do we have an ECU expert on the forum
Discussion
That is to say someone who understands the coding in an ECU as well as what it does.
My car has two precats in the exhaust manifold and a big cat inthe exhausts themselves. I want to replace the two manifolds with a specially made non manufacturer one but have been warned that this might well bring on my engine management light. Apparently the ECU compares the two lambda sensors, and if the difference is outside a range set by the car maker, then its the EML and eventually the crawl home
What I want to understand is what the lambda sensors measure, how the measurement relates to fuel air mixture, and what the warning light is all about. A tuning company say they can eliminate the light but the maker doesnt put a warning light into the system just for visual effect. Its to warn of a problem. What problem?
In short I need an idiots guide to how the ecu works.
Incidentally if the ECU is altered in any way would this be apparent to the main dealer when he links up his ODB reader - its a 2008 car so outside the new EU rules.
My car has two precats in the exhaust manifold and a big cat inthe exhausts themselves. I want to replace the two manifolds with a specially made non manufacturer one but have been warned that this might well bring on my engine management light. Apparently the ECU compares the two lambda sensors, and if the difference is outside a range set by the car maker, then its the EML and eventually the crawl home
What I want to understand is what the lambda sensors measure, how the measurement relates to fuel air mixture, and what the warning light is all about. A tuning company say they can eliminate the light but the maker doesnt put a warning light into the system just for visual effect. Its to warn of a problem. What problem?
In short I need an idiots guide to how the ecu works.
Incidentally if the ECU is altered in any way would this be apparent to the main dealer when he links up his ODB reader - its a 2008 car so outside the new EU rules.
In no way an expert but can talk about the 350/370 which uses this system . The light is to warn of reduced cat efficiency , on 350 and 370s they have the lambda each end of the cat and if you fit sports cats the downstream lambda will often throw a fault, it wont however go into limp home mode, just the CEL comes on. Its common when remapping to switch them off. The manufacturer could see if they have been switched off and would of course also notice if parts have been changed from stock , it may also impact the emissions which could effect the mot test.
A MIL lighting up on your dash means you no longer comply with legislation, specifically emissions legislation this case.
If the gas ratios are reading wrong between the two sensors (i.e. between engine-out emissions and post-catalyst emissions) then it says something is wrong with your aftertreatment system.
You basically need someone to turn off the aftertreatment control of your engine - it's part of the software that's to do with emissions control.
Go ask VW, they know how to turn it off
If the gas ratios are reading wrong between the two sensors (i.e. between engine-out emissions and post-catalyst emissions) then it says something is wrong with your aftertreatment system.
You basically need someone to turn off the aftertreatment control of your engine - it's part of the software that's to do with emissions control.
Go ask VW, they know how to turn it off
lambda sensors measure the difference in the amount of oxygen in the sampled air steam (exhaust) and the amount of oxygen in air. a lean mixture will give a lower voltage i believe, (excess air). rich mixture giving higher voltages. the signal given back to the ECU then tells it to adjust the fuelling.
ive junked both pre-cats on my mr2 but the new manifold retains the oe lambda sensors so no ecu faults are thrown.
ive junked both pre-cats on my mr2 but the new manifold retains the oe lambda sensors so no ecu faults are thrown.
GroundEffect said:
A MIL lighting up on your dash means you no longer comply with legislation, specifically emissions legislation this case.
Yeah, that. Not all manufacturers use the rear lambdas to influence fuel trims, but some do.
OP, have you got a diagnostics reader? If so, compare the fuel trims before and after unplugging the rear lambdas.
Where there is an additional sensor downstream, the ECU usually uses this as a catalyst performance check, by measuring the exhaust gas against what it "expects" to see from a healthy catalytic converter (or converters) under the engine operating conditions at that moment. Any fault in the cat (or removal of them) will cause the measured gas to fall out of tolerance, and trigger the MIL unless modified code is uploaded that ignores, or fudges the output from this sensor (so called "mapping out").
Where the upstream sensor is used to control fuelling, AFAIK, the downstream sensor is there only for monitoring / fault indication. It doesn't actually affect anything, so it can be "mapped out" safely if any changes to the exhaust are likely to cause issues with it.
Where the upstream sensor is used to control fuelling, AFAIK, the downstream sensor is there only for monitoring / fault indication. It doesn't actually affect anything, so it can be "mapped out" safely if any changes to the exhaust are likely to cause issues with it.
I was recommended this when I took the cats out of a skoda:
http://www.ebay.co.uk/itm/251317571511?_trksid=p20...
Never got around to using it, but understand it does the trick.
http://www.ebay.co.uk/itm/251317571511?_trksid=p20...
Never got around to using it, but understand it does the trick.
But does the ECU alter the mixture on the fly based on the feedback from the lambda sensors or are the sensors just there to monitor emissions? If they do alter the air fuel mixture then me messing around just having the light switched off might leave the engine with the wrong mixture.
But if the light is just for enmissions, dumping the precats could give me MoT problems
But if the light is just for enmissions, dumping the precats could give me MoT problems
I think I remember reading something about how modern vehicles have started to use information from both lambda sensors to adjust fuel/air mix and not just the pre-cat one which used to be the case. This might be bulls
t though so ymmv
If your car only uses the first sensor, an easy and easy bodge was to fit a spacer to the rear sensor which removes it from the exhaust stream. This tricks the ECU into believing that the cat is still working. They are cheap enough that it worth throwing one on to try it before messing with anything else. available here
t though so ymmvIf your car only uses the first sensor, an easy and easy bodge was to fit a spacer to the rear sensor which removes it from the exhaust stream. This tricks the ECU into believing that the cat is still working. They are cheap enough that it worth throwing one on to try it before messing with anything else. available here
bordseye said:
But does the ECU alter the mixture on the fly based on the feedback from the lambda sensors or are the sensors just there to monitor emissions? If they do alter the air fuel mixture then me messing around just having the light switched off might leave the engine with the wrong mixture.
But if the light is just for enmissions, dumping the precats could give me MoT problems
The precat sensor readings are definitely used by the ECU to alter the mixture on the fly. But if the light is just for enmissions, dumping the precats could give me MoT problems
The postcat sensor is usually just for monitoring, but as others have said above, some manufacturers also use it to provide additional fuelling control (I didn't know this).
So you want to completely change the exhaust and remove the cats...and are then worried about a warning light because you've removed those cats ?
How random.
If you're really that concerned, leave it alone.
But of course bare in mind it will not pass a legitimate MOT with no cats fitted.
How random.
If you're really that concerned, leave it alone.
But of course bare in mind it will not pass a legitimate MOT with no cats fitted.
The only people with a real view of the ECU code will be the car and ECU manufacturer. And possibly the odd tuning house who have reverse engineered the software/data flashed onto the ECU.
The 'code' will be in two parts. The actual program code, which is what tells the ECU what to do. And data which is flashed, usually, onto non-volatile storage on the main chip (usually a simple microcontroller).
Depending on how the program is written it may or may not be possible to ignore/bypass inputs for sensors, like the post Cat O2 sensor. It will differ for each ECU manufacturer and model. Some are programmed in a way that these sensors can be disabled by changing a single bit of data, others would need to be flashed with new program software.
Whether the ECU attempts to use particular sensors to trim the fuel maps will be part of the program software, but it is possible that this can be disabled by changing the data/map if they built that into the software program as a feature.
The 'code' will be in two parts. The actual program code, which is what tells the ECU what to do. And data which is flashed, usually, onto non-volatile storage on the main chip (usually a simple microcontroller).
Depending on how the program is written it may or may not be possible to ignore/bypass inputs for sensors, like the post Cat O2 sensor. It will differ for each ECU manufacturer and model. Some are programmed in a way that these sensors can be disabled by changing a single bit of data, others would need to be flashed with new program software.
Whether the ECU attempts to use particular sensors to trim the fuel maps will be part of the program software, but it is possible that this can be disabled by changing the data/map if they built that into the software program as a feature.
European On Board Diagnostics, or EOBD to you and i, mandates that the system must continuously monitor catalyst polution conversion efficiency. This is to ensure that the catalytic convertor stays functioning as it should and removes the vast majority of the polutants from the engine out emissions. To achieve this, the CatMonitor strategy looks at the output voltages from the upstream (precat/engine out) sensors and the downstream (post cat/tailpipe emissions) sensors. The health (conversion efficiency) of a 3 way catalyst is proportionally linked to the oxygen storage capability of the precious metal active materials present in the coating that covers the catalysts metalic or ceramic substrate. So, the CatMon compares up and downstream lambda perturbation amplitude and switch counts (a count being a crossing from rich to lean, or lean to rich). With a "healthy" cat, the downstream oxygen content of the exhaust gas is heavily damped by the oxygen stored in the cat, and the rear sensor will switch something like 10 or 20times less than the upstream sensor (which is constantly switching because the exhaust AFR is deliberately perturbated around the lambda bias target to improve conversion efficency).
If you remove the catalyst, the rear sensor switches everytime the upstream sensor switches, the CatMon spots this and flags a "catalyst below mandated efficiency" fault, causing a MIL to come on and an appropriate fault code to be stored in the ecu.
The system also uses the average measured oxygen concentration from the post catalyst sensor to trim upstream AFR (the so called Lambda bias) to attempt to allow the catalyst to work to the peak of it's capability. That AFR trimming is however a small value, way less than 1%, and has no bearing on engine performance etc.
The "space your rear sensor out" devices work, they simply keep the rear sensor out of the gas stream, so when you remove your cats, even though the exhaust lambda is now rapidly switching rich/lean, the rear sensor doesn't actually see that stream because it is now tucked up out the way, effectively in a non flowing little "backwater" of exhaust gas.
Some of the later systems are smart enough that they will flag not enough switches just as most systems flag too many, meaning you may have to experiment with the length of the spacer to get it to work
(as an aside, loads of people were up-in-arms about VW "Cheating" emissions tests, but have no qualms about taking cats and emissions controls equipment off their own cars, despite the fact there tailpipe emissions will immediately be equivalent to about 5000 "cheating" VW cars......... )
If you remove the catalyst, the rear sensor switches everytime the upstream sensor switches, the CatMon spots this and flags a "catalyst below mandated efficiency" fault, causing a MIL to come on and an appropriate fault code to be stored in the ecu.
The system also uses the average measured oxygen concentration from the post catalyst sensor to trim upstream AFR (the so called Lambda bias) to attempt to allow the catalyst to work to the peak of it's capability. That AFR trimming is however a small value, way less than 1%, and has no bearing on engine performance etc.
The "space your rear sensor out" devices work, they simply keep the rear sensor out of the gas stream, so when you remove your cats, even though the exhaust lambda is now rapidly switching rich/lean, the rear sensor doesn't actually see that stream because it is now tucked up out the way, effectively in a non flowing little "backwater" of exhaust gas.
Some of the later systems are smart enough that they will flag not enough switches just as most systems flag too many, meaning you may have to experiment with the length of the spacer to get it to work
(as an aside, loads of people were up-in-arms about VW "Cheating" emissions tests, but have no qualms about taking cats and emissions controls equipment off their own cars, despite the fact there tailpipe emissions will immediately be equivalent to about 5000 "cheating" VW cars......... )
Max_Torque said:
The system also uses the average measured oxygen concentration from the post catalyst sensor to trim upstream AFR (the so called Lambda bias) to attempt to allow the catalyst to work to the peak of it's capability. That AFR trimming is however a small value, way less than 1%, and has no bearing on engine performance etc.
Hmm interesting stuff, good to know! Max_Torque said:
European On Board Diagnostics, or EOBD to you and i, mandates that the system must continuously monitor catalyst polution conversion efficiency. This is to ensure that the catalytic convertor stays functioning as it should and removes the vast majority of the polutants from the engine out emissions. To achieve this, the CatMonitor strategy looks at the output voltages from the upstream (precat/engine out) sensors and the downstream (post cat/tailpipe emissions) sensors. The health (conversion efficiency) of a 3 way catalyst is proportionally linked to the oxygen storage capability of the precious metal active materials present in the coating that covers the catalysts metalic or ceramic substrate. So, the CatMon compares up and downstream lambda perturbation amplitude and switch counts (a count being a crossing from rich to lean, or lean to rich). With a "healthy" cat, the downstream oxygen content of the exhaust gas is heavily damped by the oxygen stored in the cat, and the rear sensor will switch something like 10 or 20times less than the upstream sensor (which is constantly switching because the exhaust AFR is deliberately perturbated around the lambda bias target to improve conversion efficency).
If you remove the catalyst, the rear sensor switches everytime the upstream sensor switches, the CatMon spots this and flags a "catalyst below mandated efficiency" fault, causing a MIL to come on and an appropriate fault code to be stored in the ecu.
The system also uses the average measured oxygen concentration from the post catalyst sensor to trim upstream AFR (the so called Lambda bias) to attempt to allow the catalyst to work to the peak of it's capability. That AFR trimming is however a small value, way less than 1%, and has no bearing on engine performance etc.
The "space your rear sensor out" devices work, they simply keep the rear sensor out of the gas stream, so when you remove your cats, even though the exhaust lambda is now rapidly switching rich/lean, the rear sensor doesn't actually see that stream because it is now tucked up out the way, effectively in a non flowing little "backwater" of exhaust gas.
Some of the later systems are smart enough that they will flag not enough switches just as most systems flag too many, meaning you may have to experiment with the length of the spacer to get it to work
(as an aside, loads of people were up-in-arms about VW "Cheating" emissions tests, but have no qualms about taking cats and emissions controls equipment off their own cars, despite the fact there tailpipe emissions will immediately be equivalent to about 5000 "cheating" VW cars......... )
Good post, well explained. Thank you.If you remove the catalyst, the rear sensor switches everytime the upstream sensor switches, the CatMon spots this and flags a "catalyst below mandated efficiency" fault, causing a MIL to come on and an appropriate fault code to be stored in the ecu.
The system also uses the average measured oxygen concentration from the post catalyst sensor to trim upstream AFR (the so called Lambda bias) to attempt to allow the catalyst to work to the peak of it's capability. That AFR trimming is however a small value, way less than 1%, and has no bearing on engine performance etc.
The "space your rear sensor out" devices work, they simply keep the rear sensor out of the gas stream, so when you remove your cats, even though the exhaust lambda is now rapidly switching rich/lean, the rear sensor doesn't actually see that stream because it is now tucked up out the way, effectively in a non flowing little "backwater" of exhaust gas.
Some of the later systems are smart enough that they will flag not enough switches just as most systems flag too many, meaning you may have to experiment with the length of the spacer to get it to work
(as an aside, loads of people were up-in-arms about VW "Cheating" emissions tests, but have no qualms about taking cats and emissions controls equipment off their own cars, despite the fact there tailpipe emissions will immediately be equivalent to about 5000 "cheating" VW cars......... )
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff