Hi folks. New to the forum, new to my Vette but seasoned in the garage as a heavy DIYer. Coming to this forum as I am at my wits end. 1987 C4 convertible manual. Intermittent bog down, bucking, and sag on acceleration. Some days she's strong, others I may not make it home. Replaced the fuel pump (deleted pulsator), new tank gasket, intake strainer, and fuel filter. No change. Slapped a fuel pressure gauge on the Shrader and the windshield. Key to ON, pump primes to 35ish psi (low) and for 2 seconds then drops to 17psi IMMEDIATELY on cutoff. Naturally, I think fuel pressure regulator. Pulled the plenum and regulator. Physically, it looked fine, no tears or degradation. Replaced anyway along with manifold gaskets because I had the parts. Gauge still attached. Same business but car ran MUCH rougher. Stalled multiple times, serious bucking. Looks like operating fuel pressure climbed a few to about 38ish but still IMMEDIATE drop to about 17psi after prime. This seems like a massive fuel pressure issue and am not sure where to turn at this point. All injectors OHM'd out around 18.2 but I didn't do a leak down on them. Is that the next logical step? Massive leak in sender assembly? No check valve on new pump (but issues were still there with original pump)? Clamped all permutations of send and return lines at the tank and same drop to 17psi after prime. Looking for the collective wisdom of this group. Many thanks in advance.

 

Welcome to the forum.
It could be that the new fuel pump is defective.

 

The reasons I would argue against that are 1) The symptoms were in place before I replaced the pump 2) The pump has no problem priming up to high 30s psi…it’s when it completes that prime cycle that the pressure drops to 17psi 3) One could argue that the Carquest pump does not have a check valve but I clamped all lines after prime to simulate a check valve and the pressure dove down as usual, suggesting the issue is upstream. Last night I tore apart the intake and removed the fuel rail. I’ll be replacing the injectors and we’ll see what happens. Plenum and runners to a machine shop for hot tank and glass beads.

 

Since you are clamping all the lines to and from the pump and the pressure drops to 17 psi, I'm wondering why it stops at 17 psi and doesn't just bleed off to zero.