DTC says MAP did not increase more than 1 Kpa (vacuum drop) when the EGR was commanded on. EGR opening should open the pipe from the exhaust manifold. That should cause the vacuum to drop due to CO2 and exhaust pressure entering manifold with the TB at a constant position (+/- 1% see test conditions below). I'm thinking maybe the due to Negative Backpressure EGR valve operation. Normally in a stock setup the exhaust backpressure can be up to 1 PSI . But you have a freeflow exhaust. Could be not enough exhaust pressure to correctly move the valve. Here's some reading ref shbox.com 4th Gen LT1 F-body EGR
. And following that just for info I added the GM mode6 (diagnostic test) writeup for DTC P0400.
1997 5.7L (LT1) F-car ENGINE DIAGNOSTIC PARAMETERS
MAP change > 1 Kpa.
A change in EGR flow
results in a change in
The EGR valve is closed for 1.5
seconds, and the peak change in
MAP is recorded.
A filtered value of expected vs.
actual MAP change is compared
to the fail limit.
No MAP DTC’s
No IAT, ECT, or Throttle position DTC’s
No EGR solenoid control circuit DTC’s
MAP > 24 KPa
ECT > 20°C
Vehicle Speed > 19 MPH
BARO > 60 KPa
Engine Speed > 500 rpm
Delta Idle Control < 3 steps
Delta MAP < 1 KPa before test start
The vehicle must meet stable operation criteria
for 1.0 - 2.0 seconds.
Throttle position > 5% but < 20%
Delta Throttle < 1%
Compensated MAP > 20 KPa but < 60 Kpa
Stabilization before and during the test is
required. Duration 1.5 seconds.