Dskeezy16

iTrader: (1)

Hi, just acquired an 87 trans am with a digital dash thats had a heads/cam lt1 and t56 swapped into it. My problem is that when I go 3/4 to full throttle the motor just cuts out and bucks like crazy. The fuel pressure fluctuates with the bucking, it will fluctuate from 30 psi to 40psi. I replaced the fuel pump, regulator, and filter. Im starting suspect the voltage to the pump is dropping when I got wot. Im clueless to what would cause that to happen though. Anybody have some idea of what might be happening?

Dskeezy16

iTrader: (1)

Just checked voltage. Ive got 12.2 volts at the fuel pump relay, and 12.2 volts at the connection in front of the rear axle.

AlkyIROC

Minimum voltage required for the pump to work is battery voltage.

I'm assuming EFI.
Large or small cap HEI?

Dskeezy16

iTrader: (1)

yep its EFI. Its an LT1 so its optispark. The guy we got it from sad he had just replaced it. Its definately not missing. Runs awesome until it goes wide open. Also the pump I put in is a walbro 255.

RBob

iTrader: (1)

Check the intake ductwork prior to the throttle body. Sounds like it may be collapsing under WOT (BTDT).

RBob.

Dskeezy16

iTrader: (1)

It doesnt look like theres anywhere for it to collapse. The intake is mostly alluminum.

Could a leaking pulsator in the tank cause the fuel pressure to rise and drop?

Dskeezy16

iTrader: (1)

Update: I checked at voltage at the fuel pump relay while it was bucking and the voltage would drop from 13.8 down to 11.8. So I ran a wire from the battery straight to the hot side on the relay and same results.

Dskeezy16

iTrader: (1)

Hey RBob can I send you a data log of whats happening? the voltage on this thing is actually dropping to zero sometimes and the left o2 is giving some really really low readings.

bjankuski

This is a good answer, check the filter and all the ducting. I have seen filters and ducts collapse at wot and it will cause the bucking. The fluxuating fuel pressure is a clue that this is the problem. When the duct collapses the vacuum goes up and the fuel pressure drops and the car bucks due to a loss of air.

Dskeezy16

iTrader: (1)

It was a very good thought! Ive ran the car without the air filter. Most of the intake is aluminium theres not enough rubber to be sucked closed.

I took some more datalogs and ive found that the ecm is pretty much shutting down as soon as i got WOT. I get about 2 frames of data at 100% tps and then a voltage drop down to .8 volts after that there is no data until I let off the throttle.

I checked all checked for tight wires that might get pulled on when the motor torques and I checked all ground wires and power wires going to the PCM.