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After spending many hours tracing a fault with my car I decided to post up some information from what I learned during the process.
Background:
Series II cars to meet Euro V emissions rules all use a combined diesel oxidation catalyst + diesel particulate filter in the same shell.
There are four sensors in this housing; Three EGT sensors, one pre cat (nearest engine) one post cat pre DPF (middle of Cat-DPF) and one post DPF at the back of the Cat-DPF housing.
Then there is the O2 sensor in the pipe leaving the Cat-DPF.
While these three EGT sensors have different part numbers, they are all electrically the same.
They are a high temperature NTC thermistor. The different part numbers account for the different plugs and lengths of cable.
From the WSM the resistance should be somewhere between 82 & 137 k OHMs at 50 deg C to be declared within specification.
I characterised the sensors up to 250 deg C and at 50 deg C mine were all at about 100k
To do the characterisation I used a saucepan full of canola oil and I heated it up slowly with a digital meat thermometer in the oil also.
I used oil so I could co above 100 deg C, otherwise water would do.
Here is what I recorded for the middle and back:
20 280000 260000
50 10000 10000
100 37000 35000
150 16000 15000
200 8000 7000
250 4300 4100
300 2600 2600
350 1800 1800
400 1300 1300
450 1000 1000
500 770 770
550 625 625
600 518 518
650 438 438
700 377 377
Note anything over 250 is an estimate based off the typical NTC thermistor curve for the type.
I tested one with my heat gun at about 500 deg C and its pretty close.
Reason:
OK so why would I want to do this?
My car has been modified to have the DPF taken out and there has been modifications to the fuel maps for extra torque ~40% more.
One thing I found is that the engine always seemed to go into some sort of a torque limiting mode at high loads.
It bugged me for nearly one year. I got no help from the modifiers and I didn't even bother with Mazda as they would not want to have any involvement wit modifications, I figured I would be on my own.
For another background; The DPF in these cars performs a second function, it is also a NOx converter otherwise known as an LNT. (lean NOx Trap)
To perform this function, the temperatures need to be strictly controlled.
And so while the car is operating, the ECU tries to keep the DPF entry temp and exit temp between a narrow temperature window so that the DPF can reduce NOx.
The problem being that with the fuel map modifications in my car, my post cat EGTs would probably be alot higher. Also without the DPF core being in place, the last EGT sensor would be seeing temperatures alot higher than expected thanks to no thermal mass damping plus NOx reduction being endothermic, no associated heat loss.
The result; the ECU would start reducing torque at high loads thinking the LNT function was out of range.
The 3rd EGT sensor also exists to ensure the DPF doesn't melt during a regen process, something that has been coded out of my ECU but the resulting DCT codes are now also no longer available. This might be useful for cars with operational DPFs; The last sensor performs a vital function in controlling the completion of the regen cycle.
Why is all this important?
If your vehicle is unmodified the ECU will go into limp mode if any of the EGT sensors go out of range. You can test yours with the information provided above.
If your vehicle is modified and you have a cat but no DPF you car may not like the reading the ECU receives from the 3rd EGT as it will go out of range without the DPF in place.
Depending on what changes where done in the ECU by the tuner concerned, these EGT values could be ignored or just the DCT codes ignored.
What did I do to fix it?.....
I wish I had the time to read the PID values from the OBDII port and log it, I purchased the software and had the hardware, what I didn't have is time and someone to assist with reading the values out of the laptop while I was driving.
And so I decided to take an educated guess to make the ECU think that the temperature on EGT 3 was lower that was what actually there.
To do this I connected a 220R resitor in series with the sensor. Being an NTC thermistor this extra resistance will cause the ECU to treat it as normal its just that the max temperature has been biased lower.
I made the change near the sensor itself, but this was painful as the wire being high temperature does not take well to solder and I used glued heatshrink to give the joins mechanical strength.
How does it drive: The problem "seems" to be fixed, I will only know after a long drive at high load at high ambient temps.
Its coming into winter here now and so a full test may not happen for a while.
In summary:
ALL EGT sENSORS ARE ELECTRICALLY IDENTICAL
THE THREE CONNECTORS AND WIRE LENGTHS ARE DIFFERENT
THE WIRES ARE DIFFICULT TO SOFT SOLDER
THE ECU WILL GO INTO LIMP MODE IF ANY SENSORS GO OUT OF RANGE.
Background:
Series II cars to meet Euro V emissions rules all use a combined diesel oxidation catalyst + diesel particulate filter in the same shell.
There are four sensors in this housing; Three EGT sensors, one pre cat (nearest engine) one post cat pre DPF (middle of Cat-DPF) and one post DPF at the back of the Cat-DPF housing.
Then there is the O2 sensor in the pipe leaving the Cat-DPF.
While these three EGT sensors have different part numbers, they are all electrically the same.
They are a high temperature NTC thermistor. The different part numbers account for the different plugs and lengths of cable.
From the WSM the resistance should be somewhere between 82 & 137 k OHMs at 50 deg C to be declared within specification.
I characterised the sensors up to 250 deg C and at 50 deg C mine were all at about 100k
To do the characterisation I used a saucepan full of canola oil and I heated it up slowly with a digital meat thermometer in the oil also.
I used oil so I could co above 100 deg C, otherwise water would do.
Here is what I recorded for the middle and back:
20 280000 260000
50 10000 10000
100 37000 35000
150 16000 15000
200 8000 7000
250 4300 4100
300 2600 2600
350 1800 1800
400 1300 1300
450 1000 1000
500 770 770
550 625 625
600 518 518
650 438 438
700 377 377
Note anything over 250 is an estimate based off the typical NTC thermistor curve for the type.
I tested one with my heat gun at about 500 deg C and its pretty close.
Reason:
OK so why would I want to do this?
My car has been modified to have the DPF taken out and there has been modifications to the fuel maps for extra torque ~40% more.
One thing I found is that the engine always seemed to go into some sort of a torque limiting mode at high loads.
It bugged me for nearly one year. I got no help from the modifiers and I didn't even bother with Mazda as they would not want to have any involvement wit modifications, I figured I would be on my own.
For another background; The DPF in these cars performs a second function, it is also a NOx converter otherwise known as an LNT. (lean NOx Trap)
To perform this function, the temperatures need to be strictly controlled.
And so while the car is operating, the ECU tries to keep the DPF entry temp and exit temp between a narrow temperature window so that the DPF can reduce NOx.
The problem being that with the fuel map modifications in my car, my post cat EGTs would probably be alot higher. Also without the DPF core being in place, the last EGT sensor would be seeing temperatures alot higher than expected thanks to no thermal mass damping plus NOx reduction being endothermic, no associated heat loss.
The result; the ECU would start reducing torque at high loads thinking the LNT function was out of range.
The 3rd EGT sensor also exists to ensure the DPF doesn't melt during a regen process, something that has been coded out of my ECU but the resulting DCT codes are now also no longer available. This might be useful for cars with operational DPFs; The last sensor performs a vital function in controlling the completion of the regen cycle.
Why is all this important?
If your vehicle is unmodified the ECU will go into limp mode if any of the EGT sensors go out of range. You can test yours with the information provided above.
If your vehicle is modified and you have a cat but no DPF you car may not like the reading the ECU receives from the 3rd EGT as it will go out of range without the DPF in place.
Depending on what changes where done in the ECU by the tuner concerned, these EGT values could be ignored or just the DCT codes ignored.
What did I do to fix it?.....
I wish I had the time to read the PID values from the OBDII port and log it, I purchased the software and had the hardware, what I didn't have is time and someone to assist with reading the values out of the laptop while I was driving.
And so I decided to take an educated guess to make the ECU think that the temperature on EGT 3 was lower that was what actually there.
To do this I connected a 220R resitor in series with the sensor. Being an NTC thermistor this extra resistance will cause the ECU to treat it as normal its just that the max temperature has been biased lower.
I made the change near the sensor itself, but this was painful as the wire being high temperature does not take well to solder and I used glued heatshrink to give the joins mechanical strength.
How does it drive: The problem "seems" to be fixed, I will only know after a long drive at high load at high ambient temps.
Its coming into winter here now and so a full test may not happen for a while.
In summary:
ALL EGT sENSORS ARE ELECTRICALLY IDENTICAL
THE THREE CONNECTORS AND WIRE LENGTHS ARE DIFFERENT
THE WIRES ARE DIFFICULT TO SOFT SOLDER
THE ECU WILL GO INTO LIMP MODE IF ANY SENSORS GO OUT OF RANGE.
