. Good luck.
F.J.

[WUTWZAT]

This sounds about right to me..... I think the fuel pressure is supposed to bleed down gradually back out the fuel pump with it not running. The 30 psi with it on sounds right and the 15 psi running is also on the good side. The 15 psi is a constant pressure after the motor has received what it requires to run. The 30 psi is the pressure without the motor taking any before the pump has to cycle off or revert the fuel to the by-pass.
When it is hard to start what does it do?
Does it turn over and over and over......
Does it turn over and you hear slight burst, but not strong enough to fire?
If your rail has the above pressure, check that injectors are clean and free from leaks.
Good luck
Jason

[hava life]

You might have more help in the gear heads forum.

[Oldsquirt]

I Have A Tuned Port Motor In A Street Rod And It Loses Fuel Pressure With The Key Off......The Plugs Look Ok Not Rich But Not Too Lean I Think???
When I First Turn The Key On-Fuel Pump On Press Goes To Approx 30psi
After About 30 Seconds The Pressure Drops To 20 psi
After About 1 min The Pressure Is 10 psi
After 2 min The Pressure is 0
While Running The Pressure Seems To Be Around 15 psi
If your pressure is dropping that fast, you definitely have a problem. The system should maintain pressure for hours, not minutes, although some drop is normal. I will assume you do have the pressure gauge plumbed into the fuel rail, right?
An initial pressure of 30 psi when the pump comes on sounds about right, if not just a few pounds low. Your running pressure should be in the mid-thirties(check a GM manual for exact number) on a typical EFI system. Assuming the TPI you have has a vacuum modulated pressure regulator, the pressure will INCREASE when the engine is under load(manifold vacuum decreases allowing the pressure regulator to close slightly).
It is important to know that the typical EFI fuel pump for a port injection system will have a dead-head pressure of upwards of 80 psi. These pumps have a one-way check valve on the outlet side to prevent fuel from bleeding back through the pump.
You have four common sources of pressure drop. First is the one that would be most noticeable----an external leak.
Another would be a bad pressure regulator. These can fail in 2 ways. They can stay open and let too much fuel return to the tank. They can also rupture the diaphragm which allows fuel to be pumped through the vacuum line into the intake manifold.
Fuel pump failure can cause the problem. A weak pump or bad check valve. On GM pumps there is another issue thatbears mentioning. The stock-style in-tank pumps were connected to a fuel outlet tube by a short length of hose. Gm Had problems with these not fitting tightly enough and leaking fuel before it ever got into the lines. It also mimiced a bad check valve when engine was shut down.
Last major source would be leaky injector(s) although that would not likely cause such a low running pressure.
How to test? First test is to simply remove the vacuum line from the regulator with the KEY OFF. Are there any signs of fuel in the hose? IF YES, regulator is bad. If NO, find a way to block the return line from regulator to tank. This can be tough if you have braided-stainless or hard lines running alll the way back. With line blocked, cycle the key ON. You should see a pressure very near to pump's maximum pressure(approx 80+ psi) and it should hold. If so, the regulator is stuck open and should be replaced. If NOT, move to next test.
With regulator return still blocked, find a spot in the line from pump to the fuel rail that you can pinch off. Cycle the key ON and immediately block the fuel supply hose. Does the pressure now hold? If it does, the likely cause is at the pump, either the pump itself or the attaching hose if it is an in-tank pump. If the pressure doesn't hold(and you are POSITIVE the lines are blocked) likely leak is injector(s). Note that a leaky injector will usually give a very rough idle on initial start and isn't likely to give the pressure readings you listed.
You should also consider replacing the fuel filter as a maintenance item and absolutely should replace it if the pump is failing.
Please make ALL fuel system tests using EXTREME care. We don't want any fires around here.

[Tow Guy]

Cranks About 15-20 Seconds Then Fires After Warm Fires After About 3-5 Seconds...
I'll Post In Gear head Section Too..
Thank's For The Input.

[sanger rat]

You need a sealed gas cap.

[73kona455]

is it an intank pump?.. if so check the hose that goes from the fuel pump to the metal pipe on the fuel pump hanger.. have seen them leak there and lose pressure in a hurry... what year tpi is it?.. my 85 has about 39 psi running

[Tow Guy]

Year 1986
In Tank - Yes

[BADASS38CHEVY]

check fuel filter first.then check the fuel pressure and as it goes down squeeze the fuel return line with a pair of pliers,if fuel pressure goes up most likely you have a bad pressure regulator.

[HP350SC]

Oldsquirt's diag. is right on the money.
I know on Ford's you can cycle the key to "run" a couple times to build up fuel pressure before you crank it over. Might save your starter until you get it fixed. Oh wait, maybe your setup triggers off oil pressure. I think GM did around that year.

[Moneypitt]

Those TPI cars had extensive fuel tank baffles to keep fuel around the pump at all times. What type tank are you using, and could you be sucking it dry around the pump? The pump is only suppose to run for a couple of seconds when the key is first turned on, then recover after start up. What wiring harness was used? The pressure should be well above 15 running, like 35 - 40. I ran a TPI with a vented cap and no problems...........MP

[Kindsvater Flat]

I know the TBI systems used a low pressure 12-13 psi and the later stuff uses a high pressure 56-62 psi. I also know on the high pressure that if you have 50 psi it won't start. Most of the GM systems runs for 5 sec KOEO and the fuel pump starts runs during crank. On the early if there was no or low oil pressure it would shut the pump down. The TPI requires 30-44 psi at idle and 44-48 psi KOEO.