lt1s10blazer

i have a 94 lt1 in an 87 s10. and its running rough and misfiring especially underload. when its revved high underload it falls on its face and backfires. i'm not getting any codes. it has a brand new walbro 255lph inline fuel pump, brand new fuel filter. new plug wires, new coil, new icm
I'm pretty sure it does it in open loop and closed loop.
at what temp does it change?

if u unplug one O2 sensor does it go into "limp mode" or whatever where it stays in closed? or do u hafta unplug both?


It starts good, and runs good cold. but acts up before it goes in to closed loop(pretty sure). i do have some datalogging equippment,(cable and software) but besides codes, don't know what to look at. thanx in advance

i suspect the opti, but i'm not to eager to purchase one, obviously they aren't too cheap. is there a way to test it? i saw a test on shoebox's page. but it seemed you would only need to do that test if the car is not starting at all. am i wrong? anything else i should test?

ss.slp.ls1

Missfiring and backfiring, are strong indicators of the opti. Also check your opti harness. You can test the opti per Shoebox's website regardless of starting/no starting. I would suspect your low res signals are not working properly.

Injuneer

The transition from open loop to closed loop requires three things: 1) both O2 sensors heated up to the point where they start working (~600*); 2) Coolant temp above 120-140*F (variable); 3) time since start greater than 205 seconds.

If an O2 sensor fails (or is unplugged), it simply reverts to open loop and sets a code/SES light. It doesn't go into "limp mode". Very few people actually experience limp mode, which involves the PCM attempting to run the engine with virtually no input at all from the sensors.


To understand what your data logger is showing you, read this:

http://members.aol.com/InjuneerZZ/ScanMast.htm

lt1s10blazer

thanx for the respsonses guys, looks like i will be doing that opti test on shoebox's page. and i'll go ahead and disconnect an 02 sensor and take it for a spin, j/ to rule that out for sure. i dont guess i really understood the term "limp mode" thanx for clarifying that. and i will definately take a look at the datalogging page when i get home. 'preciate it...

lt1s10blazer

ok i j/ did that opti test off of shoebox's page and the results are... 5.0 volts on terminal A the (low res signal) and on terminal D (the distributor reference low signal) it showed only .01
A= 5.0v dc
D= .01v dc
does that scream i need a new opti? or does that .01 mean its in the wires somewhere? perhaps a new opti harness? *shrugs*

ss.slp.ls1

"If you get no voltage, use the diagram and chase back toward the coil and the ignition fuse. Power for the ICM comes from the ignition fuse and through the coil, so any of that could be bad."

It says no voltage, but I would still check the coil and the ignition fuse. Also check the grounds on the coil, there should be three.

lt1s10blazer

I j/ looked at the information on testing the opti again, and i did it wrong. i was reading some post where a guy seperated the opti harness on the passenger side and and tested the pcm side of the wires. so thats what i did... but now that i actually thought bout it.... i dunno what the helll i was thinking, i'll go and check the voltage at the icm harness when i get home.

lt1s10blazer

Ok, i j/ checked the opti correctly......i got 11.4v dc on the 2 terminals and from there... did the ac voltage check while cranking and the voltage seemed to move around between 1 and 4 volts, i was using a digital gauge, which may not have been ideal.... so what does that mean? it seems to me that it could still be bad. but if it's not i'm not in a hurry to replace it

ss.slp.ls1

I'd say that the test helps confirm an opti is bad rather than proving that an opti is good. If its the factory opti, its probably due for a change anyway.

lt1s10blazer

looking at what that test checks i would hafta agree. wish there was a better test. oh well.... Looks like its time for an opti...

lt1s10blazer

OK, so i've got the opti off. and i've been doing a lil research.... i haven't gotten any dtc's for hi- res, lo- res, etc. and when i did the test on shbox's page regarding the signal getting to the coil. it passed. Looking at the cap and rotor there is corosion on the terminals... how do i know wether or not i need to replace the whole opti. it looks to me that i would be okay w/ a cap and rotor... what's the best deal on a cap and rotor? any comments appreciated

ss.slp.ls1

You could open up the bottom half of the opti and inspect the optical wheel and sensor. But if its a 94 original opti, it would be good preventative maintanence to replace the whole unit.

lt1s10blazer

i'm still debating on what i wanna do... its not a daily driver, so if i hafta tear it down later thats ok, but at the same time its not all that much fun. what do i look for w/ the optical wheel and sensor?

Injuneer

The main "external" problem with the optical sensor is rust particles building up in the housing for the emitter/receptor that shines the light through the slots in the disc. Or the disc may have buildup on it, blocking the slots. That won't check the electronics though. If it was a sensor problem, you should have been getting codes for it. That's the only codes the Opti sets.

Since you appear to have the Opti off the front of the engine, with the Opti hooked up to the harness, and the key on, put a volt meter across the ground (pink/black) and one of the signal wires (purple/white for the high res; red/black for the low res). As you slowly rotate the optical wheel, the voltage should switch back and forth between 0V and 5V. Check both signal wires. The high res will be hard to turn slow enough, since there are 360 tiny slots in the wheel.