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Cat rattles when pounded with fist - pics added?

 
Old 10-11-2010, 08:37 PM
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jwillman
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Default Cat rattles when pounded with fist - pics added?

While inspecting the exhaust on my 79 I was banging my fist on parts to check for security. When I bang on the cat dead center there is a pronounced rattle.

I did a search here and a general search in google and found that soemcats are pellet filled and might rattle. I don't thionk my exhaust is original. Does the 79 have an O2 sensor? I am not seeing one of those either.

Thanks

Last edited by jwillman; 10-12-2010 at 10:38 PM.
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Old 10-11-2010, 08:44 PM
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PorKen
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I don't think it should rattle.

This is what a '78-'79 cat looks like. '80 looks more modern and has an O2 sensor. '81-'84 have air injection piping. AFAIK, '80-'84 should fit '79.



http://cgi.ebay.com/ebaymotors/ws/eB...m=120632053507


'78 has a big body crossmember which connects the non-frame rails so I don't think a later cat will bolt on without modification.

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Old 10-11-2010, 09:26 PM
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Pt says to MD: Doctor, it hurts when I do this."

MD to Pt: "Then don't do that!"

'79 manifolds will have an undrilled flange-tab casting on the downward travel. No O2 into cat IIRC. And the stock cat is a honeycomb affair, no pellets.
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Old 10-11-2010, 10:02 PM
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jwillman
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Originally Posted by PorKen View Post
I don't think it should rattle.

This is what a '78-'79 cat looks like. '80 looks more modern and has an O2 sensor. '81-'84 have air injection piping. AFAIK, '80-'84 should fit '79.



http://cgi.ebay.com/ebaymotors/ws/eB...m=120632053507


'78 has a big body crossmember which connects the non-frame rails so I don't think a later cat will bolt on without modification.

My system Y's into a single pipe which goes another ~2 ft and matches up with what should be the cat. Makes me wonder more about the origin of my exhaust. My 79 does not have the rear crossmember nor are there holes to mount one.

Last edited by jwillman; 10-12-2010 at 09:45 AM.
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Old 10-11-2010, 11:10 PM
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please post some pictures it will make an ID of what you have possible
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Old 10-12-2010, 12:00 AM
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RyanPerrella
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in general a rattling cat means its time for a replacement.

Most cats Ive ever seen were honeycomb either precious metal or ceramic, im not familiar with pellet filled cats. Either way they rattle when they have broken apart and that means its time for some new ones.

Eventually the cats will clog up and the car wont run.
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Old 10-12-2010, 12:17 AM
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Originally Posted by RyanPerrella View Post
in general a rattling cat means its time for a replacement.

Most cats Ive ever seen were honeycomb either precious metal or ceramic, im not familiar with pellet filled cats. Either way they rattle when they have broken apart and that means its time for some new ones.

Eventually the cats will clog up and the car wont run.
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Old 10-12-2010, 08:05 AM
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Remove a few bolts at the rear flange and use the M1A1-01 visual inspection device (aka eyeball) to see if the honeycomb has clogged or melted closed. Ceramic honeycomb with metal coating. When overheated, the honeycomb sections melt closed. Usually starts at the front, but you should be able to see some from the rear if it's severe. If nothing else you can slide a piece of fine safety wire through a few of the cells and see if they pass through. Melting starts at the area in the fron where the gas first first passes into the honeycomb, so pass the wire through off to the side of the rear centered flange.
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Old 10-12-2010, 10:17 AM
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Originally Posted by GregBBRD View Post
+1
+2
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Old 10-12-2010, 10:27 AM
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Does Texas test for HC and nitrates when yearly inspection is due? I usually just punch out the cat and get a few free ponies. When inspection time rolls around you can add 3oz acetone/10 gal 100% petrol and you will blow better HC/nitrate #s than if you had a brand new cat.
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Old 10-12-2010, 12:49 PM
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oh, they might run, but when they do run, they will have flat spots in acceleration. frustrating. my 79 track car had this, we just replaced with a exhuast shop cat, hack job.
however, if you get headers, i do have a nice cat that mounts to the devek level 1 headers and then to the stock exhuast system. let me know if you are heading the header direction and are interested in the cat to fit that system.
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Old 10-12-2010, 03:25 PM
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Originally Posted by xschop View Post
Does Texas test for HC and nitrates when yearly inspection is due? I usually just punch out the cat and get a few free ponies. When inspection time rolls around you can add 3oz acetone/10 gal 100% petrol and you will blow better HC/nitrate #s than if you had a brand new cat.
Looks like no emission testing and no cat required for my 79.

Here is the verbiage from the Texas Vehicle Inspection site:

If installed as original equipment by the manufacturer, the catalytic converter will be considered a part of the exhaust emission system on all 1984 and later model vehicles. It will be inspected as a part of the exhaust system on prior to 1984 model vehicles.

The catalytic converter will be considered as a part of the exhaust system on all vehicles prior to 1984 year model and will be inspected only visually (if present) for leakage. On 1984 and later model light truck and passenger vehicles, the catalytic converter will be checked for presence and leakage. Flexible tubing which meets the requirements listed below may be used anywhere in the exhaust system.

NOTE: Inspection of exhaust systems covers the discharge of exhaust fumes and is not concerned with the noise level.

Which vehicles are required to have the emissions test?

Vehicles 2 through 24 years old OR upon expiration of the two-year initial inspection sticker.
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Old 10-12-2010, 04:02 PM
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Punch it out and weld in an adjustable washer on a thru bolt for backpressure. I've done this to many bikes an a couple of cars

Here's a very good write-up, same goes for any exhaust system... you'll just need a bigger washer
http://www.nightrider.com/biketech/exhaust.htm

Last edited by xschop; 10-12-2010 at 04:19 PM.
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Old 10-12-2010, 04:18 PM
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Neither of my OB's have cats, the 78 has a Y-pipe into a borla cat back system, and the 79 has a factory 2 into 1 muffler located where the cat would be on a US model into a resonator and a homemade RMB. Technically, I should fail the visual inspection on the 78, but haven't so far in two years. Not really sure how the 79 made it into the country without a cat being added.

I say replace the cat with a Y-pipe, and remove the air pump.
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Old 10-12-2010, 04:26 PM
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And I was just about to suggest a Y-pipe the same diameter of the front collectors.
I didn't fall victim to the "bigger is better" crowd doing the 948 swap. I ran 2.5" exhast all the way back to a straight thru Magnaflow with NO cat and I have ZERO issues throught the RPM range.

Actually the only issue I have is keeping the lightweight beast in a straight line....check the post mid-page....No cats allowed
http://944hybrids.forumotion.com/944...ight=nailed+it
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