1998 Blazer 4.3L 4wd

My blazer has had an exhaust rattle coming from the CAT for a little while but was still running fine. One time the rattle stopped and then it had some problems with engine performance (weak) and threw code P0420 (catalyst efficiency). The rattle came back and the service engine light went off. Recently, the rattle has gotten worse but was still driving good. Then I started to notice a shudder during acceleration if the engine had not fully warmed up but the SEL did not come on. Yesterday the rattle got even worse (sounds like marbles in the exhaust) and now the shudder is present at idle and when driving the shudder is worse and the SEL came on. Checked the code and this time it is a P0304 (clyinder 4 misfire). Could the CAT getting worse cause the engine to run rough and cause the misfire? Any ideas on what could be causing a cylinder misfire other than the cat?

Any help is greatly appeciated.

 

pull a o2 sensor out forward oof the Cat and see if it performs better. If so I'd look into replacing it on the off chance that it is all your problems. Jerry

 

By the time the Cat starts rattling Is a good time to consider replacing it. I bought mine for $150 installed. Magnaflow aftermarket. But the cat may not be your whole problem. If pieces have gotten in your muffler it also could be getting plugged.

 

Had same exact problem check for coolant in the oil.
what happened to mine internal intake coolant leak small at first engine suck up mostly as steam at first, it then hit hot convertor honeycomb cracking it.
that caused the rattle as leak got worse o2 sensor contamination on and off
it would set ses light then go away then missing started until one day bang and heavy misfire cyl 2 and 4 about 3 extra quarts of coolant in oil.
engine was saved didn't blow up but had to be completely disassembled and cleaned new bearing ect...

look for wetness around lower intake left/right at ends front and rear if its wet change it right away or you will regret it.
mine didn't show from outside but if I had pulled throttle body and looked into
upper intake I would have been able to see the white contamination from pcv system.

good luck

 

I had to replace my cat because it was breaking up and it seemed to be causing alot of back pressure.
I wish I would have just deleted it or just cut a hole in it empty it out and weld the hole.

 

I had an identical problem. I cut my cat out completely (no $$ for a new one ATM) and it made a world of difference. I had over 10psi before the cat and .5psi is factory. AFTER you get the cat problem sorted out, I suggest a whole tune-up. Plugs, wire, cap, rotor. the whole 9, don't cheap out. Go with Delco or Delphi for all things related to the engine on these things.

 

Zeppelinman_2000: I did have a coolant leak and had to replace the intake gaskets, and the rattle from the cat started around that time. It is all starting to make sense now.

Thanks to everyone for the replies and tips. Looks like I am going to have to replace the converter, has anyone tried the direct fit converters? or should I take it to a muffler shop and have a universal fit converter welded in? I just do not want to spend a ton of money on this right now if there is a cheaper option or a way I could replace it myself.

Thanks,

Paul

 

Just installed a MagnaFlow direct fit CAT and now it runs amazing.

 

Glad you got it fixed up! Thanks for the feedback!

 

Amazing isnt it? I also replaced my muffler with a magnaflo because chunks of the cat where in there and the salt from the roads here in PA ate through it. It sounds 100x's better and I get better mpg. I was around 16 highway

I didn't see where anyone explained the rattling in the cat is the internals breaking up and bouncing around. They can pile ontop of each other or in my case get stuck in the muffler and the curved pipe between the two.