Hey guys, first post here, so treat me gently. From what I can tell this isn't a common problem, or if it is I haven't seen it anywhere else. I've recently aquired a 76 vega with an LT1 out of a 95 camaro. The day I picked the car up it had a dead miss on one cylinder, but otherwise ran great. I had presumed going into the deal that the miss was an opti issue. This is my first LT car, and I've not had any specific experience with them, but I'm pretty familiar with the common problems. The next day the car would barely run. I had come to the conclusion tgat the fuel pump had gone out. New fuel pump and filter, same problem. New opti fixed the dead miss, but not the new problem. Car runs good for just a little bit, then will barely run at all. Falls on its face any time you're near the gas pedal. I thought maybe I had gotten a bad pump. Returned it and got a walbro 255. Same problem. Then I got to listening. The pump is plenty loud enough that you can hear it in the car. The pump runs, then just all on it's own, stops. New relay, no change. Put a meter on it and got a strange surprise. The pump runs great at ~13.5 volts, but then the voltage spikes to 14, and the pump shuts off. It does this almost rythmically. Infrequently at first, then after it has run awhile it cycles from 13-14 every few seconds. Every time it spikes the fuel pump dies, and the car nearly dies with it. If the car ever dies completely, the pump immediately kicks back on and primes the system. I'm stumped. Voltage readings were taken between the pcm and fuel pump relay, as well as between the relay and pump. Same result on both sides. Battery always has around 14v with no fluctuation.

 

The spike may be coincident with the pump be commanded to turn off by the PCM. Have you measured fuel pressure at the Schrader valve? Does it start and run when the engine temp is cold? How long does it run before it runs rough?

 

He needs to know that the pump typically only runs for about 2 seconds when the ignition is turned on. Then it runs only after the engine is started or when the ignition is cycled again. The pcm determines that the engine is running only when it sees the signals from the Opti distributor.

 

Fuel pressure is good, both static and running. And yes, I know the fuel pump is only on for 2 seconds to prime. All these measurements are taken with the car running

 

How/where did you connect the meter? Now it sounds like one of the wires, ground (at pump) or power (from relay), is loose. Can you feel the relay turning on and off? If not, the pcm isn't shutting it off.

 

As far as how long it runs good, I can pull out of the driveway and it runs like a scalded ape, drive 1/8 mile and it's crap. Have a video I'm trying to post, will leave a link when I get it uploaded

 

Measuring within 1/2 an inch from the relay, and no, the relay isn't cycling. The pump is shutting off while it still has voltage

 

So the break could be between the relay and the pump, or between the pump and the ground.

 

Power is also good at the pump. Ground is a 4 inch long wire straight to bare metal on the unibody. The pump has power. Unquestionably. It just stops running at 14 volts. It is mind boggling. I haven't got a chance to check how much current it is getting though. Walbro says it should be ~10-11 amps @45psi

 

If it stops at 14 volts, it is defective. No other explanation. I can't help any more because this defies electrical theory. Good luck.

 

This is the third pump I've had on this car in a week, and they have all done the same thing. I can understand getting a bad fuel pump. MAYBE even two. But not three.

 

I'm no expert and don't take this to the bank,but did you check the ICM?Previous posts have suggested the it or the wires going to it and from it could be bad as well.You just can't take ICM down to autozone and have it checked as it has to be hot hot hot(Like the song) to be checked.They cost about $40 some bucks and do the ICM mod at Shoebox's site.Hope this helps.How much does your car weigh? Sounds frightening fast.My02.

 

No, it's definitely starving for fuel. Thank you for all the suggestions though. But the car weighs a little under 2700. I certainly wouldn't call it frightening, but it does alright

 

video

 

What is the screech in the background as the voltage drops into the 12's? The pump?

Your posts indicate the problem occurs when the voltage spiked to 14V. But is appears just the opposite.... as the voltage drops the problem occurs. The video shows once in the 12's it stumbles and almost stalls. Am I reading this wrong?

When the voltage at the pump drops, does the voltage at the alternator drop similarly?

I think it may be some component on the engine control system that is causing the problem, possibly pulling the voltage down.