I recently rebuilt my 5.7 out of a one ton silverado. It will start and run for about twenty or thirty seconds and then stalls. The engine is slow cranking and takes some effort to start, and when running is erratic and rough for the first few seconds then evens out and sounds great. after a few seconds though the vacuum begins to drop from an erratic 15 to 20 inches to zero and then the engine stalls. I am sure the vacuum hoses are in good condition and in the correct place, and the intake and throttle body has a good seal with the proper torque. Any suggestions are appreciated on how to resolve this or diagnose the problem to make sure there will be no damage to the new engine.

 

check your vacuum hoses. vacuum shouldnt be at 0 at idle. it should be 15-20mmHg. Vacuum should be at 0 on Wide Open Throttle.

 

The vacuum hoses are in their proper places and all check out good with no leaks or blockage. The engine has between 10 and 15 hg at an idle of 1500 rpm, when allowed to idle lower vacuum drops th around 5. The engine will only run for about thirty seconds with a rough idle that smoothes out as it runs, and just before it stalls the vacuum will slowly drop to zero no matter how I try to keep it running. I am looking for any suggestions on this one because the more this engine runs like this the more risk there is due to it being at the initial start up and break in phase of rebuilding.

 

Do you have a leak around the intake manifold? Vacuum is too low at idle. It shouldnt be 5mmHg, it should be 15-20mmHg.