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How do you check your Cats?

jkailj

jkailj

Full Member
Joined
May 13, 2003
Messages
2,419
Location
Williamsburg, VA
I am getting a small misfire on cylinder 3 but it doesn't happen enough to throw a code.  It just idles rough and I see it on my scan software.  I have swapped plugs, coil packs and wires and it stays there.  I have searched and some say after swapping the cats if fixed this.  But how do I check the cats to see if they are bad?  Thanks, Jason  :wave:
 
Jason, this will get you close to what you are looking for

Warm the engine then turn it off. Remove the the driver side O2 sensor the one on the engine side of the CAT  and put in a compression tester, rev to engine to 2000 rpm if it reads more than 3 PSI you have a problem Also do the same test on the O2 sensor behind the CAT if you have more than 3 psi your problem is downstream somewhere
When CAT's go bad they add restriction.

Now with that being said you have to look at the rest of the exhaust system for restrictions before assuming it's the cat's
 
Also this is for a 04 I think close for other years too. I once had one go bad on my old blazor it was at 12 psi. HTH
 
Alot of times they break, try tapping them with a rubber mallet, if you hear a loud rattle you know its broken...Also 3 psi may be a bit high, try for less than 1 psi(.5 psi is spec) at 2500 rpm in park, on a engine at operating temp.  I have done before and after tests, after a faulty cat is replaced my exhaust backpressure gauge won't move off of 0 at 2500 rpm in park.
 
By Helm manual said 3 psi 4.8L or 5.3 L  .5 psi for 6.0 L
 
swing em by the tail and if they meow REALLY loud, they are ok!

Didit
 
Didit said:
swing em by the tail and if they meow REALLY loud, they are ok!

Didit
Click to expand...
lol.  do you still need to pressure test them?
 
There is a way, so I've heard, but I'm not going there, lol


Didit
 
MichiganSilver03 said:
By Helm manual said 3 psi 4.8L or 5.3 L? .5 psi for 6.0 L
Click to expand...
Maybe a typo in the helm manual, some info the gm's service information on restricted exhaust..

Document ID# 777339
2004 Chevrolet Avalanche - 4WD     


--------------------------------------------------------------------------------

Restricted Exhaust
Test Description
Caution: While engine is operating, the exhaust system will become extremely hot. To prevent burns avoid contacting a hot exhaust system.

The numbers below refer to the step numbers on the diagnostic table.

The exhaust system has very low back pressure under normal conditions. If the exhaust system is restricted, a significant increase in the exhaust pressure is noticed on the J 35314-A .

Removing the heater oxygen sensor (HO2S) sensor may set a DTC. When finishing this diagnostic table, be sure to clear all codes.

This step will isolate the catalytic converter from the remainder of the exhaust system.

Confirming that the condition has been fixed is essential. If the symptom still exists and the vehicle has a dual exhaust system, proceed to Step 2 and repeat diagnostic procedure on the opposite exhaust pipe.



Restricted Exhaust Step
Action
Value(s)
Yes
No

1
Did you verify the customers complaint?
--
Go to Step 2
--

2
Did you review the exhaust symptoms diagnostic information and perform the necessary inspections?
--
Go to Step 3
Go to Symptoms - Engine Exhaust

3
Remove the heated oxygen sensor (HO2S) that is in front of and closest to the catalytic converter. Refer to:
  -  Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 1 or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 1 in Engine Controls - 4.8L, 5.3L, and 6.0L. 

  -  Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 1 or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 1 in Engine Controls - 8.1L. 

Install the J 35314-A in place of the HO2S sensor.
Start the engine.
Increase and monitor the engine speed at 2000 RPM.
Observe the exhaust system back pressure reading on the gage.
Does the reading exceed the specified value?
20 kPa (3 psi)

If equipped with a 4.8L, 5.3L, or 6.0L - 3.4 kPa (0.5 psi)
Go to Step 4
Go to Step 7

4
Turn the engine off and place the ignition in the lock position.
Remove the J 35314-A .
Re-install the HO2S sensor. Refer to:
  -  Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 1 or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 1 in Engine Controls - 4.8L, 5.3L, and 6.0L. 

  -  Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 1 or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 1 in Engine Controls - 8.1L. 

Remove the post-catalyst HO2S sensor. Refer to
  -  Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 2 or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 2 in Engine Controls 4.8L, 5.3L, and 6.0L. 

  -  Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 2 or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 2 in Engine Controls - 8.1L. 

Install the J 35314-A in place of the post HO2S sensor.
Start the engine.
Increase and monitor the engine speed at 2000 RPM.
Observe the exhaust system back pressure reading on the gage.
Does the reading exceed the specified value?
20 kPa (3 psi)

If equipped with a 4.8L, 5.3L, or 6.0L - 3.4 kPa (0.5 psi)
Go to Step 5
Go to Step 6

5
Inspect the exhaust system for the following conditions:

  ?  Damage in the exhaust pipe. 

  ?  Debris in the exhaust pipe 

  ?  Muffler internal failure. 

  ?  Two-layer exhaust pipe separation. 

Did you find and correct the condition?
--
Go to Step 7
--

6
Replace the catalytic converter. Refer to Catalytic Converter Replacement .

Did you find and correct the condition?
--
Go to Step 7
--

7
Remove the J 35314-A .
Re-install the HO2S sensor. Refer to:
  -  Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 2 or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 2 in Engine Controls - 4.8L, 5.3L, and 6.0L. 

  -  Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 2 or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 2 in Engine Controls - 8.1L. 

Clear any codes.
Road test the vehicle in order to verify the repair.
Did you correct the condition?
--
System OK
Go to Step 2



--------------------------------------------------------------------------------
  Document ID# 777339

 
Wow, thank you all for the good info.  I will have to wait till I get to my home in a month or so to do that though.  Thanks again.
 
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