Closed loop question
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Race Director
Joined: Feb 2003
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From: Lake Wylie SC
Closed loop question
I have a LT1. If the O2 sensors fail, when the engine goes into closed loop could this shut down the engine? The valve seals are leaking a bit and I thought maybe the O2 sensors got clogged with oil and couldn't send the ECM correct signals. Any ideas?
Team Owner
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From: Hartford WI
It would make it run bad. If you think the Valve Stem Seals are bad, I wouldn't change the O2 sensors. I'd change the VSS and then change the O2. Either way, the O2 does get lazy and slow over time.
Last edited by aklim; Mar 22, 2017 at 03:07 PM.
Team Owner
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From: Hartford WI
If one is bad, I'd toss both
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Actually often when the O2 fails, it will not throw a code until much later. Best thing to recommend most people do is watch their fuel mileage, it will go south long before the SES light pops.
If one is bad replace all.
If one is bad replace all.
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From: Anaheim CA
If the O2 sensor is bad the ECM will never go into closed loop mode.
I don't know how the ECM determines that the O2 sensor is bad if you have multiple O2 sensors. I think it only looks at the ones in front of the cats.
I don't know how the ECM determines that the O2 sensor is bad if you have multiple O2 sensors. I think it only looks at the ones in front of the cats.
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Le Mans Master
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From: Orlando FL
There is a lot good info on the O2s. but you have made the jump for the original problem which I don’t know what it is, to O2s as a guess. O2s do get lazy (but still working) and cause all the problems as indicted already in the posts. But until they really die and stop swinging, that’s when poor mileage and codes are set. If there is no voltage swing, you will not get to closed loop but will still run and not shut down.
At this point I would put a Tech1 or OBD1 scanner on it and look at the O2 voltage swing and fuel trim numbers.
But where I am with this is, I'm not clear what your original problem is and why you have focused on the O2s
At this point I would put a Tech1 or OBD1 scanner on it and look at the O2 voltage swing and fuel trim numbers.
But where I am with this is, I'm not clear what your original problem is and why you have focused on the O2s
Team Owner
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From: Hartford WI
But until they really die and stop swinging, that’s when poor mileage and codes are set. If there is no voltage swing, you will not get to closed loop but will still run and not shut down.
At this point I would put a Tech1 or OBD1 scanner on it and look at the O2 voltage swing and fuel trim numbers.
But where I am with this is, I'm not clear what your original problem is and why you have focused on the O2s
At this point I would put a Tech1 or OBD1 scanner on it and look at the O2 voltage swing and fuel trim numbers.
But where I am with this is, I'm not clear what your original problem is and why you have focused on the O2s
That would be good. Any good scanner could do that.
Most people don't change these things until the are completely broke. We are ingrained with the 3000 mile oil change but things like brake fluid, power steering fluid, etc, are "out of sight, out of mind" and If the sensors are old, why not change them before they are completely toast?
Le Mans Master
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From: Orlando FL
But if it swings slowly and doesn't adjust the fuel trim properly, wouldn't it also affect the fuel adjustments?
That would be good. Any good scanner could do that.
Most people don't change these things until the are completely broke. We are ingrained with the 3000 mile oil change but things like brake fluid, power steering fluid, etc, are "out of sight, out of mind" and If the sensors are old, why not change them before they are completely toast?
That would be good. Any good scanner could do that.
Most people don't change these things until the are completely broke. We are ingrained with the 3000 mile oil change but things like brake fluid, power steering fluid, etc, are "out of sight, out of mind" and If the sensors are old, why not change them before they are completely toast?
I believe the window for working O2s is pretty large and as long as the ECM/PCM sees a acceptable voltage swing it's happy. How slow is to slow. If an O2 is not 100%, it would be hard to see performance or mileage changes until the normal mileage goes to 15 and that large change would be very noticeable. Probably the voltage would be steady with little change at that point.
Team Owner
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From: Hartford WI
I don’t know if you can look at a consumers scanner and visually determine if it is swinging to slow. Expensive equipment (O-scopes, fast response scanners) are required to see a time base measurement. Where do you draw the line when using a basic scanner.
I believe the window for working O2s is pretty large and as long as the ECM/PCM sees a acceptable voltage swing it's happy. How slow is to slow. If an O2 is not 100%, it would be hard to see performance or mileage changes until the normal mileage goes to 15 and that large change would be very noticeable. Probably the voltage would be steady with little change at that point.
I believe the window for working O2s is pretty large and as long as the ECM/PCM sees a acceptable voltage swing it's happy. How slow is to slow. If an O2 is not 100%, it would be hard to see performance or mileage changes until the normal mileage goes to 15 and that large change would be very noticeable. Probably the voltage would be steady with little change at that point.
Thread Starter
Race Director
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From: Lake Wylie SC
No I haven't gone the 383 route yet. My 350 is running too good to mess with it. I still have all the fixings to upgrade to the LT4 heads, intake, roller rocks and cam. It's all just sitting on my work bench 
Getting back to my problem with this other LT1 problem, I've replaced just about all the emission sensors except the PCM and O2 sensors along with a new fuel pump. I think the answer to the ability of the O2 sensor to trim the fuel system to a setting too lean to run is no. It should go to open loop and continue to run. I'd take it to the dealership, but that requires a tow as the shop is too far for the car to make the trip before it shuts off. Thanks for all the replies to my problem.
Getting back to my problem with this other LT1 problem, I've replaced just about all the emission sensors except the PCM and O2 sensors along with a new fuel pump. I think the answer to the ability of the O2 sensor to trim the fuel system to a setting too lean to run is no. It should go to open loop and continue to run. I'd take it to the dealership, but that requires a tow as the shop is too far for the car to make the trip before it shuts off. Thanks for all the replies to my problem.
Le Mans Master
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From: Orlando FL
Ok so the car dies. Back to square 1, when it dies have you check for spark and fuel pressure (if you can) at the time on the failure. I know it's a new pump so then check for spark off the coil. When you get a direction then you can move forward.
If there is no spark, the ICM is one on the main items for random spark failure. I would also look for any poor pin connections for the opti, coil, ICM and any other related ignition items. Might be hard to check the ICM for a problem like this (if there is no spark) and substitution is probably the best method for determination of a problem.
I would not focus on the opti until I knew a lot more about what was really happening, or not happening. There are other things to be considered. I would want to know if the injectors had drive. And it is also known the ECMs do cause these kinds of random problems.
If there is no spark, the ICM is one on the main items for random spark failure. I would also look for any poor pin connections for the opti, coil, ICM and any other related ignition items. Might be hard to check the ICM for a problem like this (if there is no spark) and substitution is probably the best method for determination of a problem.
I would not focus on the opti until I knew a lot more about what was really happening, or not happening. There are other things to be considered. I would want to know if the injectors had drive. And it is also known the ECMs do cause these kinds of random problems.
Last edited by pcolt94; Mar 24, 2017 at 02:41 PM.
Le Mans Master
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From: Las Vegas - Just stop perpetuating myths please.
As for O2 sensors most owners don't realize they are a normal wear item/part. Experts say replace O2's every time you replace the tires or brakes (but only for one of those not both
).Hope this helps.
