My car has original exhaust, original headers, original cats, everything almost 12 years old and run about 60k miles (but until me very badly maintained...). I've put it on tester but sensors are all working fine apparently...

After changing all the ignition parts (opti, wires, plugs) and cleaning injectors, changing filter, the car's still not running quite even and when cold, missfires like crazy (first 30 seconds when startup). It also blows out of the exhaust when doing engine break sometimes.

Could any of this be caused by leaks / clogged cats ? Car's stock no mods so far.

Thanks

 

up any ideas?

 

The only things I can think of are...

Clogged Cat
Bad o2 sensors (Should only effect closed loop)
EGR valve stuck open...

The exhaust side is relatively simple...

Edit: I don't know if it could happen on an LT1, but your symptoms sound like the timing chain could have jumped a tooth or two... It would have to have a bunch of slack for that to happen... I've never heard of it on an LT1...

 

clogged cat, yeah could be. Timing chain jumping a tooth, I doubt it. He would notice if his timing was off a tooth. Car would be running pretty bad.

I'd try gutting the cats to see if that helps.

 

Highly unlikely that a stock engine, even a poorly maintained one, has killed the cats at 60k.

Clogged cats also would not cause a symptom that goes away when the car warms up.

You need to scan the car to find out what it is doing.

 

Check your temp sensor on the water pump.