Its a 305 TBI, comp cams XFI268, 113 vette heads, 1.6 rocker arms, edelbrock 3704 intake manifold, stock 305 throttle body[305 injectors]

As soon as you crank it, fuel pressure goes up.... then straight down to 0

Should I try turning the fuel pressure up? No idea where to start.

 

Went back to the factory setting on the computer [i had it leaned out before because of my fuel system upgrades] did no good, tried a couple other BINs that came with the EBL Flash and none of them worked any better.

Car will start for like .5 seconds, then it just dies.

 

no fuel pressure bad fuel pump

 

Well the pump and everything worked perfectly before I pulled the motor. Its a Walbro 255 that I installed 6 or so months ago, and I drove with it for 3 months or so and it gave me no issues.

 

Make sure a wire isn't in the wrong place after surgery that is causing the pump not to run after the engine starts....

 

The oil pressure sender got cracked when I installed the motor, the bottom part of it. Thats the only wiring issue I can think of. Most of the wires only go in one way, and everything is plugged in, no unplugged wires [except for AIR, which I removed]

Im honestly at a total loss.
It will make proper fuel pressure, but then it just fires up and immediately dies.

 

then to me i would be the sender unit. I had the same issue happen to me when I gutted my harness. make absolutley sure that no damage came to the wires or unit or the car WILL NOT run. trust me on this.

 

Hopefully its just that pressure sender....
I went to advanced auto parts and they had to order it, im going to go pick it up tomorrow. Im keeping my fingers crossed.

 

By itself, the OPSU won't keep the car from running.

 

I thought it might, its the "3 terminal" one for the later year third gens if that makes any difference. I thought if the computer doesent see any oil pressure it will shut the car off?

Any ideas what else could be causing the fuel pump to cut out as soon as the motor starts?
I have the EBL Flash and I can see the sensors with the key in the ignition, no idea if that can help me diagnose what the problem is. Maybe if someone knows what all the sensors should be reading with the motor off and the key just in the ignition I could narrow it down.

 

The computer doesn't get any signals from the OPSU. It's only a backup for the fuel pump relay.

 

So no idea what it could be?

It starts, fuel pressure is up, then fuel pressure goes to 0 and the motor dies. All in less then 1second.

 

Check the fuel pump relay and make sure the ECM is getting reference pulses from the distributor.

 

I will check those after I change the oil pressure sensor.

Im not sure how to check for reference pulses. Mind explaining?

 

may want to check grounds on the back of the heads,jimmy

 

Everything checked out....
Still same problem. Could it be the fuel pressure? Should I turn the fuel pressure up. What psi do you recommend, right now its at like 10/11psi with a Walbro 255 pump. Should I try 15 maybe?

 

hey tim ,whats up? fuel pressure should 12-14 psi for tbi,jimmy

 

AH! is it possible that a fuel line got crushed during the work?
Pump running, plenty of pressure but no flow?
Like a stopped up filter...

 

then he would have no pressure at all if its a crushed or plugged up filter

i still say fuel pump... jump the relay for the fuel pump and see if it runs

 

Maybe this sounds like i a dumb remark but just throwing it out there, to my knowledge and from what I have been told at the machine shop is that any head with a 1.94in or bigger intake valve wont really work on a 305 because of its smaller bore. Did you just throw the heads on a factory bore 305? Could this have something to do with it?

 

It would flow a trickle enough to build pressure which the engine would consume when it started.
Besides, it ran fine before he swapped the heads and cam.

 

Did you replace the oil pressure switch? Yes it will shut down the fuel pump. I believe it will still prime, but will not run. See the attached connection diagrams, pages 4,5.
TBI_wiring_diagrams.pdf

 

Thanks a lot for the replys!

The oil pressure switch dident want to go on, so I had to go back and get another one, ill try it tomorrow.

As far as the heads go, i heard the 113 vette heads work fine on 305s, same 58cc combustion chamber. Never heard anything about the valves.

The fuel lines are steel braided to and from the tank/filter. Filter is about 4months old, as is the fuel pump [walbro 255]

All the sensors, ignition checked out fine.

The diagrams are helpful, thanks.

Ill try turning up the fuel pressure [maybe its not getting enough fuel], ill check for a kinked fuel line, and replace the oil pressure sensor. Then ill report back.

 

I had the same issue with my 88 IROC-Z. I had a head job done, swapped in a new cam, EBL Flash, full exhaust, the works. the car ran beautifully until I drove it across country and died on me.

Turns out, the car wasn't getting enough fuel pressure. The pump would prime, I jumped out the aformentioned oil pressure relay (btw, the oil pressure sender sends a signal to allow the fuel pump to keep running. its a safety thing, if the oil pressure drops, the fuel pump relay cuts the flow of fuel), and even with the pump running continuously I wasn't getting enough fuel pressure.

I did install a Walbro 255, and I thought the unit was bad. It turns out, the small rubber hose that runs from the pump itself to the steel line (inside of the tank) had ruptured, allowing for just barely enough fuel to start the motor, but not build up pressure to keep it running.

Get a fuel pressure gauge (autozone rents them) and check your fuel pressure. If its not up to par, jump out the oil sender (by taking the two wires running to it and linking them together). You should hear the fuel pump running continuously. If it still isn't enough pressure, you have to check your fuel lines, which may include pulling the tank, which is not a fun task.

 

Ive got an external pressure regulator, with a pressure gauge on it.

I said earlier, it makes 10/11psi like it always has, but ill try increasing the psi. So its making proper pressure.

 

The oil pressure switch isn't going to stop the fuel pump from running. So don't worry about it for now. For the non-believer's check how it is wired into the circuit. It is in parallel with the fuel pump relay.

Tim, when you key on the fuel pump (FP) starts and the pressure builds (this is from you first post). At key-on, engine-off the FP should run for 2 seconds then turn off. Most likely it is doing that.

Once the distributor turns (cranking the engine) reference pulses are sent to the ECM. The ECM will then turn the FP on. And keep it on until 2 seconds after the last seen distributor reference pulse (DRP).

It may be that the ECM isn't getting the reference pulses from the distributor.

Try this: there is a FP relay test lead that protrudes from the harness. Red wire with a fast-on type connector on the end. Usually about 6" long. It may be by the relays near the brake booster, or over by the A/C unit on the firewall.

Run a wire from that connector to the battery + terminal. That will hot-wire the FP on. Is the pump on and there is fuel pressure?

Now try starting the engine. If it starts and runs check the WUD for a RPM signal. Should have one. My bet is that there will be one, otherwise the odds are the engine won't be running.

Now, if the engine starts and dies, and does this continuously. IOW, each time you try to start it it just cranks, starts, then dies, it is most likely going lean, or the timing is off.

If the engine cranks, starts, and dies, then needs to sit or needs clear flood (hold gas pedal to floor while cranking), then it is too rich.

For hte too lean, can bump the fuel pressure 5 pounds or so. Or, increase the BPC vs VAC table 10% or so. Just want to get it running, so the exact values aren't important (at the moment).

Can also give the distributor a little turn in the advanced direction (counter clockwise). And try again.

If the engine is rich (needs to sit or clear flood) then lower the BPC vs VAC table about 5% and try again. If better but stilll no go then lower by another 5%.

RBob.

 

Thanks a lot Rbob!

I changed my ignition module from the stock one to an Accel one [35370]
You think that could be causing issues with the computer?
[I might try the stock one, just in case]

So ill try bumping up fuel pressure, then ill try switching the ignition module, then ill try wiring the FP test relay

Thanks a ton!

 

just a thought, you said you are getting pressure to the throttle body, are your injectors firing though? i ask because i had a similar problem where i had pressure to the tbi but the injectors werent spraying, turned out to be the fuel pump relay, i tapped it and the injectors started to fire, problem solved.

 

If the stock ignition module was a GM unit I would put it back in. Just be sure to use a thin even coating of the white heat sink compound on the bottom of it. Any more all I will use is a GM ignition module.

RBob.

 

Turned out to be a stuck IAC!
But the motor runs good now, just need to slowly start tuning it.

Now I need to figure out whats going on with my transmission, the car doesent move anywhere......