Oh guys I have ANOTHER question! lol. This one is for the exhaust experts. Was concerned about my cat being clogged up and was looking in my Haynes manual. The manual says that if you hook up a vacuum gauge to manifold vacuum, rev it to about 2000 rpms and then take your foot off gas really quick it should quickly drop back down to about 2 in hg within what you started at..ok did this and mine is pulling about 14-15 at 2000 rpms and then when I let off the throttle it jumps up 1 or 2 in hg then drops down to close to 14 or 15 again. So what do you think? The manual says that if it "seems to momentarily hover around its highest reading for a moment before it returns" then a clog is present in the exhaust system. Sounds to me like I'm gonna need some exhaust work too. So second part of this question is what CHEAP exhaust systems are good? Will probably go ahead and do the whole exhaust system (except for headers, cant afford those right now) as far as cat pipes and muffler. So what is good and cheap (or should I say economical). I'm not looking for performance exhaust just basic close to stock or cheaper upgrades.

 

Inaccurate test.... better way is to drill a small hole in the exhaust, before the cat, and put a pressure gauge on it. Anything over 2 pounds at idle is too much.

 

I don't know where you got that info from but it's wrong.And not accurate like HeyYou said. At idle what ever you see, lets say 20Hg, when you slowly rev the engine to 2500rpm, the vacuum should go up.

 

Was flipping through my Haynes Manual (#30041 Dodge Pickups 1994-2001) last night and that's where I found it. Had never heard of it before either. I'm sure you've got one lying around. It's at the very end of the emissions and engine control section. Check out page 6-43. Tells you how to do the test I posted about. Thanks for the info that it's not accurate. Will do the "drill a hole" test. Thats why I like this forum; you guys have had more experience with these trucks/motors/trans than I have. I mean I'm not a newb mechanic just haven't screwed with some of the stuff. Like I said I had never heard of checking a cat that way but thought I would try it.
So I guess that begs another question, not to change the topic, but makes you wonder about some of the other info in there. I've always had good luck with using Haynes manuals although there are quite a few little things missing sometimes but makes me wonder...
Thanks for the help guys.

 

I've heard of it.. here..

I don't know how accurate it is, but it's spoken about in a lot of publications..

the other one to use to determine excessive backpressure is the rapid accelerating one.. if you bury your vacuum and see a slow raise, it is supposed to indicate the same..