Plugged exhaust?
Plugged exhaust?
Thread Starter
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Tuned
Joined: Jun 2003
Posts: 489
Likes: 1
Plugged exhaust?
Trying to eliminate why my truck is running rough at idle. One thing that has come to mind is a possible plugged exhaust. It sounds like I have some exhaust coming out from the engine/headers...I just thought there might be a small leak and it doesn't bother me. But I have twin pipes coming out the back and just realized that I feel very little pressure coming out of the pipes...it's a 93 w/302, shouldn't it be pushing some serious air out even at idle? it does kind of sound like a diesel when I am driving it...maybe the cat is plugged. How would I test to verify this?
Post Fiend
Joined: Nov 2002
Posts: 9,987
Likes: 7
From: Memphis, TN
Plugged exhaust?
To find an exhaust leak, have an assistant hold a rag TIGHTLY over the end of the tailpipe while you listen for the growing hissing sounds as pressure builds inside the exhaust system. The muffler has a weep hole, so don't worry about that leak, and it's common for the clamps not to seal, so just concentrate on leaks FORWARD of the cats.
To check for an exhaust restriction, use a vacuum gauge.
To check for an exhaust restriction, use a vacuum gauge.
Thread Starter
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Tuned
Joined: Jun 2003
Posts: 489
Likes: 1
Plugged exhaust?
Thanks for the info, but finding the leak isn't my concern as much as it is finding if the cat or something else in the exhaust is plugged. I feel very little coming out of the tailpipes and wonder if this could cause rough idle/missing. I was thinking if I pulled the o2 sensor and ran it I could see if all the pressure is relieved...any other ways to determine if the cat or exhaust is plugged...would it throw a code if it were?
Lead Driver
Joined: Sep 2002
Posts: 9,498
Likes: 2,020
From: North Texas
Plugged exhaust?
The removed O2 sensor will eventually throw a code...it took me about 20min for me to throw one when I drove open long tubes to my exhaust place. 
You will be fine to pull it and see if that helps.
How many miles are on your truck now? What sort of exhaust system do you have on there?
You will be fine to pull it and see if that helps.
How many miles are on your truck now? What sort of exhaust system do you have on there?
Senior User
Joined: Jul 2003
Posts: 299
Likes: 2
From: central valley California
Plugged exhaust?
the best way to know if your cats are plugged is to take it to a muff shop have them blow a small hole before the cat and do a pressure test. If the test is 3 or higher then you have a pluged cat. I do believe that oem set up is a two cat system on your truck out to a single pipe out and or a dual pipe system.
Thread Starter
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Tuned
Joined: Jun 2003
Posts: 489
Likes: 1
Plugged exhaust?
pulled the o2 and it runs better...everything is coming out of it.
Measured the vacuum at the tree beforehand and had about 18" at idle, it would drop to about 10" when I ran at about 2000rpm. Looks like the cat, or something behind it is plugged.
The exhaust is a single muffler and twin pipes come out after a Y...I think the whole thing is custom...the muffler looks new...I've only had the truck for 1 month.
Measured the vacuum at the tree beforehand and had about 18" at idle, it would drop to about 10" when I ran at about 2000rpm. Looks like the cat, or something behind it is plugged.
The exhaust is a single muffler and twin pipes come out after a Y...I think the whole thing is custom...the muffler looks new...I've only had the truck for 1 month.
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Lead Driver
Joined: Sep 2002
Posts: 9,498
Likes: 2,020
From: North Texas
Plugged exhaust?
So you muffler is new, but your cat & Y-pipe assy. is still OE? How many miles on it?
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Thread Starter
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Tuned
Joined: Jun 2003
Posts: 489
Likes: 1
Plugged exhaust?
99K miles on it and it is the original 2 converter system. Did some additional diagnosis on it. When I measure the vacuum, I get 19" while at idle...if I rev it to 2000 or 2500 (in park) it pretty much stays the same (varies a little, but pretty much the same). Now, if I drive around with it, it drops to between 5" and 7" at a constant speed. I pulled the o2 sensor out and it seems to run better (smoother), but the vacuum is between 8" and 11" when I do the same type of driving. Does it make sense that this happens only when the engine is under load vs. in park? For some reason I would expect the results to be the same in park as driving if it was an exhaust issue. I'm guessing the small difference between o2 in verses out is because the o2 sensor hole is so small it makes little difference.