Alight well as some of you know I have been trying to get my new motor running for sometime now. This weekend I got everything wired up and installed and I went to start the car. Well I am getting funny fuel pressure. When I turn the key my fuel pump puts out 6psi(which is what the regulator is set at) and then it loses pressure pretty quickly. I can get the motor started but only for 2 seconds or so and then the pump loses pressure and air shoots through the carb. Anyone know whats going on? I appreciatte the help.

 

Try running the engine with 12v applied to the pump terminal of the ALDL, terminal "G" or the lower left hand terminal. How long does it run now?

Does the pump at least prime? If so, the relay is good. If It runs with 12v directly applied, try replacing the oil pressure switch.

 

Yes the pump primes. when I turn the key in the on position I can here it and it pressure guage reads 5psi but when the pump stops primeing the pressure drops. Also I dont think its the oil pressure switch because I took that out when I put a mechanical oil pressure guage in about a year ago it worked fine. Also I dont think this would effect pressure when the car is in the on position would it? Where is the ALDL connector at I havent ever used it so I dont know where it is. I will try your idea and get back to you. Thanks man.

 

ALDL is on the driver side, under the knee bolster (interior). if you peer under there, you will see what looks like it should have a little connector going to it. Thats the ALDL. Apply 12v to the bottom left hand terminal.

A few nights ago, i ran a fuse tapped wire from the fuse box to that terminal on my caprice to test the pump....and then i remembered the Caprice has a mechanical pump...Oh the things i can do when im sleep deprived

Here's how it works: When the pump primes, the ECM supplies power to the pump, through the pump relay. That pressurizes the system enough so that the engine can run and generate oil pressure. The pressure switch then takes over. It sees that there is adequate pressure and "opens" allowing power to the relay. If that switch detects a sudden drop in oil pressure, or it doesn't work, it cuts off the power supply and the pump stops.

On a side note, did you put the switch back it when you were done? (I've seen it happen)

 

Alright I hooked a wire from the aldl to the battery and guess what I got constant fuel pressure. WHOO HOO. But what does this mean? I really dont want to hot wire my fuel pump. Oh and by the way thanks alot stekman you help me out a ton since I started this project on friday. I really appreciatte man.

 

No problem at all, I'm glad i could help out. Besides,one more running third gen is one less in the junkyard.

As long as the ECM is in functional condition, there is no need to hotwire the relay. By completing that simple little test, you have completed the process of elimination. Observe: for the factory fuel system (EFI), there are a few main components.

-Pump
-Relay
-Relay fuse
-Oil pressure switch (not to be confused with the sender for the gauge, although, for you, they may be the same piece - a combined sender/switch)
-ECM

Obviously the car runs, so the ECM is good. The pump supplies good pressure, so the pump is good. The pump primes and whatnot, so the relay and fuse are good. You haven't blown the fuse yet for the relay, so the wiring is good. By process of elimination, the only major component left is the pressure switch. But, it may not actually be shot. Before you go replacing it, look at the connector. Is it properly connected?

 

here is what I experienced with my TPI to carb conversion and it sounds like the same issue you have.


The ECM only runs the pump for the prime. after that the fuel pump is activated by an oil pressure switch. This oil pressure switch is located above the oil filter on the drivers side of the block. The oil pressure sensor on the back of the block by the distributor is the one that runs the gauge. The first one I mentioned above the oil filter is the one that actually runs the fuel pump. This is the only thing that tells the pump to turn on. Once this switch sees oil pressure it will activate the pump. Let me know if you need any more info about this switch.

Hodge

 

As mentioned the oil pressure safety switch will shut the pump off at the relay once the oil pressure drops below a certain level. Many people who have done this swap have experienced problems similar to yours. There are a couple of ways around this. One is to bypass the relay by hooking the green and white wire to an "ignition-hot" power source. This is what I did, and it works fine. I have also heard about certain ways to 'fool' the switch into thinking it has adequate pressure by wiring resistors inline, and even using certain types of oil filters (?). I don't know much about these other two but if you do a search you will probably find more info. than you need. Maybe someone else who has experienced this will read this thread.

 

why would you need to fool the oil pressure switch into thinking that there is oil pressure. I you have the switch(sensor) in the block above the oil filter and you have the factory wiring hooked up to it then the pump should work fine.

I went this route because if there is an accident and the pump is wired to an ignition hot lead then the pump will continue to run. If you use the oil pressure switch the way it is intended to be used an you have an accident, once the motor cuts out so does the fuel pump.

 

Well I got the fuel pump problem taken care of for now and the engine is fired up. I actually had a few overheating problems with the motor yesterday and I got burned pretty bad but other than that the motor is good. I just need to tune the carb, and work out the problem with it getting so hot. Then I am going to drive it. Man I am excited I havent driven this thing in about 5 months and it sounds awesome. Thanks for all you help guys.