hi everyone this would be my second post on this site..lately my monte has been having problems with acceleration and hesitation bad rotten egg smell and sometimes it wont downshift on the highway when wot..


went to the junk yard and put on the stock ss exhaust with a u bend on my car compared to the 3 inch cat back i had with no u bend..
the car ran much better at city driving..

long story short the past 4k miles my car idle surges to 800- 1200rpms while crusing 50mph espcially when it gets hot, and i have a lean code after i reset the codes my scan tools shows my bank 1 o2 voltage drops all the way to 0.02 jumps randomly to .800
and my long term fuel trim is +40%
short term fuel trim is +28%
i have to turn the key for the fuel pump about 12 times and almost wot to get started after driving it for 10 mins...
does not over heat at all* stays between the 1/4 mark and almost half on temp gauge

i have tried sea foming it
changed my fuel filter,
new fuel pump
cleaning maf sensor
got my injectors ultrasonic cleaned on ebay...
180 degree thermostat new
iduruim ngk plugs (i change every year)
new plug wires
cleaned tb/egr/
tried every fuel injecor cleaner there is
new radiator new coolant the green stuff washed all the orange stuff out...

and took i to a shop to get it back pressure tested and valaa
they told me i had over 18psi at 3000rpms
and 6psi at idle
. and that my cat is severly blocked
tried selling me a new cat for 380$ and welding would cost $525 total
(rip off)???

*the plan*
me and my dad were looking at 3 inch off road down pipe for the 3800
and my dad said he could weld/fab one up for my car and get rid of the cat and u bend delete and run it into the stock ehaust after a huge cherry bomb as a res

now my question is is it possible for the back pressure to be 18psi
and would a 3 inch down-pipe with no cat or u bend be to big for a monte carlo ls 3.4 to the 2.5 inch stock exhuast..

or should i run a 2.5 inch down pipe with no cat no u bend to stock exhaust ?
if you are confused with my post please feel free to ask because i need help lol thank you

BTW we live in Michigan so there are no emission testing

 

Stick with a 2.5" with cat. The 3" will move the power band up the rpm scale and you will lose some bottom end power. This is why you noticed the car didn't run as well in city traffic when you had the 3" ZZP downpipe installed. Even though you don't have an emission test, if you eliminate the cat it will set a P0420 code and activate the CEL. This will force the PCM into open loop mode instead of operating in closed loop mode. This will have a direct impact on drivability and fuel mileage.

 

According to the shop book, the P0420 code does nothing more then light up the CEL and store freeze-frame data of the condition. I does not change open/closed loop as a result of the code.

But having a 3400, unless you have plans to turbo or super charge, I see no reason to do a 3" DP. As mentioned, it's going to cause some issues. A good 2.5" is all you should run with. For the cost that shop was quoting you, I bet you can get a nice pipe some where that bolts right in. ZZPerformance.com is the place that comes to my mind, maybe even WBodyStore.com. And I just checked, neither sell DPs for a 3400

Maybe a stock one from RockAuto.com???

As for the P0420 code, I would recommend you look into a method to NOT throw that code (just so you don't get in the habit of ignoring the CEL).

 

The book may not mention it, but the sequence of events caused by P0420 will force the PCM into open loop mode. A cold engine operates in open loop (default tables) until the engine comes up to temp. Once up to temp the PCM switches to closed loop (adjustment via sensors) so it can monitor the O2 in the exhaust stream and adjust the air/fuel mixture according to engine load. If P0420 has been set the PCM cannot do a comparison between the readings of the upstream and downstream sensors. Since it cannot do the comparison the PCM is forced to remain in open loop mode (default tables). The engine runs richer then needed which results in lower mpg and drivability problems (hard starting, hesitation, erratic shifting, etc).

 

my dad already went to the junk yard and cut the flange off on the stock down pipe...
i am going to use 2.5 inch from the header to the mufflers.
NO CAT..i have read on a forums where they had the rear o2 sensor spaced from the exhaust and the trick is as long as the o2 sensor gets some heat no cel
and possibly 2 40 series mufflers
and anyone know any good resonators(dont want rasp) up to 17 inches in length under $90??

quick question since my car has had multi lean codes. i pulled the injectors out and ultra sonic cleaned for 2 days.. them just to be on the safe side.
while checking them injector with a multimeter i got these strange#
# 1,2,3,5, were at 12.8 ohms
while 4, was at 14.0 and #6 was at 14.2 ohms
i redid the test 4 times and got the same results.. are my injectors within spec.
should i replace the other 2?
and ill upload a pic of my downpipe when my dad welds it..

also can i bypass the coolant line that runs thought the tb? while taking my uim off i forgot the hoses and yanked it off...ughh
big thanks to
03SSLE
&
The_Maniac

 

[IMG]www.bigdaddiesgarage.com/images/o2-simulator-big-daddies.gif[/IMG]

 

You can bypass the coolant lines going to the TB. Best way is to get a heater pipe from a 94/95 3100 that did NOT feed coolant through the TB (fyi, if you are not aware, the 3100 and 3400 are for the most part, the same engine).

As for the O2, I have read lots of success spacing the O2 out of the exhaust stream for cars with non-OEM or high flow cats. I never read of anyone doing it when there is no cat (so I have no idea if there is a chance for success or not). Now, it looks like that company wants $50 for that. If you are looking to do that trick, just buy the $5-$10 spark plug non foulers from the auto parts store. take one and bore it out to fit the O2. Using 2 total to space it out of the exhaust stream.

And as for the injectors, I heard of OHM testing them. I have heard that test is not 100% reliable. Also, if the injectors were ever changed with non OEM, there is no way of telling for sure if that test is valid.