Hello everyone, been chasing down a fuel problem I am having, I have a 1997 automatic that is running extremely rich, there are no cats on the car.

- DIC is throwing no codes that have to do with fuel, air, or exhaust
- I just replaced the 2 front 02 sensors and they did not seem to help.
- Car is stuck in Open Loop
- I am getting no feed back on short term fuel trims from front 02's
- All 02's seem to be in the .7XX to .8XX range
- Scan tool is showing rear 02's STFT at +99% ???

I do have an appointment with a tuner on the 21st of September however I'd like to get this repaired myself before that date.

Please see photos of the live date.
Any other info needed please let me know!
I'm really at a loss here on this one.

Open loop

0% on all FT data?

Maf, IAT, TPS

02 Data

02 Data

02 Data

 

Have you driven the car or are you just idling in your dreveway? Do you have headers. If the sensors don't heat up they won't start to switch and if they don't switch you will never go into closed loop. I would check the O2 heater circuit and you can also check the O2 sensor voltages with the car off and not running. They should be about 0.500 VDC. That will tell you if the ECU can read the O2 sensors.

 

Hmm so i checked the 02 sensors with the engine off and the key in the on position and the front 02 sensors read about 0.002 and slowly dissipated to 0 volts. When you say check the 02 heater circuit are you referring to ground G105 which the i believe all 02 circuits are grounded to.

edit: I do not have headers

 

ok another update, I drove the car and 02 sensor voltages are adjusting however the car still will not go into closed loop. Car got up to temp (over 200 degrees)

Is it possible the pcm is not reading the temperature of the 02 sensors?

 

Well the reading of 0 volts with the key in run position and engine not running is not good. The PCM provides a bias voltage of around .500 VDC and it should read that with the key on engine not running.You can measure the resistance looking in to pins A and B of the O2 sensor plug. Not sure what it should be but it should not be "open" I would check mini fuse 15 in the under hood fuse box. G105 is the ground for the heated O2 sensors. Do all 4 O2 voltages read 0 with the key on engine off? The O2 heaters only have a voltage on them when the engine is running and it switches on and off. Not sure what this voltage should be on AC or DC mode.

 

The front o2 sensors are not heated.

 

Not true. The front sensors are heated.

 

At a quick glance it looks like a MAP sensor issue !!...18 inHg at idle ??...my 01 reads about 10.3 inHg (my scan tool numbers below)..my Baro reading is 29.5 so if you subtract 29.5 (a little low because of a tropical storm moving through) - 10.3 it equals 19.2 which would be my intake vacuum...very close !!...18-22 are good numbers. If you are close to sea level your intake manifold vacuum would be around 10 inHg !!...is your car speed density tuned right now ???...if so the PCM thinks your car is under acceleration (low manifold vacuum) so your injector pulse width is increased hence your O2’s are seeing a rich signal...it is the effect but not the cause !!...don’t understand why your calculated low is only 3% ??...car is staying in OL either because of the rich O2’s or because the PCM thinks you are under load. Also with that scan tool go to into Generic OBD2 and see if you have any DTC’s stored there ??...if you don’t know it’s the engine symbol when you first turn on the scan tool...you don’t enter the VIN or the car year etc...to check heater circuit function with a scan tool the bias voltage is .450 volts on a cold O2 sensor and will decrease if you leave the key on...if you pull the heater circuit fuse you will read this bias voltage but if the fuse is in and with the key on that heater circuit will start to heat up...if you see .450 Mv with the fuse out the wiring is good on all the O2’s...after a few minutes it should read less than 200mV if the fuse is installed !!...the fuel trims are at 0 because you are in open loop BTW !!

 

This chart show the behavior of the O2 voltages of my O2's after engine start .
The voltages drops down like Rob says during open loop period and start cycling when the PCM enter closed loop.
My MAP is 12 inHg

Here is also the schematic for your O2's so you can check /measure the wirings .

 

as C5 Diag said, the map reading is high, so is tps, which should be showing the throttle plate closed (ECT says the engine is at full op temp) and the maf reads high too. Which is strange since rpm is idle, or a tad above. Since it is a no cost effort, maybe cleaning the TB might be a start. Something is apparently feeding air, but it appears to be metered, so maybe not a vacuum leak. A vacuum leak would be unmetered air so you should see lean DTC codes. So maybe either map or maf is lying to the pcm.

Something seems amiss with throttle position, so at least pulling the air horn and checking to see if it is open a bit, which it should not with the engine fully warmed up and no throttle input. A ring of black goo where the blade seats is a clue to a dirty TB

fuel trims are set by maf input to the pcm, corrected by feedback from the O2 sensors

I just replaced a maf on my c5 because it was reading high, and driving rich ltft’s. Then stft’s were compensating to drag the total trim to about Zero. My first experience with one reading high, but my clue was that as i revved the engine, the maf response was non-linear wrt rpm. New maf corrected that. Never had the first DTC, but the CAT readiness monitors would not complete. They did complete after a new maf and a drive cycle. Justin Miller on YouTube has a great vid about testing maf sensors.

 

I think his TPS is OK...mine reads about 7% and you KNOW my TB is sparkling clean...LOL !!...but yes the MAF is just a little high but if he is SD tuned that could account for the rich condition...further testing would have to be done to nail this down !!...”calculated load“ number is “proprietary” info by the manufacturer so we could only compare one car to another..there is no service info on what it should be !!

 

i wonder if the screen in the maf is gone. z06’s don’t have a screen, and maf readings are a bit high than the approximate displacement= g/s at idle.

my tb is clean too, so if i get a chance, i’ll check my idle at temp settings. I will be in closed loop, though

 

Thank you all for the helpful information as soon as I get home this afternoon I will be running some of the tests you all recommended and Will post my results. I believe the answer is right around the corner.

 

Great !!...the first thing is look at the O2 heater circuit...pull the fuse...hook up your scan tool and turn the key ON...all 4 O2’s should be reading 450mv or so...if they do the heater circuit is fine...that bias voltage is the only way the PCM knows there is a wiring issue...it’s not smart enough to know if the O2 is rich or lean !!!...a lot of circuits on the car have a bias voltages to detect wiring issues especially DTC’s that day “circuit high” or “circuit low” etc...

 

Ok update... With fuse 15 out of the fuse box and the key on engine off, I was reading all 4 02 sensors at the correct voltage of around 450mv. So I believe as stated that circuit is working correctly. A few other notes

Key on engine off MAP is reading 29inH ?? is this okay?? (started the car with the fuse 15 circuit out and my map was still at around 18inHG)

Key on engine off TPS at 16%

Someone asked earlier about MAF screen and my car does have the screen still.

This question may be a long shot but is there any way to tell if my car has a SD tune over the a normal OL/CL tune?

 

Yes, that is a normal baro/atmospheric pressure. 29.92 inHG is "standard".

If the car was SD tuned, P0103 would have be active, or some other MAF fault, in which case the PCM would be reverting to VE/speed density fueling only.

But SD and open loop are not synonymous. If the MAF was failed and it was running SD, it would still be running closed loop by default. Unless tuned to run open loop all the time as well.

A car could also be tuned to run MAF, open loop. I don't know why anyone would do such a thing...open loop only is really only useful for cars with a really wild aggressive cam setup or a racecar without O2 sensors etc.

18 inHG is still higher than expected for idle. My cammed motor is still 14-15 inHG at idle.

 

OK, as was mentioned 29.9 is standard at sea level. Here are my numbers KOEO....my key was on for about 90 seconds until I got my Verus hooked up...you can see my O2’s...another 30 seconds and all of my O2 heaters would have been below 200 Mv.... that a book number. I really don’t like diagnosing Corvettes with a tune...too many PCM parameters have been messed with...normally if a MAF issue is suspected like if it’s over or under reporting you disconnect the MAF connector and see if it runs better in SD...fuse 15 isn’t going to affect the MAP numbers !!...I’d like to see what your vacuum reads but that’s how it’s calculated without a vacuum gauge...BARO - MAP (inHg) equals intake manifold vacuum.

 

I was under the assumption that the PCM does not apply heater voltage to the O2 sensors when the car is not running i.e. KOEO. So I checked my 04 C5. As expected all four O2's were reading 0.442 volts. O2 heater current was 0 amps. So no heating occurred. Both stayed at .44 volts for a few minutes. The bias voltage reading goes away as the O2 sensor heats up and it's resistance goes down. So if the car has been running before these readings are taken the bias voltage could be anything. The bias voltage is most useful when the engine has not been run and the O2's are cold.

 

The heater circuit is powered as soon as the key is turned on and that bias voltage will decrease as you saw on my screenshot. I always test this heater circuit function after the O2 sensor has cooled off sufficiently...did you do this check on your O2 when they were cold ???...you can disconnect the sensor and check your power and grounds with the key on. The heater circuit is not PWM...the circuit is power side switched so the power comes from the fuse and the ground is a block ground...some heater circuits are PWM and the ground is a PCM ground that will turn on and off !!

 

I think 2004/P59 PCM is the only year where the heater circuit is indeed grounded by the PCM. I put a P59 in my 2003 and had to disable the O2 heater codes.