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  #1  
Old 02-03-2010
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The great O2 sensor debate

Since I put the 5.0 in my truck I've been running without ANY 02 sensors.

I started thinking about it the other day, and figured it puts the truck in "limp mode" since it fails to see the circuit completed.

So I grabbed my 02 sensors, plugged them in, and zip-tied them to the frame.

Was this a good move?
Remember, I have a custom true-dual exhaust, no crossover, no H or Y pipe, and no catalytic converters. Just manifolds, pipes, and Flowmaster 40's.
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Old 02-03-2010
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what computer are you using to run the 5.0? in my mustang i ran without them (un knowingly) and it ran fine lol. but this is an 88 with no post cat o2 sensors or maf.
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Old 02-03-2010
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Running with them hanging will cause the computer to think the engine is running lean, so it would richen the fuel mixture and it could foul the plugs or wash out the rings. I'm not sure what it would do with them unplugged.
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Old 02-03-2010
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With them unpluged the ecu would return to base timing. With the o2's tied to the frame would do the same thing since the o2's need to hit a certan temp to work.
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Old 02-03-2010
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Originally Posted by 87xlt View Post
With them unpluged the ecu would return to base timing. With the o2's tied to the frame would do the same thing since the o2's need to hit a certan temp to work.
Which is why they are 4-wire O2 sensors with a separate heating circuit altogether.

With them unplugged, the ecu goes into "limp mode", correct?

I defiantly don't want to be running too rich. I'm gonna go pull the codes now and see what the computer tells me. That AutoXRay was the best investment ever!
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Old 02-03-2010
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Originally Posted by Needforspeed3685 View Post
Which is why they are 4-wire O2 sensors with a separate heating circuit altogether.

With them unplugged, the ecu goes into "limp mode", correct?

I defiantly don't want to be running too rich. I'm gonna go pull the codes now and see what the computer tells me. That AutoXRay was the best investment ever!
Yes it stays in "limp mode" just like when you first start your truck but it never sees whats comming out of your engine so it reverts to make it run mode. Just weld in a bung for the o2's and call it a day. What year is your engines ecu pre or post 96?
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Old 02-03-2010
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What is the difference between limp mode and open loop?
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Old 02-03-2010
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Same thing just different wording.
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Old 02-03-2010
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there is open loop and closed loop. open loop is what they used on verry early FI systems, no O2 sensors. it adjusted the amount of fuel by engine temp, rpm, tps, iat, and iap. the more advanced closed loop is what used on all FI systems today along with all the other sensors it also has O2 sensors that tell the computer how the motor is running rich/leen to correct it self.

limp mode is just a basic fuel map that will allow the engine to run but it will have a lower max rpm and low performance.
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Old 02-03-2010
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Originally Posted by EricRanger View Post
there is open loop and closed loop. open loop is what they used on verry early FI systems, no O2 sensors. it adjusted the amount of fuel by engine temp, rpm, tps, iat, and iap. the more advanced closed loop is what used on all FI systems today along with all the other sensors it also has O2 sensors that tell the computer how the motor is running rich/leen to correct it self.

limp mode is just a basic fuel map that will allow the engine to run but it will have a lower max rpm and low performance.
Eh open loop is only for when you first start up your truck that allowes your o2's and other sensors to get to proper operating temps. Then your ecu turns to closed loop thuss relying on your o2's ect for engine timing ect. Sure your not thinking of old school mech injection? Also (correct me if im wrong) there is no such thing as "limp mode" only open/closed loop.
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Old 02-03-2010
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Open loop:
The air/fuel ratio is taken from a PCM table that is based on load and RPM and corrected for other factors like barometric pressure, coolant temp, intake temp, etc. Open loop is commonly used at WOT or near WOT, during some decelerations and before the oxygen sensors come up to operating temperature. It is also used during the execution of certain OBDII monitors.

Closed loop:
The air/fuel ratio is controlled around stoichiometry by the PCM based on a feedback loop using input from the upstream oxygen sensor(s). Closed loop is typically employed during idle and part throttle operation after the oxygen sensors have warmed enough to make them effective. Closed loop operation can produce very high efficiency in a 3-way catalytic converter.

Limp mode:
This is the PCM's "last resort" attempt to keep the engine running and the vehicle moving. Most sensors are ignored and the PCM uses very crude spark and fuel delivery. Electronic automatic transmissions use only 2nd or 3rd of the forward gears and torque converter clutch operation is disabled. In limp mode, performance, driveability, fuel economy and emission control are severely compromised. Basically, the air/fuel portion of limp mode is open loop taken to its most primitive level.
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Old 02-03-2010
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Quote:
Originally Posted by Needforspeed3685 View Post
Since I put the 5.0 in my truck I've been running without ANY 02 sensors.

I started thinking about it the other day, and figured it puts the truck in "limp mode" since it fails to see the circuit completed.

So I grabbed my 02 sensors, plugged them in, and zip-tied them to the frame.

Was this a good move?
Remember, I have a custom true-dual exhaust, no crossover, no H or Y pipe, and no catalytic converters. Just manifolds, pipes, and Flowmaster 40's.
Without any oxygen sensor input, the PCM will go to the open fuel tables, light the CEL and store trouble codes in the PCM.

I'm sure that zip tying the sensors out in the air will not improve anything and it should actually cause the engine to run richer than ideal - at least for a while until the PCM hits the clip and reverts to open loop.

The best fuel control strategy should be to weld O2 bungs into the down pipes near the exhaust manifolds, install the upstream sensors and let the PCM use closed loop when appropriate.
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Old 02-03-2010
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Originally Posted by rwenzing View Post
Without any oxygen sensor input, the PCM will go to the open fuel tables, light the CEL and store trouble codes in the PCM.

I'm sure that zip tying the sensors out in the air will not improve anything and it should actually cause the engine to run richer than ideal - at least for a while until the PCM hits the clip and reverts to open loop.

The best fuel control strategy should be to weld O2 bungs into the down pipes near the exhaust manifolds, install the upstream sensors and let the PCM use closed loop when appropriate.
First of all, I'm so glad to see you've chimed in on this thread, Bob. It was because of you and your information that I was able to complete the V8 swap as well as many other electrical modifications. I appreciate all your knowledge and think you are a genius!

Now, do I understand correctly you're thinking I should install only 2 of the 4 total O2 sensors, those being the ones that usually are BEFORE the catalytic converter, and leave the two After-Cat sensors unplugged?

I'm starting to think investing in a decent catalyst system and installing O2 sensors would be the best route for fuel economy and performance.
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Old 02-03-2010
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Originally Posted by 87xlt View Post
Eh open loop is only for when you first start up your truck that allowes your o2's and other sensors to get to proper operating temps. Then your ecu turns to closed loop thuss relying on your o2's ect for engine timing ect. Sure your not thinking of old school mech injection? Also (correct me if im wrong) there is no such thing as "limp mode" only open/closed loop.
Hey i just gave some input. Im not a car mechanic. I am a certified powersport technician. Most EFI motorcycles run an open loop system. It wasnt untill California passed their emission laws then O2's were put on motorcycles. And there is a "limp mode" at leas there is in the powersport world.
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Old 02-04-2010
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Quote:
Originally Posted by Needforspeed3685 View Post
Now, do I understand correctly you're thinking I should install only 2 of the 4 total O2 sensors, those being the ones that usually are BEFORE the catalytic converter, and leave the two After-Cat sensors unplugged?

I'm starting to think investing in a decent catalyst system and installing O2 sensors would be the best route for fuel economy and performance.
You can go either way (cats or no cats) and I doubt that it would affect fuel economy enough to be noticeable. The fuel control is based almost entirely on input from the sensors upstream of the cats.

I prefer to run with cats and O2's because they make such a huge difference in emissions without much impact on the performance of a mild street engine. The trick is to place the cats close enough to the engine to keep them hot enough to work at idle but far enough away that they are not damaged by heat at part throttle and WOT. I took the easy way out and used the stock front cats in the stock location and the rears in a near stock location.

Obviously, a Ranger V8 is not about fuel economy. I haven't bothered to measure FE in town but it's not great, partly because it's almost impossible to keep your foot out of it. At steady speeds on the highway, my 5.0 economy does not seem to be much lower than than the 4.0 was. I think that it's helped by the lower revs of the automatic's overdrive ratio.
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  #16  
Old 02-04-2010
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Originally Posted by rwenzing View Post
Limp mode:
This is the PCM's "last resort" attempt to keep the engine running and the vehicle moving. Most sensors are ignored and the PCM uses very crude spark and fuel delivery. Electronic automatic transmissions use only 2nd or 3rd of the forward gears and torque converter clutch operation is disabled. In limp mode, performance, driveability, fuel economy and emission control are severely compromised. Basically, the air/fuel portion of limp mode is open loop taken to its most primitive level.
Ahh so there is such a thing as limp mode ok learn something new everyday :).
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