I started getting exhaust fumes inside and thought maybe my
cat was clogged? I also had a prob with lack of power and hesitating/stumbling when I step on it.
Well I'm not a mechanic but I must have thought I was a detective
because I put all the symptons together and thought for sure I
had a bad cat, I called the muffler shop and they said they can
test the cat for back pressure, and they did.
They told me the cat was bad because the back presure was way
too high 7.5 lbs and it should only be 2 - 2.5 lbs they also said
its possible the cat isn't causing the problems But it's bad
I had it replaced but it didn't solve any of the problems and
thats ok if it was bad, but I thought I saw a post about back pressure normally being 7 lbs
What is the normal backpressure? and is that the correct way
to diag. a cat?
Thanks again
Forest I can't run

 

year/engine?

Did you do something before this started? It could be an exhaust leak among other things. More info would help.

 

94 Z28 LT1 I didn't have a exhaust leak b4 they replaced the
cat and after I only got 1 mile away and it was rattling and making
so much noise I thought the cat was falling off! I turned around and when I pulled in the drive they could hear me coming and put
it right back on the lift did some welding and banging and I was off. (and now I think I have a exhaust leak)
Is there any reason I would feel airflow when I stick my arm under the car near the front of the cat? Would I be able to feel
any air flow from the cooling fans? I'm thinking that I shouldn't
feel any airflow at all except at the exhaust pipes?

 

If the car is cool (fans off and the cats arent radiating heat) and you start it and let it idle, you should feel NO air from around the cat. Only from the fans and the exhaust tips. Feeling air while stopped is either heat radiating off of the pipes or from the fans... hence to try while it is cold. If that is the case, yell at them to fix it...

But going to a shop where you leave and the cat is falling off IMHO does NOT inspire confidence about the quality of skill of the technicians. Did they remove the Y-pipe before welding, or did they weld it on the car? Are they drunk? I'm guessing it's a really crappy weld job. Does the car idle perfectly smoothly? The hesitation could be something else, and if it's bad, exhaust leaks usually arent the culprit espicially if it is after the o2 sensors.

 

I think they used a torch and grinder to remove the cat and the
only welding I saw was all done on the car. I bet if I had it up on a rack I would see a bad weld and thats where the leak is.

My main concern has been the overall performance the car runs so bad I don't want to drive it back to have the cat checked for a leak.
The dealer seems confident that the problem is a bad PCM
So I'll have to wait until I have the $ and then contact pcm4less
I still would like to know about the back pressure for the cat is
7 lbs high and indicates a bad cat?
It's ok if they put one on if I needed it as long as they clean up the bad weld. But if they sold me a cat I didn't need and did a lousy job installing it I'm really going to be pissed and I don't care how its runnnig cause I'll go back and get my money.
I'm really fed up with shody work and it's not just mechanics
I don't know how these people keep a job! And I've been out of
work for 2 months.

 

problem is while welding on the car you cant get to the spots where they come together on top, because you just cant get there!

May I reccomend a Lt1 scanmaster? They display a million things and DTC codes. More then worth the $200, considering most places (aside from autozone that does it for free) charge $100 or more for a diagnostic check.

EDIT: I dont know if 7lbs is right or not. Someone else chip in here?

 

Idle backpressure should not exceed 1.25 psi and 3 psi at 2000 rpms.