Chevy Impala SS Forum banner

1 - 11 of 11 Posts

·
Registered
Joined
·
845 Posts
Discussion Starter · #1 ·
My car didnt pass emissions. Since I have thoroughly gone through all of the sensors and major tune up items in the engine compartment, I suspected it was the cat which I replaced when I first got the car three years ago. My suspicions were right, I got a new OBD II california compliant part installed and it passed emissions with flying colors.

Since I did this my car has been heating up way faster and almost over heating. This is accompanied by a very obvious egg like smell. Now, since alot catalytic converters last many years I also suspected that there was something else wrong that made the part go bad.

My question is what is it that makes a cat go bad? to lean? to rich? what can I look for to correct this prospective issue? any help would be greatly appreciated
 

·
Registered
Joined
·
731 Posts
One thing that will kill a cat in time is an extra rich condition.

Did you replace the O2 sensor? If it has a lot of miles on it, it could be "dead" and running the engine pig-rich.

Also, assuming you are tbi, did you check the CTS in the intake manifold? If it gets out of calibration it can run the engine extremely rich.

The worst case scenario is a bad head gasket, that will kill a cat as well, but you would surely have noticed other symptoms by now if that were the case.
 

·
Registered
Joined
·
845 Posts
Discussion Starter · #3 ·
O2 has been replaced about a year ago, as well as the CTS about six months ago in efforts to correct a bad running problem I have had for a while. While neither of those fixed the problem they both made it slightly better and then it went back to almost stalling and backfiring at random. Its definitely not the head gasket, there are no symptoms to suggest that. Is there a special way to install the CTS or the O2 that would throw it out of whack if not done correctly?
 

·
Registered
Joined
·
199 Posts
A rich mixture by itself will not cause a cat to melt unless there is additional oxygen in the exhaust to support combustion. This typically happens if there is an exhaust leak or a cylinder misfire. An exhaust leak before the O2 sensor will cause the O2 sensor to command a richer mixture, plus it will introduce additional air into the exhaust. A misfire will inject a slug of unburned air/fuel past the exhaust valve. In both cases, there is both unburned fuel in the cat AND enough oxygen to support combustion. Other possibilities include an AIR pump that runs all the time or an AIR pump check valve that is leaking. Any of these can also cause a backfire as combustion that takes place inside the exhaust is still sometimes taking place when it reaches the tailpipe. The result is a nice satisfying BANG! out of the exhaust, sometimes with muffler shrapnel to boot.
 

·
Registered
Joined
·
845 Posts
Discussion Starter · #5 ·
A rich mixture by itself will not cause a cat to melt unless there is additional oxygen in the exhaust to support combustion. This typically happens if there is an exhaust leak or a cylinder misfire. An exhaust leak before the O2 sensor will cause the O2 sensor to command a richer mixture, plus it will introduce additional air into the exhaust. A misfire will inject a slug of unburned air/fuel past the exhaust valve. In both cases, there is both unburned fuel in the cat AND enough oxygen to support combustion. Other possibilities include an AIR pump that runs all the time or an AIR pump check valve that is leaking. Any of these can also cause a backfire as combustion that takes place inside the exhaust is still sometimes taking place when it reaches the tailpipe. The result is a nice satisfying BANG! out of the exhaust, sometimes with muffler shrapnel to boot.
This is really informative, and much appreciated. I only had backfiring with the old cat, now it just runs a little better but stalls from time to time, im assuming from whatever is causing this issue. No exhaust leak, Ive had a back pressure test done and everything checked out. Can a distributor that is in bad shape cause a misfire? would an airpump that was on all the time be obvious?
 

·
Registered
Joined
·
5,693 Posts
Can a distributor that is in bad shape cause a misfire? would an airpump that was on all the time be obvious?
Yes, the dist. can cause a misfire, so can just the cap or rotor. Yes, you can hear the air pump run, might not be obvious unless you listen for it, but it runs at start-up for a minute or 2....at least mine does.
 

·
Registered
Joined
·
11,473 Posts
On a TBI motor, if the ground at the thermostat is faulty, it can cause the engine to run rich. It is the ground for the ECM and the O2 sensor. I fried a cat because of this problem. The car ran max rich, and would not idle properly.
 

·
Registered
Joined
·
1,739 Posts
On a TBI motor, if the ground at the thermostat is faulty, it can cause the engine to run rich. It is the ground for the ECM and the O2 sensor. I fried a cat because of this problem. The car ran max rich, and would not idle properly.
That's a good tip. I've been struggling passing emission for the last few years myself with my 91 L03. Even though I get good mileage (20mpg out of each tankful) and runs great.
I've replaced, now, everything, sensors, engine, tbi, dist, cat, etc. except the air pump and it's check valve with no improvements.
But I have on a few times dealt with that/those ground wires when unscrewing the thermostat bolt. Last inspection the CO was high pointing to a rich mixture.
Have to check them more closely. If I recall right, there are two ring terminals, one with one wire, the other with two going to it. Do you know which are for what?
 

·
Registered
Joined
·
11,473 Posts
There are 2 from the ECM on one lug, and about 6 other things connected through a splice slightly upstream from the other lug (G103). You should check the ground at the rear of the PS cylinder head. It also has ECM grounds, and numerous sensor grounds (G104).

You can find the locations and specifics in the ground distribution section of the FSM.
 

·
Registered
Joined
·
22 Posts
My 92 caprice had a problem with stalling. Had it retimed and it ran good for a while then started stalling again. Mechanic pulled the distributer and found out the magnets around the distributer shaft had cracked and were jumping around. Had to replace the distributer shaft and it ran great.
 
1 - 11 of 11 Posts
Top