Is it possible for both cats to be clogged and not throw any CEL codes? I am having some air flow issues and I suspect one or both cats, but I'm not throwing any codes.

 

Yes, it's possible.
On OBD-I vehicles (95-down) cats do not throw a code when plugged.
On OBD-II vehicles (96-up) a plugged cat may throw a code when it gets bad enough to affect the post-cat O2 sensor.

 

Are you having trouble reaching higher rpms? Does it want to die after idling for a while? What makes you think it might be clogged?

Do you own a vacuum gauge?

 

Yep, I've got a vaccum guage. It just feels like it's bogging down and I can't seem to find any other issues; air/fuel mixture, etc. I don't know how to explain it, it just feels like it's not gettin' enough air through the exhaust. Also, I pulled all the carpet so I can Line-x the floor and now I can hear a faint rattle coming from the passenger side right where the cats are and I'm pretty sure it's the screen rattling.

 

I'm not having a problem with higher RPM's and it doesn't die when idling, it just feels like something's holding it back!

 

bogging down can also be a spark issue. On my old truck i had a spark plug wire rubbing on a piece of metal and it over time rubbed the rubber of the wire and the spark was arching onto the block and not giving me any spark to 3 of my cylinders, it would start up, then bogg down unless i hammered the gas to keep it running. The minute i let off the gas it stalled. An rattleing can just be a heat shield somewhere. Alot of people's heat shields on there muffler, near the gas tank or near the bottom of there cats will rattle agenst something, then one day stop because it has probably fallen off on the road somewhere.

 

I've checked every wire and everything else I can think of, so I started suspecting the cats. They look OLD and are covered in rust deposits, so I just started thinking they might be clogged. I hit 'em with a rubber mallet and I don't hear anything loose, but I can't find anything else wrong with this thing. I did look at the engine running in the dark the other night and saw that the ignition module was glowing a little bit, but I just replaced it a few months ago, so that shouldn't be going bad already.

 

With a vacuum gauge, there are two easy tests you can run to determine if your exhaust is restricted (cats clogged).

First test:
1) Attach vacuum gauge to manifold vacuum source.
2) Observe vacuum at idle.
3) Snap throttle to WOT and release, while watching the gauge.

The vacuum gauge should drop to almost zero when you hit WOT. When the throttle snaps closed immediately after, the vacuum should read 4" to 6" higher than what it did at idle. It should then settle back at the same reading it was at idle before the WOT snap. This whole exchange should go from idle reading to idle reading in 2-3 seconds, tops. If it takes longer for the gauge to return to the same idle reading, your exhaust is restricted. The longer it takes, the worse the restriction.

Second test:
1) Attach vacuum gauge to manifold vacuum source.
2) Warm up engine.
3) Observe vacuum at idle.
4) Leave engine idling for 10 minutes. Do not touch anything on the car during this time. In fact, go inside and fix some coffee.
5) After 10 minutes, observe the vacuum.

If the vacuum is the same or higher, you don't have any resctrictions. If the vacuum gauge has dropped, you do have a restriction. The more it drops, the greater the restriction. If it drops 1" or more, you have a serious restriction. 2" or more and you might as well have the exhaust welded shut. Keep in mind that a rich exhaust will clog up cats in a hurry.

 

THANK YOU! I didn't know there was a test for that. I thought you just had to hit the cats with a hammer and if you get a rattle, then you replace it. Do you know where is the manifold vaccum source is located on a '92, 5.0, manual transmission? Thanks again. -Al

 

Find your fuel pressure regulator, and follow the vacuum line that attaches to it back to it's source. If there is an extra nipple at the source, plug your vacuum gauge in there. If all the nipples are used, you will need to use a T and install the vacuum gauge inline.

 

Another test that seems bizzare but I've seen several on other forums confirm that it works:

Remove the O2 sensor and run the truck. You'll throw a code for the mixture, but just the hole in the exhaust will reduce the restriction (if it exists) enough to tell there is an exhaust restriction.




And BTW Pkrwud,

Per the second test, step 4. What if you don't drink coffee?

 

I also forgot to mention that my gas mileage went from about 13 to about 8, almost overnight. I don't know how I forgot that, since I'm reminded everytime I crank out $4/gallon.

 

The vaccum line from the fuel pressure regulator goes leads to the top of the intake. There are a couple of other vaccum lines leading out of the intake and one open line that is capped. Is that where I hook up the guage?

 

I finally got a CEL this morning on the way to work. I put the code scanner on it and I'm getting a 31 and a 32, which I got a couple of weeks ago. I already replaced the EGR, the EGR sensor, all of the vaccum lines and the little actuator that the vaccum line goes to when I got the codes the last time. At least that explains the bad gas mileage. Any ideas?

 

Sounds perfect. Hook up the vacuum gauge and start the engine. If it reads between 17 & 19, and is steady, you've found a good port.


One thing at a time, lol. Do your exhaust tests first, and post the results.





Who said it was for you??? lol.


Still the smarta$$, I see.