I have searched the forums and haven't found the exact issues I am chasing down right now.

I have an '02 LS1 and 6-speed from a Z28 installed into my '95 Chevy Caprice. I did all of the work myself...including the wiring harness. Here is what is in my car now: TR 224/112 cam, Pacesetter Long Tube headers, 30# SVO injectors, and 55-60psi at the rail. I had Bryan Herter of PCMforless write the PCM code for me (he has done a great job on numerous LT1 PCMs for our club, and is a great guy to boot). I currently have 2 PCms for this set-up, one that is a 'nomal' set-up, and one PCM that uses the same code as the other...but is forced to stay Open Loop.

The car runs good for the first minute or so, then barely runs and is horribly rich when it goes into closed loop. I was originally getting a code for a bad RH O2 sensor, so I replaced both of them (new AC Delco sensors for the '02 Z28 Ls1 ). The running issues continued with the same results. I unplugged the O2 sensors from the wiring harness, and with the engine being held in Open Loop...it ran much better. I ohmed-out the wiring harness, and it checked out fine. I even went as far as building a 'slave set' of O2 harnesses to plug into the PCM connectors and the O2 sensors, but have had the same poor Closed Loop running condition.

I had Bryan Herter modify my PCM code and load a spare PCM that would keep the engine in Open Loop. With this PCM, I can drive around with no SES lamp, and the car performs reasonable well, but is actually running rich somewhat.

Yesterday I pulled all 8 spark plugs (about 5 minutes in my car) and they all looked to burning properly...each plug looking nearly identical to any of the others. After reinstalling the spark plugs, I checked the fuel pressure at the rail, and found about 55psi at idle. I unplugged the vacuum line (using '98 Vette rails with the same on-rail regulator...my Caprice had the 2-line fuel system already) and the pressure went up to 60 psi and stayed there. So, I don't consider fuel pressure an issue, likewise with each plug looking fine, I don't think I have a bad injector at all.

What my big issue is...using a Snap-On scan tool (MT2500) I borrow from a friend, the O2 sensors stay at .030 or so (even while under load) and don't dither at all. I was used to seeing LT1 O2 sensors (on my old engine) going high - low, low- high very quickly. Scanning my car now... they just sit low...until I rev the engine, then they go high (.800 or so) when I let off the gas again.

I don't understand what could be causing the O2 sensors to respond so strangely.

I have check-tightened the header screws at the heads, and the collectors are modified to bolt directly to my cat-back exhaust (minus the cats) so they use the factory 2-bolt 'ball-n-socket' sealing method, of which there are no leaks. I replaced the new O2 sensors last night, with some used ones (my LS1 donor car had only 13k miles on it) and the O2 sensors still stay low.

I know the car can run much better than it is right now (forced to stay Open Loop), and I can probably get better mileage. I would also like to get the slight smell of running rich eliminated. BTW, my car weighs nearly 4300 lbs, and ran a best of 13.5 at 106 with 3.73 gears spinning the tires badly, and also getting nearly 22mpg on the trip to the track and back.

Any help you guys might be able to provide is greatly appreciated. Thank You,

Gary
Albany, NY

 

Have you checked all the wiring to make positively sure nothing is shorting out. Only time i see my o2's behave like that is when I run the open headers with them connected. o2 sensors think there is a big *** leak

 

HumpinSS,

Thanks for the reply. I have checked all of the wiring, and even built two 'slave' O2 sensor cables to bypass the engine harness completely. This way the O2 sensors plug directly into the POCM pins, and the O2 heaters get their power and ground from good sources as well. No change if the way they behave.

Gary

 

Did you use a voltmeter to make sure they're getting a 5 volt reference voltage?

 

If you have heater power on an O2 sensor off the
car, and waft a propane torch onto the tip you
should see the voltage indicate; unlit torch or a
yellow flame should make it swing all the way past
0.9V. Blue flame should be about stoich and give you
half a volt or close.

You can test sensors this way, there were some
good writeups on it way back in the '80s I remember
when people were into hacking homebrew EFI.

Probably the thing to do is, put the sensor back in the
pipe but break the harness signal and return; measure
the O2 output voltage open-circuit with a high-Z DMM
and see if it is alive (should be), then reconnect to the
rest of the harness and see if the voltage gets snuffed
(which would indicate a short / misconnection or
at least an improper load).

 

Thanks for the replies guys. I borrowed an ocsilloscope from a friend to figure out what signals I have with regards to the O2 sensors. I won't be able to set up the scope until sometime Monday, as I am off to the Drag Races tomorrow morning to check out the 4.56 gear upgrade I did Wednesday night. I will keep this post...posted on the progress.

I know that there is a High and a Low set of pins for the O2 sensors. With purple wires being one and tan wires being the other. Should I be able to see a reference voltage from one of those wires to chassis ground, and what would the expexted voltage be? I have an '02 Camaor Factory Service Manual, but I don't see any voltages mentioned to check the output of the PCM to the O2 sensors.

Gary

 

I was wrong before, the reference voltage is 450 mv. I'm looking at a vette wiring diagram and the heater circuit is brown for power, black for ground. The reference voltage is either gonna be on the purple or tan wire, but I can't tell which one from the wiring diagram so just try both i guess.

 

The O2s on these cars should have O2 low side
(tan/wht B1S1, B1S2; tan B2S1, B2S2) distinct
from chassis ground. Tying the PCM reference to
chassis might inject noise or offsets. Especially
if block <> sheet metal ground, which is another
good thing to check - ground braids and more
than one.

B1S1/B1S2 purple/wht, B2S1/B2S2 purple is the
O2 voltage high side. The quantity the PCM
is using is (high-low).