Catalytic Converter Replacement
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GM Enthusiast
Joined: Jan 2007
Posts: 3,982
From: Long Island, New York
Catalytic Converter Replacement
Hi all. Question about when to replace a catalytic converter. Is there a mileage/age when they should be replaced?
I've seen cars that ran pretty well with the original cat on it and no issues/check engine light/etc. Then the cat was replaced and there was a noticeable difference in the gas mileage and power. Any thoughts?
I've seen cars that ran pretty well with the original cat on it and no issues/check engine light/etc. Then the cat was replaced and there was a noticeable difference in the gas mileage and power. Any thoughts?
Registered User
Joined: May 2011
Posts: 2,339
From: Denver, CO.
You can do a pressure test on the exhaust in front of the cat. If you have more the about 1 to 1.5 psi pre cat at 2500rpm, then its restricted and should be replaced. If not, and it passes the local e-test, then no need to replace it.
1974 DELTA 88
Joined: Jun 2011
Posts: 569
From: Yavapai County, Arizona
You can also check for a blockage with a vacuum gauge.
Hook up a vacuum gauge to direct manifold vacuum, start the engine. Note the vacuum reading at idle. Hook up tach, if needed.
Rev the engine to around 2000-2500 RPM, the vacuum reading should initially drop when revving then once you are holding the throttle open steadily it should return to within 2-3 inches of the idle reading. If the vacuum reading stays low with the engine at speed you likely have a restriction.
Even better if you can get a long hose for the vacuum gauge and bring it in the vehicle with you. Go drive it and when you feel the power loss you suspect from a exhaust restriction, you should notice the vacuum reading drop, but RPM stay relatively the same.
This is due to exhaust not being able to leave the cylinders fast enough with the engine at speed or under load. When the exhaust valves open, the back pressure from your plugged exhaust prevents the cylinders from emptying completely. Then on the intake stroke the cylinders aren't empty so you don't get quite the vacuum you should. Also screws up your air/fuel mixture.
No need to drill holes in your exhaust.
Good luck!
Hook up a vacuum gauge to direct manifold vacuum, start the engine. Note the vacuum reading at idle. Hook up tach, if needed.
Rev the engine to around 2000-2500 RPM, the vacuum reading should initially drop when revving then once you are holding the throttle open steadily it should return to within 2-3 inches of the idle reading. If the vacuum reading stays low with the engine at speed you likely have a restriction.
Even better if you can get a long hose for the vacuum gauge and bring it in the vehicle with you. Go drive it and when you feel the power loss you suspect from a exhaust restriction, you should notice the vacuum reading drop, but RPM stay relatively the same.
This is due to exhaust not being able to leave the cylinders fast enough with the engine at speed or under load. When the exhaust valves open, the back pressure from your plugged exhaust prevents the cylinders from emptying completely. Then on the intake stroke the cylinders aren't empty so you don't get quite the vacuum you should. Also screws up your air/fuel mixture.
No need to drill holes in your exhaust.
Good luck!
Last edited by AZ455; Feb 8, 2012 at 08:34 AM. Reason: Added Additional Information
Registered User
Joined: Nov 2010
Posts: 601
"CQR's" got the best method, but you'll sometimes need to weld in a test port ahead of the cat, to hook up a gauge. If you're gonna do that, good idea testing the exhaust air/fuel ratio, with a wide band 02 monitor, as a lean ratio will kill a new cat in short order. New cat designs flow much better than the older ones, and aren't that expensive any more. Suppose you could start by standing behind the car when up to temperature, with someone working the gas pedal a bit. If the cats bad, usually smelly exhaust, and slow throttle response.
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