91 LT1 Egr problems

We have one last caprice in our fleet with 86000 miles in it. The problem is that it runs rough when in drive and a little sluggish when taking off from the start.
Things that were replaced were plugs,wires,egr,egr solenoid..etc..

My problem is that today i figure i would go and redissect the egr system again. I applied vaccuum to the egr and the engine would not stall or stumble which i was always taught would happen if the passages were not clogged. Weird thing I noticed was that if the engine was off it held steady vaccuum. If i start the engine it does not hold vaccuum at all..It is like it got sucked out. I pump up the vaccuum and it goes back to 0 instantly. Engine off it holds.

Your title is somewhat confusing. If it is a stock car, there is no such thing as a 91 LT1. So either someone drop an LT1 into it aftermarket, or else it does not have one.

Regardless of the engine in place.... Putting vacuum to the EGR and the engine not stumbling usually indicates either a plugged passage somewhere or a bad EGR valve.

My error in thinking it was an LT1 and I stand corrected.

I replaced the EGR and it will hold vacuum till the sun comes up but I found it odd that when the engine is off it would hold vacuum and as soon as it is started it loses all vacuum and even trying to manually apply vacuum to it is futile because it goes stright to zero. Maybe it is time for the scrap heap.

That is because the EGR valve is probably a negative backpressure type, with a pilot valve which dumps main EGR vacuum after exhaust pressure has increased to a predetermined point (calibrated by a spring under the pilot valve). Either the EGR valve is failing, or it the wrong type (if it has been replaced) or there is some exhaust restriction creating excessive backpressure.

http://72.19.213.157/files/EGRNeg.gif