Clogged Cat?

[PCGUY112887]

96 3100 120k miles, car usually drives fine but sometimes shutters when downshifting from OD to third to go up a hill. Or if I get on the gas to pass someone, RPM's get over 3k and the car shutters and the RPM's go up a bit, then down a bit, then up a bit again, and down a bit again. Speed bairly goes up.

Thought maybe clogged injectors... but i've had them cleaned plus I run fuel injector cleaner every so often, no help. I thought my trans was going bad but I read that when a trans slips, RPM's go up and you don't move... my RPM's go up and down.

Thought maybe a tuneup was in order, but I did plugs and wires about a year ago... using Rapidfire plugs, they should all be fine. Did a topend clean with seafoam as well just before that.

Had my LIM replaced this winter, no more leaks aside from my engine leaking oil. I don't know what else it could be aside from a clogged cat? No SES lights when she shutters away.

[3400-95-Modified]

Shuttering is typically caused from a bad TCC lock up solenoid or bad/worn tcc section of the valve body. So the trans could be on its way out.

Clogged cat you will really feel, Car will have little to NO power all the time, and not rev worth of crap.

[tractorman]

Agreed. What you're describing doesn't sound at all like a clogged cat. In my experience, an engine with a clogged cat will run smoothly enough- with very little part-throttle power and/or response.

[IsaacHayes]

My friends 2001 impala didn't run smoothly with a clogged cat. But I can't say what the RPMs were doing. But it seemed almost random on how the engine ran. It had no power of course, but the power fluctuated from very very low, to almost you can't tell.

Does the car haul when you floor it? If not then possibly cat, but the RPM thing is probably trans related. Hopefully it's just the TCC solenoid.

[PCGUY112887]

If I floor it, it bairly moves. RPM's shutter and take FOREVER to climb.

[IsaacHayes]

Ok, thats a clogged cat. Loosen the bolts on the down pipe, go for a short drive (not too long don't want to melt stuff) and floor it and see if it moves a lot faster. Then that will confirm it.

Do you hear a pinging type of noise when under load or flooring it? That's the downpipe opening and snapping shut from the back pressure.

Man, my original cat has 200,000 on it. But the car moves good. I wonder if I'd gain any with a new one, or if mine's just still in great shape.... LOL

[IanSzgatti]

how about doing a fuel pressure test to confirm your pump is up to the task? I would suspect some kind of check engine light if the downstream o2 sees converter efficiency is low from being clooged all to f%&@ but who knows, they dont always throw a code i guess. I'm also not certain if 96s have the downstream o2...haha, but OBD2... it might

[PCGUY112887]

It does.

And fuel pump is new... fairly new, about a year old.

No "pinging" sound when the engine is under load. Went out on the highway yesterday, it was SO hard to get to 70mph! Had to stay at about 60 most of the time, it just couldn't make it to 70.

Still no SES lights or anything telling me there is a problem. I don't know how something could be killing performance so badly, and not give an SES.

[TazMan]

I had a car with a bad cat years ago. I didn't get an SES light till it was so clogged it didn't go over 15 mph. When It first started happening, it felt like a transmission problem too. What I didn't realize till it got worse was that I was pressing the gas alot harder to go the same speed as I usually do. This was causing the transmission to shift much later and rougher. I didn't have a downstream O2, but I think that sensor checks for to much harmful emissions. If the exhaust isn't flowing, the downstream sensor still wouldn't see any pollution, and it wouldn't cause a code.

As a matter of fact my Van has no cats. The 2 upstream sensors were no good. I replaced them and put the bad sensors downstream. Since they aren't working, they don't see any harmfull emission and I don't get any SES light. Tractorman, if you read this, thanks for that tip. It worked perfectly.

One last word of advise, if you do disconnect the downpipe, as Isaac said, only go for a short drive. I did this in my cavalier. It drove great, but 2 blocks later, flames were coming out from under the hood. By the time I got the fire out everything under the hood was so melted it just wasn't worth fixing.

[IsaacHayes]

My friends Impala had no SES code. Only time was for EGR position cause the back pressure was forcing it open.

Yeah I said short drive.. hehe just like in front of the house, turn around come back. Like a block is all you need to feel the difference.

[3400-95-Modified]

There is really nothing for the ecm to check to see if the cat is clogged.... Do you have a downstream sensor?? if not then that code is shut off so it can’t even tell if it’s getting enough flow, it would probably come up with insufficient activity.

[IsaacHayes]

I don't think the downstream will ever set a code if the cat is clogged. At least the 2001 impala didn't. It only had a SES randomly then it would go out. And that was the EGR because there was so much back pressure it was forcing it open. You could see it got super hot the metal casing was discolored!!

I've even seen older 3.1mpfi cars with clogged cats and the EGR has totally melted off!!!!!

I guess my 200k cat is ok cause I run it WOT all the time, keeps it nice and hot and clean? hahahhaa

[Baldeagle242]

You can have a cat that has come apart on the inside. If the honeycomb crap is broken up it could be moving around inside the cat and when it gets turned sideways it stops or slows the exhaust flow.
Another way you can check to see if the cat is clogged is to remove the O2 sensor that is right in front of the cat. This is easier than taking the cat itself off (22mm IIRC) If there is more exhaust coming out the small hole than out the end of your pipe chances are you have a clogged cat. Just make sure the hole does not blow right up onto your floorboard.