Ok, this thing has me beat. 2000 454 mag mpi (mefi3). starts, then when settling to idle it chuggles and stalls. Brand new longblock. old one also did the same thing. I tried known good ignition modules from properly running engines, reset the distributor to #1 tdc countless times, tried a different wire harness, ECM, brand new OEM injectors, tested and verified to spec at a reputable injector shop. I took a complete intake with tps, iac, and IAT from a running engine and no change. Also a new alternator, pickup coil, and ignition coil. and I spent 2 days trying every position possible on the distributor and no change. When the engine starts the vacuum jumps to about 20 and then settles to about 16 and then it starts to chuggle, the iac opens, the vaccum drops and the engine stalls. Fuel pressure is at 43 psi. Running it off a remote tank with fresh fuel. No codes. 5.19 volts on the 5v reference circuit.
I can keep it running if I pump the throttle. Also today morning I unplugged the map sensor and it started and stayed running, albeit a little too fast. Then a little while later I tried starting it with the map sensor plugged in and it also started and idles but too slow and a little lopey. I thought it might not have enough timing so I advanced the distributor but that did not help. I set the dizzy back and tried running with the map sensor disconnected but now no change. I think I tried everything here I could use some help.

EDIT: I forgot to mention that I tried throwing in the cheap chinses knockoff injectors from the running engine. That made the engine run excellent. I took them to the injector shop and was told that the knockoffs had the same flow rates as the OEM injectors, but with a much larger flow variance which should theoretically make the engine idle worse... also they told me the impedance was the same as OEM.

how did you set base timing,

Just to clarify:

You had an issue with the 'old' engine...so you decided to refresh?

After all the changes...you're having the exact same issues and symptoms?

What is the same on your current setup from the old engine? (Parts etc, )

It is either your MAP sensor or the mechanical fuel pump....yes mechanical fuel pump....I know its injected but check the mechanical pump output. If the mechanical pump does not feed the VST/CoolFuel etc correctly and consistently it can drive all sorts of weird symptoms.

VZM,

Primarily replying to follow....

Layman's prediction.... A new MAP sensor will solve the problem.

Thanks. Brad.

​​​​​​​verified damper marks, set damper to 0 and set rotor on the dizzy to #1. I cant get it to run long enough to go into base mode though and look with the timing light. Judging by the spark advance graph I have it pretty close to where it wants to be, at least to self sustain.

I couldnt get the old one to run right, then I found out that someone had dressed an L29 with MAG parts, I figured that must be the issue hence the new longblock. but still no dice. Swapped all the peripherals over, including the ecm, 2 harnesses, intake, sensors alternator and starter.

No mechanical pump on this engine. Its a cool fuel gen 2 motor. Just the single electric. Fuel pressure stays around 43-45 psi at the rail. Tried the MAP sensor off the engine that runs well and that didn't do anything. Also went to mess with it in the morning today and now it wont stay running even with the map sensor unplugged. Plugged in or unplugged no difference.

Since everyone seems to be saying its the map sensor, I will go grab one right now and throw it in to see what happens.

isnt there a gasket under the map is it in place