Ok, I am trying to diagnose a clogged cat before I do the irrevesible and rip it off. In another forum, I was told one check was to look at the map sensor readings as an indicator.

The issue with the car is that it will only rev at idle to 4000 rpm in nuetral, and then will start bucking and shaking BADLY. In gear, it idles ok, but any more throttle and it will fall on its face badly.

So here is what I am seeing for KPA readings...

Key on, Engine off- 79 KPA (This is in Colorado at 6600 feet)
Engine running at idle- mid/low 30's kpa

Revving car in nuetral results in KPA increasing. At 2500 rpm, the KPA reads upper 40's and lower 50's, adding throttle to get it to the 4000 rpm point where it refuses to go any further results in readings in the upper 60's and lower 70's, almost atmoshperic pressure.

BTW, fuel, ignition, valvetrain and compression have all been checked and are all spot on.

Are these readings indicative of a clogged cat?

 

Sure looks like a plugged exhaust.

Last month a customer came in with a similar situation. 1989 Pontiac Gran Am Quad 4. I tried taking the 02 out to relieve some back pressure, it made little difference. Only when I released the exhaust pipe from the manifold did it become obvious the cat was plugged.

Try letting one side of the Y pipe loose.

Mark