My car has been acting up really bad. it's been getting a really bad miss but mostly when it's cold. The opti is about 8 months old, I've checked all my ignition and it's fine. I looked at my readings on my friends tech 2 scanner and my fuel trims are all out of wack. fuel pressure test is fine, and the opti had signal on the tech 2. The only thing I've done to my car that I think could cause it is I gutted the cat a few weeks ago.

 

How about the rear O2s?? If they haven't been accounted for (ie removed/simulated/PCM mod) then they are probably throwing the fuel tables for a loop.

My two cents....

-Ivan

 

Check for crossed or burned spark plug wires.

 

it's not the wires or plugs. I have been reading alot about the downstream o2 sensors and I don't think that they really have too much to do with fuel

 

The rear o2s are there to be sure the cat is working properly if the cat is gutted or is not working properly the pcm will throw a code but it will not effect the way the car runs

 

Does the scanner show both bank trim numbers out or one bank. Check the injectors with a noid light for signal and listen for them firering with a t.scope. Good Luck with it.

 

Failing O2s will throw the fueling off, but not until the car goes closed loop. If it starts fine, then starts acting up a few minutes later, look at the O2 mV signals, or cross counts. Should be switching high to low to high pretty fast, several times a second.

 

Since you have access to the Tech 2, use the GM enhanced parameters capability to read the misfire count by individual cylinder. That might point you to a weak cylinder, or a pair of cylinders that are cross-firing.

Also, if you could be more specific about what is "all out of wack" with the fuel trims, like actual numbers for the various fuel trim cells, both banks, it would provide info that people might be able to use to help you track down the cause of the problem.

Its not due to the after-cat O2 sensors.

 

The fuel trims were -3 and -4% but from what I remember there were I think 4 different readings, I can't remember if it was long and short term or something. Like a dumbass I didn't write anything down lol. But I did look at the misfire chart and it was just cyl. 1 and 4 that were missing. Everything else looked great on the tech 2, but I didn't really pay much attention to the upstream o2 sensors just thinking it were the rears. Checking the injectors, fuel pressure test, and a vacuum test were the very first things I did. I can't get the tech 2 today but I'm going to see if I can get a different scan tool to use. Now being a little more educatued on what the rear o2 sensors do I'm going to look at the upstream's. That's more what I'm thinking it is since the car only acts up in closed loop.

 

yup in the upstream o2s check for a voltage signal that fluctuates between .2 to .9 volts or so this will tell you that the computer is switching from lean to rich to try to maintain a stoichiometric reading of 14.7 to 1 if the signal fluctuates the o2s are good if not it could be o2s or something else good luck

 

I got on a scanner but it just reads codes and some stored data. My fuel trims were bank 1 Short term -7% and long term 0%. Bank 2 short term -6.2% and long term -2.3% . I still don't know anything about the o2 sensors but there's no code for the upstream one's.

 

Short terms vary all over the place, and they are supposed to. 0% and -2.3% long terms are actually pretty good. A single frame of data doesn't tell you much. The fact that the O2 voltage is "switching" does not prove you have good readings from the sensors.

You need to get the long terms for a variety of operating conditions, representing the various "cells" defined by RPM on one axis and engine load (MAP) on the other. But so far, yours don't appear to be a problem.

For help with LT1 scan data, read this:

http://www.injuneer.com/ScanMast.html

 

now I have an opti low res code so I guess I'll jsut pull that damn thing off and get another since it's still under warranty

 

Clear the code (pull "PCM BAT" fuse for 30 seconds, key off) and see how fast it comes back. It isn't unusual to find P1371 as a stored code.

 

Ok, I'll give that a try but after looking at everything I can't really think of anything else that it could be. The engine dies and won't restart sometimes, has a slightly rough idle, sputters under a load, it goes crazt if I get it to 4500 rpm and it won't go any higher than that and runs like crap when it's warm. At lewast to my knowledge most of those point to my opti and since all I'm going to have to pay for is water pump gaskets I'm just going to start with the opti since it's my cheapest option