manifold vacuum reading not changing

[JohnBarnes]

I am reading 17 in of Hg at idle, and when i step on the accelerator to raise the engine speed to 2500 rpm the vacuum reading remains unchanged. i expected it to drop.

something isn't right.

I have no pull when driving, engine output sucks. but it idles fine.

need some help diagnosing this.

don't know if pistons rings, valves or HG leak at this point. any ideas welcome.

1996 LC, 217K miles

 

[quasi]

you reading should drop as soon as you open the throttle, are you checking vacuum near the booster, after the check valve? this would not represent manifold vacuum. find a spot on the manifold. you should read 17-19 inches at idle on a healthy engine. and make sure whatever you read, it is stable, an irratic vacuum reading indicates an unhealthy engine.
cheers

 

[baldilocks]

How old are you cats? Possibly plugged?

 

[JohnBarnes]

i am taking the vacuum measurement from the EVAP purge vacuum supply. it is reading correct when I am idling and i checked that the throttle plate is opening when i stomp the pedal. i even took off the breather hose from the air cleaner to the throttle to check that my throttle cable was working. The cats are at least 6 years old. could it be higher than normal exhaust back pressure? should i check that, and what reading should i expect?