in2pro

Here is a link on how to interpret a vacuum gauge and its readings... click on the scenario buttons for each condition....
You can pick up a vacuum gauge at most auto parts stores, Harbor freight and Wally world(Wal-mart)

http://www.secondchancegarage.com/public/186.cfm

BigDaddyO

OK, i'll pick up a vacuum gauge and test that first.

"Some manifolds incorporate a plug that may be removed for such purposes" Does my Suburban have this plug? if so, where can i find it?

Thanks,
Mike

MDTAHOE

99.9% the time catalyst efficiency codes are a bad cat. Testing it with a vac gauge will not tell you anything. Your converters are not clogged or restricted (that is what you test for with a vac gauge). Converter efficiency codes mean the cat is not cleaning the mixture properly. The computer looks at the voltage on the O2 sensor in front of the cat and compares it to the O2 behind the cat if the voltage are about the same then the cat is not cleaning the mixture as it should thus a PO420 (bank 1) or PO430 (bank2). If the voltage on the rear O2 sensor is lower than the front it means the cat is working. There are only 2 ways to truly test the cat for THIS problem. #1 would be to use a 5 gas analyzer to perform a oxygen capture test (best way). #2 would be to have a scan tool that shows live data and look at the front and rear O2 sensor. On that note this is probably not causing your slightly rough idle. A clogged or restricted cat would cause a loss of power not a rough idle. The 5.7 motor always has a very slight rough idle it really depends on how rough yours truly is. Also cats do not clog for no reason. If a cat melts something else caused it to happen. As far as your idle what is the condition of the cap , rotor, wires and spark plugs?

MDTAHOE

Due to the way the computer monitors the cat eliminating cat is not a good option. Also using a cheap cat may cause the light to come back on sooner than later. In case you did not know Bank 2 cat is on the passenger side, bank 1 is the drivers (you have 2).

in2pro

MDTAHOE, does that mean his problem could be flaky O2 sensors?

as for the vacuum check I think the idea was to eliminate a vac leak causing a rough or unstable idle....

BigDaddyO

I already picked up the Vacuum gauge at Napa on the way home so i'll be testing that no matter what. I'm also going to test the fuel pressure just because it can.

Now I just have to figure out where to plug it into the engine for the vacuum test.

As for the cap , rotor, wires and spark plugs, the guy said he replaced them all when it was inspected 3 months ago but I haven't had time to check them myself.

MDTAHOE

You can not test for a vac leak with a vac gauge. The only way it would cause vac to be low would be if it was so big you could hear it or if it made the fuel mixture so lean that it would drive vac down. It could technically be caused by a flaky O2 however a bad o2 would generally set a rich or lean code. In all my years of working on performance problems ( what I do everyday) ( I have a $10,000 scan tool ( Snap On Verus) to do testing) almost all if not all converter codes have been bad cats. I do perform a O2 capture test on all them to make sure before replacing.

73shark

If your '99 is any kind of fuel injected, I don't think you want to check fuel pressure w/ your new vacuum gauge. It's usually for mechanical fuel pumps on carb engines. I could be wrong tho as haven't checked vacuum gauges for years.

silverado13

alright, i had a vacuum leak and tryig to use an excellerant to find it didnt work but its much more simple then that, your vacuum system runs from the back of your truck all the way into the engine compartment, on the left side of the plastic part of the intake manifold there is a sensor with 3 small hoses/tubes coming from it, one is just a breather, the second is about a 4 inch tube/hose that runs back into the atmospheric pressure sensor and the last (the front one) actually runs backward and around the throttle body and then comes around the other side of the engine and goes to the EVAP canister (the cylinder on the right that is right behind the radiator) a couple hoses come out of that and run towards the gas tank and are mounted against the frame, so just follow all the hoses starting from that first sensor all the way back to the gas tank, if it is a "large" vacuum leak then the problem should be pretty easy to spot, your just looking for cracked/damages hosing and make sure your EVAP canister doesnt have and holes in it.
i had a small vacuum leak and found mine pretty easily, the way this causes a problem is because if the right seal isnt made throughout the lines, then the atmospheric pressure sensor isnt getting the proper reading, in my case, i waited to long to fix the leak and when the sensor wasnt getting the right reading, it was sensing that i was at a higher elevation then i was, and simple science will tell you that air is more dense the higher you are, so the computer tries to compensate for the "more dense air" and pumps more fuel then needed, because i waited so long it ended up frying all 4 of my O2 sensor which caused my truck to run even worse, not to mention that replacing 4 O2 sensor isnt cheap, this was also causing misfiring.
so id defidently check that out, to test to see if you have oil leaking into the exhaust, simply unbolt the pipes from the exhaust manifold and stick your figure into the pipe, if you have oil in your pipe then thats your problem, if not then its not
so that covers 3 of the 4 possiblr problems, as for the last, if the other 3 possibilities check out and your systems are fine, then it is most likely your converters, pricing for converters is strickly based on the vehicle, i know the converters for a 2003 dodge ram are like $2500 A PIECE, ive never priced them for a vehicle like mine or yours but it is what it is
as far as going to the junk yard and getting them, unless the yard is just not up with the laws and regulations, you can not buy them used from junk yards due to emission regulations, i know my local junk yard cuts all the converters off of all the vehicle and if for some reason the vehicle would still have them on it, it is either because they were not able to get it off without taking apart part of the vehicle (like i seem them still on late 90's model blazers) or because they just missed it, the bad thing is that catalytic converters contain a small amount of platinum in them and can actually be sold to metal yards for a pretty penny
as for going with out them, i do not know what state you are in but i know in louisiana, it is illegal to go without them, but realisitically i dont know how you would get caught, its not like a police officer is going to look under your vehicle
as far as performance without them, if you had open headers (just exhaust manifold) then you would run into problem with not having enough back pressure and that can lead to engine failure and potentially blowing your engine, but if you have a full exhaust system minus converters then you will have enough back pressure, a friend of mine had a 2000 gmc sierra and put a full exhaust system but left the converters off and he had significant power increase, but the problem with that is you have to leave your O2 sensors off because again they will get a false reading and most likely tell your computer to pump to much gas through the sytem, but the increase power is because there was just less resistance so you have been flow
now if you dont have converters then your engine light will never go off and you will not be able to get the vehicle inspected and thats just compounding problems
mine advise would be to do the other test first and if the test dont show a problem then i would defidently replace the converters, i do not know the price or your financial situation but if money is a problem and the price is to high then i would look around for someone who has a similar vehicle that got into an accident and is going to total the vehicle, from there you can try to negotiate to buy them from that person, if you do that then no one will be able to tell that you bought them used and i dont think that there is any "official" law about buying them privately

yours truly
silverado13

JimJ

Simple Science will tell you that the higher you are the LESS dense the air is. I think you got your wires crossed.