Driving with O2 sensor disconnected?
10-25-2017, 06:38 PM
#1
Drifting
Thread Starter
Driving with O2 sensor disconnected?
For my 1988 951, I've ordered a new O2 sensor -- see thread:
https://rennlist.com/forums/944-turb...l#post14559446
It will take a few days to arrive. In the meantime, anything wrong with driving around with a disconnect O2 sensor? Seems to run the same
https://rennlist.com/forums/944-turb...l#post14559446
It will take a few days to arrive. In the meantime, anything wrong with driving around with a disconnect O2 sensor? Seems to run the same
10-25-2017, 07:24 PM
#3
Drifting
Thread Starter
Ok.. I guess the next Q is how long can I run "rich" before damaging the catalytic converter? After all.. it's the original unit lol
I drive aprox. 60km a day for work. The new sensor might not arrive till Nov 6th
And I'm curious... what is the disconnected O2 sensor supposed to read at idle (on a well tuned engine with no vac leaks)?
I wonder if you can use a good O2 sensor to help fine tune the motor?
I doubt many of us have the electronic analyzers and such!
I drive aprox. 60km a day for work. The new sensor might not arrive till Nov 6th
And I'm curious... what is the disconnected O2 sensor supposed to read at idle (on a well tuned engine with no vac leaks)?
I wonder if you can use a good O2 sensor to help fine tune the motor?
I doubt many of us have the electronic analyzers and such!
10-25-2017, 10:54 PM
#4
Drifting
Thread Starter
Well.. seems this has been covered before, sorry. Idle reading should be .4-.5v ... which is where I'm at disconnected.
I guess I should have disconnected the O2 sensor before the test ha ha
Wish I didn't remove the CO testing tube from the crossover pipe though; would be good to have a shop check that now. Maybe I'll put it back
I guess I should have disconnected the O2 sensor before the test ha ha
Wish I didn't remove the CO testing tube from the crossover pipe though; would be good to have a shop check that now. Maybe I'll put it back
10-26-2017, 12:12 AM
#5
Three Wheelin'
This is not entirely true. The factory DME is programmed to ignore the O2 feedback (enter "open loop" mode) at above about 75% load as determined by the TPS signal, at which point the fuel map is set to be on the rich side to cool the combustion chambers to stave off detonation. So, no difference there whether the O2S is connected or not.
At idle and part throttle/light load conditions in which the O2 sensor feedback would normally be applied (closed loop), the base fuel table is set to be more or less stoichiometric anyway as that is where combustion is most efficient---no reason to run rich for "safety" during those conditions.
However, given the age of these cars and plethora of owners and their modifications, etc, there is a lot of opportunity for sub-optimal engine tuning during normal running. If your O2 sensor is known to be working well and providing a stoichiometric A/F ratio at idle and the idle quality drastically changes when the O2S is disconnected, then the tune isn't right (could be vacuum leaks, misadjusted idle speed or mixture, clogged idle bypass in AFM, or many other issues). There are a lot of variables here!
A factory-type "narrowband" O2 sensor is not a good tuning tool at all, because it is only accurate in the narrow range right around 14.7:1 AFR (stoichiometric for gasoline) and not anywhere else. Even if they were accurate for tuning, a digital storage oscilloscope (DSO) would be needed to monitor the O2S signal as monitoring the readings of a DMM (even that of a quality automotive DMM such as a Fluke) is useless as it will only provide an average voltage of the switching signal . . . . and yes, an average reading of 0.4-0.5V would seemingly indicate a more or less stoichiometric AFR, but does not show whether O2 sensor is "lazy" or biased in either direction as a DSO trace would.
Wideband sensors and controllers are readily available and quite affordable, and are a wise investment even for an otherwise stock 951, as it will provide valuable AFR data to help diagnose any running issues.
In closing, if your engine runs similarly in open loop mode in a quick drive around the block as it did with a working O2S, then you'll most likely be OK in the short term and not cause much damage to your cat. If it runs erratically or you smell a very rich mixture, you might have a deeper problem . . . .
At idle and part throttle/light load conditions in which the O2 sensor feedback would normally be applied (closed loop), the base fuel table is set to be more or less stoichiometric anyway as that is where combustion is most efficient---no reason to run rich for "safety" during those conditions.
However, given the age of these cars and plethora of owners and their modifications, etc, there is a lot of opportunity for sub-optimal engine tuning during normal running. If your O2 sensor is known to be working well and providing a stoichiometric A/F ratio at idle and the idle quality drastically changes when the O2S is disconnected, then the tune isn't right (could be vacuum leaks, misadjusted idle speed or mixture, clogged idle bypass in AFM, or many other issues). There are a lot of variables here!
Ok.. I guess the next Q is how long can I run "rich" before damaging the catalytic converter? After all.. it's the original unit lol
I drive aprox. 60km a day for work. The new sensor might not arrive till Nov 6th
And I'm curious... what is the disconnected O2 sensor supposed to read at idle (on a well tuned engine with no vac leaks)?
I wonder if you can use a good O2 sensor to help fine tune the motor?
I doubt many of us have the electronic analyzers and such!
I drive aprox. 60km a day for work. The new sensor might not arrive till Nov 6th
And I'm curious... what is the disconnected O2 sensor supposed to read at idle (on a well tuned engine with no vac leaks)?
I wonder if you can use a good O2 sensor to help fine tune the motor?
I doubt many of us have the electronic analyzers and such!
Wideband sensors and controllers are readily available and quite affordable, and are a wise investment even for an otherwise stock 951, as it will provide valuable AFR data to help diagnose any running issues.
In closing, if your engine runs similarly in open loop mode in a quick drive around the block as it did with a working O2S, then you'll most likely be OK in the short term and not cause much damage to your cat. If it runs erratically or you smell a very rich mixture, you might have a deeper problem . . . .
Last edited by Droops83; 10-26-2017 at 12:39 AM.
10-26-2017, 01:37 AM
#6
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Not exactly how it works.
The European versions with out cats didn't have o2 sensors. So the plug was looped so the DME knew it didn't have one. I have the oem plug somehwere. I dont remember which one it loops. I think i have the plug at home so when i get home i can check.
The European versions with out cats didn't have o2 sensors. So the plug was looped so the DME knew it didn't have one. I have the oem plug somehwere. I dont remember which one it loops. I think i have the plug at home so when i get home i can check.
10-26-2017, 02:14 AM
#7
Three Wheelin'
Not exactly how it works.
The European versions with out cats didn't have o2 sensors. So the plug was looped so the DME knew it didn't have one. I have the oem plug somehwere. I dont remember which one it loops. I think i have the plug at home so when i get home i can check.
The European versions with out cats didn't have o2 sensors. So the plug was looped so the DME knew it didn't have one. I have the oem plug somehwere. I dont remember which one it loops. I think i have the plug at home so when i get home i can check.
I guess the bottom line is that any 1980s-era Bosch fuel injection system is tuned to run at or near stoichiometric at the idle and part-load ranges, and richer at higher loads. This ensures smooth running and good fuel economy across the board, except at higher loads when engine protection is paramount. These systems are all batch-fired, so some fuel enrichment is needed at tip-in, and this is somewhat counteracted by a closed-loop O2 sensor system.
In a US-spec car, the catalytic converter will not be as efficient when the O2S is disconnected, as the system is designed to oscillate between a slightly rich mixture to curb NOx emissions and a slightly lean mixture to quell HC and CO emissions, with the average being 14.7:1. But, an otherwise well-tuned engine should not run rich enough to damage the cat if the O2S is disconnected.
But, as I mentioned in the previous post, a "well-tuned" 951 engine can be somewhat difficult to achieve given the amount of variables present!
By the way, I work on Porsches and BMWs for a living and know these systems quite well . . . .
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10-26-2017, 10:42 AM
#8
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Well, the non-cat variants work exactly as I described the above US versions when in open-loop mode (O2 sensor disconnected), so I'm not sure what your point is . . .
I guess the bottom line is that any 1980s-era Bosch fuel injection system is tuned to run at or near stoichiometric at the idle and part-load ranges, and richer at higher loads. This ensures smooth running and good fuel economy across the board, except at higher loads when engine protection is paramount. These systems are all batch-fired, so some fuel enrichment is needed at tip-in, and this is somewhat counteracted by a closed-loop O2 sensor system.
In a US-spec car, the catalytic converter will not be as efficient when the O2S is disconnected, as the system is designed to oscillate between a slightly rich mixture to curb NOx emissions and a slightly lean mixture to quell HC and CO emissions, with the average being 14.7:1. But, an otherwise well-tuned engine should not run rich enough to damage the cat if the O2S is disconnected.
But, as I mentioned in the previous post, a "well-tuned" 951 engine can be somewhat difficult to achieve given the amount of variables present!
By the way, I work on Porsches and BMWs for a living and know these systems quite well . . . .
I guess the bottom line is that any 1980s-era Bosch fuel injection system is tuned to run at or near stoichiometric at the idle and part-load ranges, and richer at higher loads. This ensures smooth running and good fuel economy across the board, except at higher loads when engine protection is paramount. These systems are all batch-fired, so some fuel enrichment is needed at tip-in, and this is somewhat counteracted by a closed-loop O2 sensor system.
In a US-spec car, the catalytic converter will not be as efficient when the O2S is disconnected, as the system is designed to oscillate between a slightly rich mixture to curb NOx emissions and a slightly lean mixture to quell HC and CO emissions, with the average being 14.7:1. But, an otherwise well-tuned engine should not run rich enough to damage the cat if the O2S is disconnected.
But, as I mentioned in the previous post, a "well-tuned" 951 engine can be somewhat difficult to achieve given the amount of variables present!
By the way, I work on Porsches and BMWs for a living and know these systems quite well . . . .
Not exactly, no.
US spec and Euro spec have the same DME.
To run with out the o2 sensor, you need to loop the plug. I have the factory plug in front of me. with it plugged in, the DME knows there's no o2 and it'll run at around 12-13:1 for when it in closed loop mode. Having the o2 un plugged the DME is still looking for a reading and it can cause it not to run properly at all.
10-26-2017, 11:50 AM
#9
Drifting
Thread Starter
This is day 3 driving with the O2 connector disconnected. I've put about 180kms mixed highway / city.
There is no funny smell and frankly, the car seems to run, if anything, a little better.
That odd "downshift shudder" is less noticeable too.
After changing AOS seals, many vacuum lines, cleaning injectors, new air filter & plugs, etc., I measure 16-17 on a quality vac gauge at warm idle off an intake port and the car runs better than before the work.
This emissions failure came as a surprise, so I'm hoping it's simply the O2 sensor. I'll find out soon enough!
I have noticed that with the O2 disconnected, this tank of gas seems to be disappearing quicker than normal--about 20 miles off at the half-way point.
Given that you can't tune with a stock O2 sensor, I guess the best procedure for a DIY is take it in to get O2 checked via factory pipe and adjusted to .6 as per workshop manual.
Or buy a wideband sensor, which I just discovered is about the same cost as the stock replacement: https://www.lindseyracing.com/LR/Parts/GENGAUGEIM.html
Crap! Should have bought one. I assume with this, I could tune by measuring the voltage reaction?
The connection is different and would need modification, correct?
There is no funny smell and frankly, the car seems to run, if anything, a little better.
That odd "downshift shudder" is less noticeable too.
After changing AOS seals, many vacuum lines, cleaning injectors, new air filter & plugs, etc., I measure 16-17 on a quality vac gauge at warm idle off an intake port and the car runs better than before the work.
This emissions failure came as a surprise, so I'm hoping it's simply the O2 sensor. I'll find out soon enough!
I have noticed that with the O2 disconnected, this tank of gas seems to be disappearing quicker than normal--about 20 miles off at the half-way point.
Given that you can't tune with a stock O2 sensor, I guess the best procedure for a DIY is take it in to get O2 checked via factory pipe and adjusted to .6 as per workshop manual.
Or buy a wideband sensor, which I just discovered is about the same cost as the stock replacement: https://www.lindseyracing.com/LR/Parts/GENGAUGEIM.html
Crap! Should have bought one. I assume with this, I could tune by measuring the voltage reaction?
The connection is different and would need modification, correct?
10-26-2017, 08:53 PM
#11
Drifting
Thread Starter
10-26-2017, 11:37 PM
#12
Three Wheelin'
True, you can't get a WBO2 sensor to interface with the stock DME, but there is no reason to do so. You also wouldn't want to mount a WBO2 sensor in the factory location, is it would be killed by the heat.
Any aftermarket WBO2 sensor setup comes with a controller that is totally separate from the factory DME. Get a muffler shop to weld a bung in the location shown here:
https://www.lindseyracing.com/LR/Par...2LOCATION.html
Wire up the controller and calibrate the WBO2 sensor per the manufacturer's instructions, figure out a place to mount the gauge and enjoy the accurate A/F ratio monitoring!
Any aftermarket WBO2 sensor setup comes with a controller that is totally separate from the factory DME. Get a muffler shop to weld a bung in the location shown here:
https://www.lindseyracing.com/LR/Par...2LOCATION.html
Wire up the controller and calibrate the WBO2 sensor per the manufacturer's instructions, figure out a place to mount the gauge and enjoy the accurate A/F ratio monitoring!
10-26-2017, 11:50 PM
#13
Three Wheelin'
Not exactly, no.
US spec and Euro spec have the same DME.
To run with out the o2 sensor, you need to loop the plug. I have the factory plug in front of me. with it plugged in, the DME knows there's no o2 and it'll run at around 12-13:1 for when it in closed loop mode. Having the o2 un plugged the DME is still looking for a reading and it can cause it not to run properly at all.
US spec and Euro spec have the same DME.
To run with out the o2 sensor, you need to loop the plug. I have the factory plug in front of me. with it plugged in, the DME knows there's no o2 and it'll run at around 12-13:1 for when it in closed loop mode. Having the o2 un plugged the DME is still looking for a reading and it can cause it not to run properly at all.
However, I must disagree with your assertion about the DME "looking for" an O2S signal if the coding plug isn't installed, and this is based on a vast amount of experience of driving all sorts of '80s Motronic/L-Jetronic cars with the O2S disconnected for diagnostic purposes over the past 12+ years (944 NA and Turbo, 911 Carrera 3.2, BMW E30, even my own Alfa GTV6). The same basic fuel map is used regardless of open or closed loop; when the O2S is connected, the injector pulsewidth is simply modified beyond what is dictated by this base map, as determined by feedback from the O2S voltage signal and by the need to oscillate the A/F ratio between slightly rich and lean of stoichiometric to optimize catalyst efficiency.
If the O2S is disconnected, the system is in "open loop" mode, same is it is when the engine/coolant temperature (NTC2) signal is below a certain level and/or the load signal (as determined by a basic on/off switch in the 944 NA or an analog sensor in the 951). It is that simple. The basic A/F ratio is determined by the base fuel map of the DME, but is also affected by the state of tune and mechanical health of the engine. On a perfectly tuned and healthy engine, the base fuel map will provide a slightly rich mixture in the normal range, but nothing excessively rich.
10-27-2017, 07:00 AM
#14
Drifting
Thread Starter
The catalytic needs this oscillation? Then, while I wait for the new one, I should plug in my poor-performing O2sensor instead of running disconnected?
10-30-2017, 01:44 PM
#15
Addict
Rennlist Member
Rennlist Member
When the O2 sensor is disconnected (3-pin connector near AOS), the DME defaults to about 0.45V (I just verified mine at idle to be a stable 0.46V), so it essentially "tricks" itself to believe that it runs stoichiometric and no reason to change (dither) the AFR. In essence running on the base map.
This has previously been discussed and documented.
Laust
This has previously been discussed and documented.
Laust
