[Bowtieguy]

I have a 97, z28, ragtop with only 35k.
For the past few months I have been trying to get it stop missing and running rich after warm up. The car starts fine and runs fine for the 1st few minutes then it starts to miss and run rich. With negative fuel trim as much as 36. And high o2 sensor waveforms .7-8. After warm up the car backfires and has sooty tail pipes. The car is pretty much bone stock with Edelbrock Shorty Headers and an SLP Loud Mouth Exhaust.

These are the things I have done:
Fuel Pressure Test 45 lb
New injectors
New Coil
New Opti Spark
New Plug Wires
Relatively new fuel pressure regulator
Ran manifold vacuum test 21lb rock solid needle
Unplugged CPS Crank Position Senor - no affect apparently only sends codes for misfires to ECU
I have run a scan tool scan toll and all the sensor are operative
Carefully looked for vacuum leaks

The issue is it starts great and once warm runs rich, misfires and backfires!

Any suggestions ?

 

[cocobolo95]

If your o2 sensor is stuck in the .700-.600 volt range then the O2 sensor, it's wiring, ground, or connectors is probably bad.

Bank 1 O2 sensors should change voltage readings continually. O2 readings should go above & below the .450 volt mid line on a regular basis. O2's stuck at one range of voltage are not working correctly.

Your 97 has 2 bank 1 O2's that should change readings often, and an after cat bank 2 O2, which doesn't change readings very much.

The fact that car runs good in open loop mode, that is what PCM uses when engine is cold. Then it runs bad in closed loop operation , this is PCM mode when warmed up That is why I suspect O2 and/or it's related components.

When engine is cold, PCM fuels engine with mostly the Map, TPS, & CTS. Plus it uses tables stored in memory to run engine.

When engine warms up, PCM depends on sensors for fueling info. The bank 1 O2 sensors are among the most important sensors giving PCM info. The other sensors are important but O2's are the king of the hill.

 

[Bowtieguy]

If your o2 sensor is stuck in the .700-.600 volt range then the O2 sensor, it's wiring, ground, or connectors is probably bad.

Bank 1 O2 sensors should change voltage readings continually. O2 readings should go above & below the .450 volt mid line on a regular basis. O2's stuck at one range of voltage are not working correctly.

Your 97 has 2 bank 1 O2's that should change readings often, and an after cat bank 2 O2, which doesn't change readings very much.

The fact that car runs good in open loop mode, that is what PCM uses when engine is cold. Then it runs bad in closed loop operation , this is PCM mode when warmed up That is why I suspect O2 and/or it's related components.

When engine is cold, PCM fuels engine with mostly the Map, TPS, & CTS. Plus it uses tables stored in memory to run engine.

When engine warms up, PCM depends on sensors for fueling info. The bank 1 O2 sensors are among the most important sensors giving PCM info. The other sensors are important but O2's are the king of the hill.

Thanks you are the first person who has been helpful !!!!

I replaced B1S1 and still get no wave form. I pulled the sensor out and tested it with a voltmeter and torch and observed the expected changes in value with the application and removal of heat. It's obvious that this problem is related to bank one because bank 2s wave form is normal. Is being stuck at .8 some kind of a baseline or default? if there was a wiring problem wouldn't sensor show no voltage on the scan tool.

After warmup bank one shows -36 fuel trim with a flat line --------
@ .8 wave-less ) s1B1 the car usually throws PO 300 and backfires after about 2-3 minutes

 

[cocobolo95]

There are several things that will keep O2 sensor in the above .7 volt range. 1st is a heater that is not working. You proved that your O2 sensor works by heating it and checking voltage.

If O2 heater is not working, voltage can remain high. 2nd thing is bad wiring & connectors. An open in the signal wire will not allow O2 signal to reach PCM. So PCM won't be able to adjust fueling.

If you don't have a factory service manual, get the 96 manual at the following link. It will have code descriptions, troubleshooting steps, wiring diagrams, etc. And 96 manual will cover 99% of 97 model. www.mediafire.com/?40mfgeoe4ctti

 

[spfmarco]

I have a 97, z28, ragtop with only 35k.
For the past few months I have been trying to get it stop missing and running rich after warm up. The car starts fine and runs fine for the 1st few minutes then it starts to miss and run rich. With negative fuel trim as much as 36. And high o2 sensor waveforms .7-8. After warm up the car backfires and has sooty tail pipes. The car is pretty much bone stock with Edelbrock Shorty Headers and an SLP Loud Mouth Exhaust.

These are the things I have done:
Fuel Pressure Test 45 lb
New injectors
New Coil
New Opti Spark
New Plug Wires
Relatively new fuel pressure regulator
Ran manifold vacuum test 21lb rock solid needle
Unplugged CPS Crank Position Senor - no affect apparently only sends codes for misfires to ECU
I have run a scan tool scan toll and all the sensor are operative
Carefully looked for vacuum leaks

The issue is it starts great and once warm runs rich, misfires and backfires!

Any suggestions ?

Look for an exhaust leak. Twice in 27 years my two exhaust bolts on the driver side came loose which caused the engine to run rich. It was actual drawing air into the manifold when cylinder #7 valves were closed. My scan tool should a lean condition on bank #1.