Hey guys, couple of weeks ago I took my 94 for a drive and after one stop light it started behaving like it never has before. It felt like only 20% of previous power was coming to the wheels. I was suggested it could be clogged cat, bought new one, went to shop car overheated before getting to shop. Took the cat off at home, it was prefectly fine inside. Now looking for some directions on what to look for next.

So to sum it up:
-94 Z28, approx 165K on engine
-Very weak throttle response, and hard to get moving after the full stop
-Engine doesnt rev as high, 3000-3500rpm at most when under load
-Exhaust sounds silenced

Any suggestions would be greatly appreciated.

 

That still sounds just loike a clogged cat. Poor power, less sound from the exhaust, and very high temps. You pulled off the cat. How do you know it's not restricted? What I'm getting at is, it's made with a honey comb structure inside. If your only looking at the inlet and outlet, that's only 10% of what's really in there. You can try running the engine without the cat. It may not be drivable, but if you can put it in gear and tape the throttle, you should notice a difference right away.

If you already put the cat back on, you can pull out the upstream O2 sensors and run the car, or take it for a quick drive. The O2 sensor holes will relieve some backpressure and restore power if that's really what's holding the engine back. I've diagnosed plenty of restricted cat's with this test.

There other things that can reduce power, clogged fuel filter, air filter, low fuel pressure or volume, and weak spark. Those are just a few off the top of my head, but none of them will give you all the other symptoms. I say double check the cat first.

 

Hm never thought of it, taking out O2 sensor sounds like a great idea. I didnt pull the cat off completely, I only disonected the pipe that goes to mufflers and looked at the rear end of cat. Some air was passing by, but Im not sure how much should pass on idle, I could hold my hand though at the back of cat ( I thought it be much hotter). Thanks for advice!

Edit: One thing that is still unclear to me is why would I have such a problem building up rpms even if it was cat, and is it possible just to be beacause of the clogged cat...

 

Is the SES on?

 

my car was a dog and overheated when my cat was clogged, put a new one on immediately before you overheat badly and blow a head gasket

 

A clogged cat stops the exhaust flow. No exhaust flows out, no fresh air comes in through the intake valves. Volumetric efficiency approaches "0". Engine can't make power unless there's lots of fresh air to combine with fuel. Take a car without a cat, and shove a potato into the exhaust pipe.... same effect.... isn't going to run too well.

 

My car had exactly the same symptoms,very low power,wouldn't rev past 3000 rpm under load.Also the manifolds were glowing red after a couple of miles.Changed the cat/y-pipe,as suggested by the helpful guys on this forum, and all was well again.
My car would rev cleanly up to the limiter in neutral but not under load,I couldn't get past 40 mph.Upon removing the cat I gave it a good shake and bits of the honeycomb were rattling about in there.
Run the car without a cat and see what happens .
Regards,Mark.