Well heres the back story:

A couple weeks ago my rotor blew up. Last weekend I replaced the opti. Drove it from Phoenix with no problems. I get back into town and hear a god awful racket which turns out to be a bad pass side cat. I take the car to the exhaust shop to remove the cats and weld in some straight pipe.

The guy at the shop could not get the drivers side bolts lose (fear of breaking the studs to the maifold) to disconnect the cat from the manifold. So as of right now I have a cat on the drivers side and straight pipe welded in on the pass side.

I left the shop yesterday and when I was pulling into my apartment I notice a stumble/hesitation at ~2000 rpms.

Would this be because I have the cat still on the DS?

It wasn't doing this until after I had the PS cat removed.

So you know:

When we replaced the opti we removed the AIR system. We went and got the AIR, cats/rear o2's, and EGR (the actual EGR is still on the car) tuned out of the PCM.

Any help is appreciated. Thanks.

 

Would this be because I have the cat still on the DS?

Maybe, if the remaining cat is fubared too. If the spark timing was bad enough to wipe out one cat, it would likely kill the other in pretty similar time. That is not a 'one bank' kind of failure. Both cats are probably pretty cooked if the car was real rich or misfiring enough to kill one. Can they just cut the pipe instead of pulling the manifold?

Also, I'd look for an exhaust leak where they were messing with the manifold bolts. Maybe they got a few bolts out, hit the stuck one, and reassembled it poorly. Check the O2 wires and plug wires, make sure none got moved and burned. Also triple check to make sure you got all the ign wire ends well seated to the opti - I had the boots slide down once on reinstall, and they were like springs that unseated a couple of the clips to the opti terminals.

Monitor the O2 data if you can and look for significant differences between banks (left/right).

A great way to look for exhaust leaks is the old hose-to ear technique.

Good luck.

 

Thanks for the reply. From what he said, he couldn't get the bolts loose where the DS cat is bolted to the manifold. IDK if there is a way to cut the cat out without removing the bolts and dropping the cat out.

I will double check the plug wires and the o2's later today.

 

I found out when I installed my longtubes that the passenger side cat was gutted on my Z-28, but not the driver's side. It ran normally the whole time leading up to that. I don't imagine having that one still in there would be giving you problems unless the cat was started to disintegrate.

It's been a while since I was under the car with stock manifolds, but maybe they could cut the Y-pipe below the cat, smash it to bits with a rod (I used a jack handle one time) and vacuum out the pieces. After that, weld the pipe back together and you're on your way?

 

I'm kind of leaning toward my DS cat being bad. I was just trying to find out if it would cause a stumble. I did a search and it seems like a low end stumble is a PITA to get fixed.

I haven't thrown any codes and no SES light has come on yet. So I am hoping that it's a good sign that it's not something besides the cat.

I am not to sure if I want to hollow out the cat that way Alan, but I will be trying to get those bolts loose this weekend so I can get that cat removed.

 

If the other one is clogged, falling apart, it could be. If the timing/fuel map are different enough from one bank to the other, it could run rough under load. Fix the cat and go from there.

 

The computer would throw a code if it were a misfire or something along those lines correct??

 

Hit the bolts with PB blaster now. If you can do it without incinerating your arms, try to pull them out after the engine is hot. If not, wax them while they're hot - the trick of holding paraffin to a hot bolt for lube recently worked great for me.

 

Even if the bolt break while removing the exhaust from the manifold, they can be replaced.

 

Stumble or hesitation at such a low rpm sounds a lot like an electrical issue to me. You just replaced the opti but you may want to make certain all wites are connected tightly, same with plug wires and anything else you can find.

A plugged cat will kill power but usually not create a stumble condition.

Doe the problem only happen when the car is warmed up or does is happen when cold as well?

You need to actually diagnose the issue. I'm just throwing out ideas without any concept of what yoy refer to as a stumble... I'm guessing it is a low rpm "miss" in which case a loose plug wire, loose or cracked spark plug, burn't plug wire, or opti issue are the first places I would look. For me, if a new issue comes up after I have changed or "fixed" something I usually go back an examine my work because the most recent change is what often causes the new problem.

 

The problem is not there the first couple munutes after start up. It shows up after it's warmed up a little.

A description of the issue:

It is sluggish on acceleration from a stop up until ~2k. I can hear a change in how the exhaust sounds.

It was fine for the ride back from Phoenix and was not there until I had the exhaust shop do it's thing. That's why I was leaning towards the cat still being there.

As far as a miss:

Wouldn't a miss throw a code? I have no codes thrown and no SES light on. I will go back and check my wires again and see if those are the issue.

 

The catalytic convertor in my blue car crumbled to bits. I remember it very well. The problem was power on the high end, though, not the low end. As long as I didn't try to exceed ~2,000 RPM, I could limp it along. If I tried to get the revs higher than that, however, it would fall on its face.

I was getting a misfire code when I burned through a plug wire on my Z-28. I don't know if it will always necessarily throw a code, though. I would check out all your plug wires and make sure none of them are burnt. Also, check your coil wire too. You might as well. Make sure all your plug wires are seated properly.

 

If it takes a few minutes for the problem to start, that sounds like a closed-loop issue. Which points to a bad O2 sensor signal.

 

Thanks for the replies guys. The "stumble" seemed to disappear in the last couple days.

Then today on my way to work, the car died on me and wouldn't start back up. I had a look and it seemed like I had some oil on my opti harness, so I have to find out where it came from.

My harness isn't in the best shape so I am going to get another one and see what happens.

 

I don't want to start a new thread so I'll ask in this one.

If something leaked onto the harness and is causing my no start, do I need to remove the opti again and open it up and clean it? Would I be able to just change the harness, clean out the connection area (with what I am not sure), and plug in the new one and hope for the best?