I keep getting a o2 sensor code and a check engine light, and my mpg has dropped. I was told that stock o2 sensors might give me trouble with headers because the heatup and cool down so fast. I'm running a 1996 LT1 in my 69 and headman mid lenght headers. O2 sensors are mounted on the collectors. Also was told you could buy dummy o2 sensors that read correctly all the time that would eliminate this problem. Is this true and is it a good idea to use them and where to you get them? Mine are 4 wire sensors.:confused:

I keep getting a o2 sensor code and a check engine light, and my mpg has dropped. I was told that stock o2 sensors might give me trouble with headers because the heatup and cool down so fast. I'm running a 1996 LT1 in my 69 and headman mid lenght headers. O2 sensors are mounted on the collectors. Also was told you could buy dummy o2 sensors that read correctly all the time that would eliminate this problem. Is this true and is it a good idea to use them and where to you get them? Mine are 4 wire sensors.:confused:

if they are 4 wire sensors they're heated, and should work fine with headers. I would not use sims because then there is no real feedback to the computer to adjust the fuel mapping. Sims are generally used on systems with (4) O2 sensors; 2 in front of the cat and 2 after. In that case the sims work fine on the rear ones as you still have the fronts to read the a/f, and the rears will just agree it's fine. You might try replacing the O2's, they do go bad. What are the codes you're getting?

Jody

bought software to read what computor is doing, it shows the volts on the o2 sensors are moving up and down a lot. any where from 100-900, and when the volts move the fuel trim moves from -8.5 to + 9.5, while at idle. when drops to lower end car runs really rough. Should o2 volts move this much. my code sheet shows 352-552 if over more than 5 sec triggers code. 134,153. Also hotter engine gets more it moves. rougher it runs. electric fan comes on at 195, goes off at 185. when fan is on doesn't move as much. Any ideas on what it should do.

bought software to read what computor is doing, it shows the volts on the o2 sensors are moving up and down a lot. any where from 100-900, and when the volts move the fuel trim moves from -8.5 to + 9.5, while at idle. when drops to lower end car runs really rough. Should o2 volts move this much. my code sheet shows 352-552 if over more than 5 sec triggers code. 134,153. Also hotter engine gets more it moves. rougher it runs. electric fan comes on at 195, goes off at 185. when fan is on doesn't move as much. Any ideas on what it should do.

O2's will cycle up and down at idle, that's normal. Have you tried replacing the O2's?

Jody

O2's will cycle up and down at idle, that's normal.
?it is the shape of this that is critical
0s sensors do not last for ever, if the wave is not right or the range not correct replace it with a model compatable with the orginal...like spark plugs manufactures supply brand lists of compatable sensors.
Also anylisers are not to taken as 'read'...often a code can come up identifing a fault, but it takes hands on experianced mechanics , to read between the the lines, wave forms etc to actually ID the real problem.

bought software to read what computor is doing, it shows the volts on the o2 sensors are moving up and down a lot. any where from 100-900, and when the volts move the fuel trim moves from -8.5 to + 9.5, while at idle. when drops to lower end car runs really rough. Should o2 volts move this much. my code sheet shows 352-552 if over more than 5 sec triggers code. 134,153. Also hotter engine gets more it moves. rougher it runs. electric fan comes on at 195, goes off at 185. when fan is on doesn't move as much. Any ideas on what it should do.

How old are the O2's? How long have you had the headers on? Did the problem start when you installed headers?

Are you getting the codes you mentioned?

134 & 153?

The trims drive the O2 voltage reading up and down with approx 0.45V as the mid point which is where minimum emissions are - the stoichiometric
point - 14.7:1, as they go from -ve to +ve and back again.

This is closed loop mixture control. The O2's are essential for keeping this right.

134 is for insufficient activity which is lack of "switching" up and down on bank 1. This can occur if the O2's are clogged or at the end of their life.

152 is Heater performance on the same sensor. Or lack of. Bank 1 also.

It sounds like yours are switching ok, but headers can introduce a delay to the travel time of the exhaust pulse which the PCM sees as a problem, plus the heat up ("light off") time can be slower.

What do you mean by this:

"when drops to lower end car runs really rough"

Do you mean that the O2 voltage drops down toward zero and stays there? That could cause rough running as the mixture goes too lean.

Yes when the fuel trim numbers go to the neg. side the car runs rough. The sensors and headers were put in new 1900 miles ago when i built the car this winter. Seemed to run better then, then now. Seems to run better cold then when engine gets hot. When eletric fan kicks on the drivers side smooths out at about 450 (o2 volts) when kicks off and starts to warm back up around 195-200 starts to run rough. Fan make some difference on pass side but not as much as drivers side. what would cause this to fluxuate this much. First tank of gas ran though the motor mpg was around 30, (273 gears) now mpg around 23-25.

Yes when the fuel trim numbers go to the neg. side the car runs rough. The sensors and headers were put in new 1900 miles ago when i built the car this winter. Seemed to run better then, then now. Seems to run better cold then when engine gets hot. When eletric fan kicks on the drivers side smooths out at about 450 (o2 volts) when kicks off and starts to warm back up around 195-200 starts to run rough. Fan make some difference on pass side but not as much as drivers side. what would cause this to fluxuate this much. First tank of gas ran though the motor mpg was around 30, (273 gears) now mpg around 23-25.

When the rough running starts, is the O2 sensor voltage still fluctuating/cycling at all? Or does it just go down towards zero and sit there?

Has the PCM been tuned by a tuner?

And...what DTC number is it posting?

When it comes back and sits at 450mv, is it still cycling then, or just sitting there?

Alright. Stock pcm targets 453 mV as stoich. It will switch going lean to rich at stoich thus a narrow band. At idle the PCM will vary fueling signifcantly this means nothing do not look at it, its doing this to change fuel instantenously to improve catalyst efficiency.

The car will run rough when voltages drop low because you don't have enough fuel to support complete combustion.

If you sensor is climbing to 453 mv or right around there and holds constant for a while you have a dead short in the circuit. 453 is the default reading.

If you are running a 4 wire sensor it will be heated and should be reading correctly it will just take it longer to light off. The PCM doesn't give two S***s about where its located in the exhaust as long as there is suffcient heat for accurate readings. Not sure what pulses have to do about o2 reading. Pulses are pressure pulses which results in the exhaust sounds and not fuel and constiuent propigation throughout the exhaust system. Plus the time for the exhaust to reach it is insignificant hundredeths if not thousandths of a second different because of the location. Its most likely faster than the refresh rate of you 96 pcm.

The two readings (bank to bank) should be within 10% of eachother long terms between banks shouldn't vary by more than 2 points. IF it does you have signifcant engine problems. Injector clogging, leaking head gasket or intake manifold or exhaust. Lower compression from running lean could also cause issues due to ring or piston damage. Each side should be consistent.

IF you have bosch o2 sensors this could be the problem GM cars and PCMs hate bosch POS's although they make good widebands. Also if you car is running rich primarily you could have hurt the 02's. They are very easy to test though.

Remove them and get a propane torch heat them up clean some carbon off then take a DMM and check the voltage. Use the second flame from the torch and keep it on the 02 sensor right in the second blue tip and the DMM should read in the 400's which will indicate good 02 sensors. Turn your car to the run position and test voltage at the heater circuit power source and then use the DMM to check for continuity across the ground for the heater. You will not see it fluctuate and I am not sure how you can look at the wave sign of the sensor and indicate its failure.... THis is new to me maybe I am not the experienced mechanic. If the sensor is bad it won't switch period regardless. It drops efficiency and usually will indicate lean mixture and the PCM will dump fuel resutling in a signifcant economy and power loss as the long terms tend to climb on the bank with the bad o2. Not only is there no wave function for a narrow band senor the rate at which it switches is so quick that most logging software won't even pick up the switches you just see the mV jump all over the place. IF you have software such as inca you can view the "switching" but none of you have this software so you cannot analyze the curves. Plus the sensor "switches" which if you looked at any switch is simply on or off you can't evaluate the performance of an on or off switch or its wave characteristics. Way over analyzing.

THE PCM cannot correctly diagnose heater control on these vehicles. It has no idea I don't know how some one says it does. THe power comes from the fuse block which powers the other engine controls. 04 and up has heater control diagnostics beacuse the heaters are run directly off the PCM and the PCM sends a voltage from one of its pins and then can determine the heat by the resistance value of the sensor. There is no way for your 96 to do this so it will not know if it is getting hot enough.

IF you take the o2 sensors out of you exhaust and turn the key to run and hold them they should heat up pretty quick and if you can't touch them without burning yourself they are fine. Another way to check i wouldn't do it that way except to indicate they are heating correctly.


Did you do any wiring on this car or did anyone else?? Your fan is screwing with the ground circuit on your 02 sensor. IF you had a wideband i garauntee your AFR would be similar with fan on and off. Somehow this circuit is crossed into your o2 wiring. This is my thoughts.

My apologies on my misstep here, as we work on later model PCM/ECM's I had stupidly missed the fact that we were talking about a 96 PCM and there was no O2 heater control current/cat temp monitoring :).

FYI travel time from exhaust port to O2 sensor does make a difference with later/tighter emissions standards and better PCM's. If the O2 sensor feedback is too far behind the short term fuel trim +/- fueling changes the PCM will decide that the O2 is slow.

Good logging software shows this relationship well when graphed.

Short term trims cycle late model narrow bands at around 1 time per 2 - 3 secs at idle, and 3 times per sec at 1500-2000rpm cruise. (one GM example).

Of course this is just interesting tech stuff, doesnt really help the problem.

Another way to check a narrow band removed from the exhaust but connected to vehicle power so it is hot is that it should read low around 0 volts when in free air, and go high to 1 Volt when an unlighted butane lighter is used to flow raw gas into the sensor.

Great idea to use a gas torch to indicate the stoich mid point. :)

Great feedback about the 96 PCM's radcannon. Thx.



If you are running a 4 wire sensor it will be heated and should be reading correctly it will just take it longer to light off. The PCM doesn't give two S***s about where its located in the exhaust as long as there is suffcient heat for accurate readings. Not sure what pulses have to do about o2 reading. Pulses are pressure pulses which results in the exhaust sounds and not fuel and constiuent propigation throughout the exhaust system. Plus the time for the exhaust to reach it is insignificant hundredeths if not thousandths of a second different because of the location. Its most likely faster than the refresh rate of you 96 pcm.

Remove them and get a propane torch heat them up clean some carbon off then take a DMM and check the voltage. Use the second flame from the torch and keep it on the 02 sensor right in the second blue tip and the DMM should read in the 400's which will indicate good 02 sensors. Turn your car to the run position and test voltage at the heater circuit power source and then use the DMM to check for continuity across the ground for the heater. You will not see it fluctuate and I am not sure how you can look at the wave sign of the sensor and indicate its failure.... THis is new to me maybe I am not the experienced mechanic. If the sensor is bad it won't switch period regardless. It drops efficiency and usually will indicate lean mixture and the PCM will dump fuel resutling in a signifcant economy and power loss as the long terms tend to climb on the bank with the bad o2. Not only is there no wave function for a narrow band senor the rate at which it switches is so quick that most logging software won't even pick up the switches you just see the mV jump all over the place. IF you have software such as inca you can view the "switching" but none of you have this software so you cannot analyze the curves. Plus the sensor "switches" which if you looked at any switch is simply on or off you can't evaluate the performance of an on or off switch or its wave characteristics. Way over analyzing.

THE PCM cannot correctly diagnose heater control on these vehicles. It has no idea I don't know how some one says it does. THe power comes from the fuse block which powers the other engine controls. 04 and up has heater control diagnostics beacuse the heaters are run directly off the PCM and the PCM sends a voltage from one of its pins and then can determine the heat by the resistance value of the sensor. There is no way for your 96 to do this so it will not know if it is getting hot enough.

That makes sense on the new computers. I could easily see the 04 Aussie PCMs determining discrepancies because of the higher bandwith and processing capabilities. If it is far enough back I could see the possibility of errors especially under part load with less air flow and where closed loop really functions. The 96 is to slow though it shouldn't care.

Now that you mention it too. I do remember watching the GM's at idle switch alot slower. Must be because catalyst efficiency at idle is so high because of low flows. The cruising was definitely refreshing faster.

And out of curiosity what logging software do you have?? I was using Data Masters and it was going so slow at some points that I couldn't see the o2 switch it was grabbing a reading at every wave change. Not only that I cannot dictate what I am graphing. I haven't seen anything better though.


Sorry for thread hijack.

That makes sense on the new computers. I could easily see the 04 Aussie PCMs determining discrepancies because of the higher bandwith and processing capabilities. If it is far enough back I could see the possibility of errors especially under part load with less air flow and where closed loop really functions. The 96 is to slow though it shouldn't care.

Now that you mention it too. I do remember watching the GM's at idle switch alot slower. Must be because catalyst efficiency at idle is so high because of low flows. The cruising was definitely refreshing faster.

And out of curiosity what logging software do you have?? I was using Data Masters and it was going so slow at some points that I couldn't see the o2 switch it was grabbing a reading at every wave change. Not only that I cannot dictate what I am graphing. I haven't seen anything better though.


Sorry for thread hijack.

FYI image of an idle log showing O2 voltage and short term trim activity with the trim stepping up/down and driving the mixture and O2's providing the feedback of same to the PCM.

http://www.fquick.com/garages/viewgallery.php?action=viewimg&id=282992

The image insertion script is failing, so the image may not show, so it is here on the link. A screenshot.

http://www.fquick.com/garages/viewgallery.php?action=viewimg&id=282992

This is via EFILive. Arguably the best for logging/presenting (and tuning :) ) that we have seen. The screen has 24 data items shown. Can do more. It records frames at around 25 per second across 30 or 40 data points so is pretty quick.

You can download a demo copy at www.efilive.com (http://www.efilive.com).


:)

Both are in the midlength header collectors....If it's using a '96 ECM, didn't the switch to OBDII also move one of the sensors to behind the catalytic? I didn't see where John answered if he's using a program other than stock.

The simulators they make only replace the after cat sensor.

Both are in the midlength header collectors....If it's using a '96 ECM, didn't the switch to OBDII also move one of the sensors to behind the catalytic? I didn't see where John answered if he's using a program other than stock.

The simulators they make only replace the after cat sensor.

Yeah around OBDII time an extra O2 was added after the cats. EPA mandated I think due to needing to raise a fault when the cat or front o2 had died.

Sorry for the long delay in response. There is alot of information that i don't know if i completely understand, But i will continue to study it to try and figure it out. But i did understand that bosch o2 might be my problem, I replaced the sensor and ran good for about 200 miles before it came back. I will test the sensor and if bad i will try delco. The engine was completely rebuilt before installing, New delco 24 injectors. o2 sensor i took out had a lot of carbon on it.

Excellent write-up.

Thanks Everett#2390. Yes! Drive a 68! :)

Link for idle logging pic fixed:

Shows:

An idle log showing O2 voltage and short term trim activity with the trim stepping up/down and driving the mixture and O2's providing the feedback of same to the PCM.

FYI image of an idle log showing O2 voltage and short term trim activity with the trim stepping up/down and driving the mixture and O2's providing the feedback of same to the PCM.

if pic missing see:

http://www.fquick.com/garages/viewga...wimg&id=282992

This is via EFILive. Arguably the best for logging/presenting (and tuning ) that we have seen. The screen has 24 data items shown. Can do more. It records frames at around 25 per second across 30 or 40 data points so is pretty quick.

You can download a demo copy at www.efilive.com.

FYI travel time from exhaust port to O2 sensor does make a difference with later/tighter emissions standards and better PCM's. If the O2 sensor feedback is too far behind the short term fuel trim +/- fueling changes the PCM will decide that the O2 is slow.

Good logging software shows this relationship well when graphed.

Short term trims cycle late model narrow bands at around 1 time per 2 - 3 secs at idle, and 3 times per sec at 1500-2000rpm cruise. (one GM example).

http://www.fquick.com/images/vehicles/full/17927282992.jpg?1253001529