ok the problem started when me and my friend broke my old TBI unit off my 92 Firebird after we replaced it from a 92 camaro’s TBI unit weird idle problems have been happening like when the car is at idle it will alternate rpm under 1000 RPM and its killing my performance we cant get the fuel pressure to move from 7 PSI. What we have done to the car to try and fix it is, replaced all the vacuum lines to the TBI, rebuilt the TBI, changed the fuel filter, changed the fuel pump, and set the IAC. My friend has the same problem with his 89 firebird but he knows it’s a vacuum leak. Anyways any help is appreciated!

 

The fuel pressure should be a steady 9-12 some odd psi. There could be a restriction in the fuel system or the spring in the regulator could be bad. I guess some how the pump could be bad but thats unlikely as you replaced it.

The only thing that controls the fuel pressure is the regulator. The vaccum or the state of the engine wont effect the fuel pressure unless the fuel drawn by the motor exceeds the pumps capacity at that pressure.

 

Like 2 or 3 weeks ago we put an adjustable fuel pressure regulator on. Today when we rebuilt the TBI we put the best stock injectors my friend had in and changed pretty much the whole regulator to another one he had. So we put the regulator on all the way up and still it's at 7 PSI and is still doing the idle alternation. The pump in the car is for TPI because I want to convert over to that sometime the TPI fuel pump operates at a higher PSI than TBI fuel pump. Could that contribute to the problem??

 

About the only thing I can think of is that maybe there is a restriction somewhere, incorrect installation, or maybe the screen on the pump pickup is clogged with sedement and severely blocking the fuel flow. Its unusual that the pressure doesnt rise up at least a little when you close the regulator all the way. If there was a restriction dead heading the pump would make the pressure come all the way up to the pumps maximum pressure. Try running the pump with the car off and the regulator to the max and see if the pump can build any pressure with no fuel flow. This can be done by pulling the connector off of the oil pressure switch and jumpering it. This will close the circuit and run the pump.

Really stupid question but your measuring the fuel pressure off of the larger of the two fuel lines, right? Im sure you are but I jsut want to make sure. Also just as stupid but which way are you turning the screw on the AFPR? CCW should raise the pressure and CW should lower it.

 

Just a few things to add. You didn't by chance put your fuel filter in backwards did you?

Dimeneted, don't you mean CW rasies the pressure? The GM vacuum unit is as such. Every 2.5 turns CW gives about 1psi boost.

 

I thought he had just a plain AFPR. On the DIY ones and probably alot of the aftermarket units they use a standard type screw to raise the spring seat so if your looking underneath at the screw itll be CCW to raise the pressure.

Donno about the vac units though. Havnt used one yet so your probably right.

 

Ha I never thought about it that way. I geuss I should have stated this. If you take the regulator off (in order to access the screw) CW will increase the pressue. If you can get an allen screw or any other toll under there without removing the top of the pod than CCW would be increasing as well. I assumed that all regulators (minus the holley) had to be removed before adjustments could be made.

 

Heh... I guess all things are relative. I found my stock regulator in my junk drawer and looking at the screw its CCW to raise the seat and CW to lower it. I guess its different for some of the other regulators.

 

I just looked at the fuel filter to see if it was put on backwards and its not. When we did the fuel pump we also replaced the screen. When we turn the car to the on position the pump turns on for a few secs and the PSI goes up to 7 PSI thin the pump turns off and the PSI drops to 0. I asked my friend if it could be the fuel pump relay he said can’t be but now his stating to think it might be. My car doses have some electrical problems like a couple things in the interior and when I press the hatch button the break light comes on but when I use my emergency break it doesn’t come on. What do you think? Could it be the relay?

 

If it is loosing pressure something is not right.
Granted I am by no means a TBI expert, but on the high pressure fuel systems, if the pressure drops significantly with any quickness, that is a bad thing. It means the drain valve on the pump is stuck, or malfunctioning and releasing pressure when it should not be.
SO of course when the line pressure comes up upon running the motor, the drain back valve opens, and releases the lines pressure, by shear luck the pump can overcome it somewhat.

 

Its common on alot of the tbi cars to loose pressure after shutdown with an old pump. Im not familiar with the pumps construction but I would think that there would be a check ball acting as a one way valve that keeps the fuel pump from regurgitating back into the tank. On the older pumps it doesnt seem to be a problem but on a new pump it should hold the pressure. If its not visibly leaking, the regulator spring is nearly pressed solid, and it still wont build pressure then the problem may lie with the pump itself.

 

ok the old pump was putting out 7 pis and the new is still at 7 psi so it must be something with?????

 

The pump is new and before we replaced the old pumps pressure was 7 PSI and with the new pump it’s still at 7 PSI. 2kickassbirds is the person who helped me with it. But how could it be the pump when it’s new?? And dose the same thing the old pump did?

 

Just because it is off the shelf does not mean it is good.
Trust me on that one. I have gotten plenty of bad parts off the shelf.

 

Im having a similar problem. a month or 2 ago I noticed that when i take my gas cap off it no longers hisses to relieve built up pressure. I had planned on doing some fuel system mods before that and went ahead and did them.
Vacum advance fuel pressure regulator.
vacum delay valve.
autometer fuel pressure gauge.

after installing those I see that when I start up my car it goes to 9si, but then slowly creeps down to about 6psi when the car is running.

also after I drive the car I smell gas.. like gas is leaking but I can't find any gas leak anywhere, and I have checked the fuel lines from the throttle body all the way back to the gas tank with it running and found no leak anywhere.

any idea?
fuel pump wearing out, or very small leak somewhere just enough to bleed of pressure?

 

My friend and I have fixed the problem. The problem was that the little rubber hose between the fuel pump and the fuel sender had a cut in it so we replaced it and now it works fine. So I had a fuel leak but it was internal. When the gas cap doesn’t hiss any more it means your gas tank has little holes that may not let gas out but just the pressure. But you may want to look at the rubber piece between the pump and the sender for your psi problem.