Ron78Z&01SS

Car is a 96 Z28 A4, bone stock with 104,000 miles.

I'm pretty sure my passenger cat is clogged, but funny thing is, it only clogs up when it's warmed up, then if you let it cool down, it's fine until it warms back up.

First indication of a problem was driving home one night and car stumbled & stalled for no apparent reason. Fired back up & drove less than 2 blocks to the house. A few weeks later took it out for a auto parts run and leaving the store had a hard time starting it. Got it fired up and while driving it would stumble, not go over idle and sometimes die. It got bad enough to where it completely died on me and couldn't get it restarted at an intersection. Apparently during all of this the passenger AIR check valve off the exhaust manifold blew out, the hose caught fire and spread to the hood cowl weatherstripping (that was fun ).

Got the car towed home and replaced the AIR check valve hoping that would fix it, but it didn't.

Current symptoms are:
If cool, the car fires right up and will rev and drive normally. You can feel a slight vacuum @ the AIR check valve. Once it warms up though you can hear the rt check valve "chugchugchugchugchug" and you can feel a slight amount of pressure escaping which gets worse the longer the car runs. The car will occasionally slightly surge at idle and if you attempt to rev it stays around idle or stalls completely if you don't let off. The car will also overheat. The longer it runs, the more extreme the symptoms become. If you let it completely cool down, it fires right back up and once warm the symptoms begin again.

As far as I'm concerned, the rt cat is clogged, but I think it's a bit strange that it seems to unclog when it cools down. I thought once a cat clogged, it basically melted and stayed clogged. I've had the car for the last 3 years, have put about 13,000 miles on it and have NEVER had a rotten egg smell.

What do you guys think?

maverickmk

when things heat up they expand and of course when they cool they contract if that particular cat gets a lot hotter than the other one then it is clogged up, make sure your getting good flow with your coolant, ie. check for clogged radiator, water pump failing, air in the system, etc

Guest47904

I think you are confusing a problem with closed loop with a cat changing. If you suspect a cat is the problem, unbolt it and try it with it disconnected.

Never seen a cat change it's characteristics.

Mr.GearheadZ28

If they're the culprit and your state doent have emissions laws then I'd pull out the hacksaw. edit: didn't see above post^

deerslayinrenek

A clogged cat will be hotter right?....if so could the temps be checked on both cats?.....Just an Idea

Z28Freak86

Cats are made externally of metal, but inside its made of of a honeycomb ceramic, which should not expend from heat. if the cat is clogged, its most likely from something else, such a carbon build up on the surface. but yea just try and remove it and see how it runs.

jclark311

If its the 04 z06, its covered by warranty by GM. ALL EMISSIONS PRODUCTS ARE 10 YEAR 100,00 Mile warranty, Its a federal law.

Guest47904

That's kinda like writing in a letter " I would have sent money but I already sealed the envelope" LOL

Injuneer

....

Wild Willy

GM did do a courtesy extended warranty on the 4-th gen Camaro- I had one replaced on my car that I bought used at 78,000 miles- at very close to 100,000 miles- and the dealer told me when he did the work, for free, that the other cat had already been warrantied. Have to give props to this upstate Chevy dealer, in Scotia, NY for great service on both my LT1 Caprice and now my Z28- he did a warranty recall on my S10 blazer, too-

Ron78Z&01SS

Finally decided to fix the 96 Monday once and for all. Even though I was convinced the cat was clogged, it still didn't make any sense to me that the cat would clog then unclog when it cooled off.I didn't want to gut the cat for no reason being that it would be a HUGE hassle, make the exhaust noisier & smell like cr@p.

The replies of Z28Freak (carbon build-up) and Speedy (closed loop) and an IM from Roycontreras8 (similar symptoms on his even after his cat was replaced)got me to thinking. According to my Helms manual it goes into closed loop @ 140*F, so I went to the garage with my scanner with the intention of paying close attention to the Engine Coolant Temp (ECT) in relation to when the problem starts, and also to comparing the Bank 1 & 2 O2 sensors throughout the warm-up cycle.

ECT got above 140* and everything was fine, didn't see any obvious problems with the O2 readings. Then I noticed that as the car was warming up, the ECT on my scanner was reading noticeably lower than what the inst panel gage was reading. Problem seemed to begin when the difference was approx 30* and up. Shut it down, got to looking in my Helms, and sure enough, a bad ECT sensor can cause a hard/no start, rough idle, stalling, and little/no acceleration when increasing throttle.

Picked up an ECT sensor ($12), swapped it out, and went to fire it up and....no start...WTF??? Checked it with the scanner and had a -40*, plugged the connector into the old sensor and still had a -40* reading. Started shaking wires looking for an intermittent break while watching the scanner. narrowed it down to an area of wires covered in electrical tape. WOW.........some jack-@ss at some time in the car's life had to replace the ECT connector and did it by just twisting the wires together!! I CORRECTLY spliced both sensor wires, and the car fired right up. The ECT on the scanner is dead on with the inst panel gage as well. Let the car idle for 20-30 minutes with no issues. Took it out on a 20 minute test drive at full throttle, part throttle, you name it. NO ISSUES AT ALL. Left the Z06 in the garage and drove the Z28 around earlier today as well with no problems, runs like a champ!

I'm guessing that it was running pig rich due to the false low engine temp reading. This in turn was loading up and eventually causing a restriction in the cat......maybe?

....point is it's fixed now

Thanks to the guys who helped me think outside the box

Wild Willy

Excellent! Good detective work there- Glad you got it sorted out- Maybe some clues there for me on a '97 Blazer that I have been chasing an intermittent check engine light for 2 years on- One of my symptoms is that the instrument panel guage reads low, while I know the heat is fine- and the scanner shows correct operating temp of the engine- I have replaced the one temp sensor on top of the water pump, still looking for whatever one is throwing the wrong into to the gauge- Sorry for the hijack, just thinking aloud- mine generates an intermittent miss, most noticable on a warm re-start-