What I posted was the only thing I saw about the values of those sensors.
Disable the EGR system the (mostly) Toyota way
- Thread starter Ebag333
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The text in quotations is from the 96 LC FSM:
P0402 on an OBDII truck is "exhaust gas recirculation flow excessive detected". The detection logic for it is either "EGR gas temp sensor value is high during EGR cut-off when engine is cold" or "EGR valve is always open".
In the case of this mod, chances are it is the former. With the jumper wire in place the computer sees little/no resistance so it thinks there is flow (little/no resistance = high temperature/flow) and throws the code (P0402).
The resistance values listed in my 96 LC EWD are the same as Ben's.
P0402 on an OBDII truck is "exhaust gas recirculation flow excessive detected". The detection logic for it is either "EGR gas temp sensor value is high during EGR cut-off when engine is cold" or "EGR valve is always open".
In the case of this mod, chances are it is the former. With the jumper wire in place the computer sees little/no resistance so it thinks there is flow (little/no resistance = high temperature/flow) and throws the code (P0402).
The resistance values listed in my 96 LC EWD are the same as Ben's.
96r50- thanks for this great info! I'm going to go ohm out the temp sensor to check whether it is ohms or kohms and post back here. Looks somewhat linear so I'm expecting somewhere around a 180-300KOhm reading at room temp.
Update: just measured it and it is at 250 Kohms at room temp (approx 70F), and it dropped as expected with a flame under it. So it is definitely Kohms, which means the EGR temp sensor readings in the OBD2 EWDs have a typo in them.
Now, looking at the temp ranges, do you think it's probable that the ECU might throw a CEL even if we keep the temp sensor resistance in one of the acceptable ranges? For example, if we choose a 3kOhm resistor (keeping the temp sensor reading in the "high-temp" range), do you think it might throw a P0402 because of the "EGR gas temp sensor value is high during EGR cut-off when engine is cold" logic? Might it be safer to use a "mid-temp" or "low-temp" resistance value instead?
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- Thread starter
- #144
Doubtful. Several people are running higher kOhm resistors without issues.
The EGR system is not picky. The fact that I ran for almost 1k miles with it jumped and no CEL's (and the ECU flagging green) shows that much.
Ebag333- who are the people with OBD2 trucks running kOhm resistors without issues? The only people I've heard running without issues are the OBD1 folks. I don't know if I'd be so sure the OBD2 trucks aren't sensitive to the resistance values- the fact that your truck came up with the same P0402 code after 1k mi. only tells me you eventually had the same problem the rest of us did- it just took longer to show up. The fact that the P0402 code logic includes "when engine cold" makes me wonder what resistance range to use. I'm going to make up 3 sets and try 'em all.
Update... 100 miles, several on and off cycles, no codes!! will keep posted!!
- Thread starter
- #147
I don't remember off hand and it is very possible that they are OBD-I and I'm misremembering them as OBD-II.
For what it's worth, all my EGR codes have shown up on the freeway. I've never had one show up otherwise, so I don't think the ECU logic provides for it to.
I don't have the PDF in front of me, but there is some logic such as throttle position that controls when it shows. I don't fully follow all of it, but what I do seems to indicate that it can only throw an CEL when it should be operating, that is cruising speed, low throttle, etc.
This is from my FSM, for OBD1.
69 – 89 kW at 50°C (112°F)
11 – 15 kW at 100°C (212°F)
2 – 4 kW at 150°C (302°F)
Edit: Brett's values are found in the 1995 manual.
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tblume posted this in another thread:
"EGR System Malfunction: 60 seconds after start of EGR operation, EGR gas temp is less than 158F and the following conditions:
a) Engine coolant temp: 127F or more
b) Engine speed: 1200RPM or more
c) Intake air temp 32F or more
- 2 trip detection logic"
Does this apply to OBD2 vehicles, as well?
"EGR System Malfunction: 60 seconds after start of EGR operation, EGR gas temp is less than 158F and the following conditions:
a) Engine coolant temp: 127F or more
b) Engine speed: 1200RPM or more
c) Intake air temp 32F or more
- 2 trip detection logic"
Does this apply to OBD2 vehicles, as well?
Some notes from a quick test I did today:
1. EGR temp sensor set to open circuit
2. EGR temp sensor set to short circuit
1. EGR temp sensor set to open circuit
- resistance = very high
- EGR temperature read by ECU = LOW
- P0401 EGR insufficient flow
2. EGR temp sensor set to short circuit
- resistance = very low
- EGR temperature read by ECU = HIGH
- P0402 EGR excessive flow
- Thread starter
- #151
I don't have a '93/'94 FSM in front of me, so I can't attest to OBD-I, but OBD-II isn't that simple.
The EGR codes are controlled by the temp of the coolant and air on startup (mostly so it doesn't error if it's too cold out), the throttle position for P0402, the vehicle speed for P0401, and run time. And that's just to enable that those codes will be checked.
The run test is based off a whole slew of other scenarios for the pass/fail, which is why so many different things can cause the P0401/P0402 codes.
I've attached the technical doc for the EGR system from my '97. It shows the criteria required in order for the ECU to test the EGR system.
Ebag333- excellent! Thanks for posting that. Relevant info for OBD2:
1. EGR Temperature Detection Method
This method is used on the VSV Cut-off Control EGR System. An EGR
temperature sensor is installed in the EGR passageway. During normal
EGR flow, the temperature of the EGR temperature sensor will rise at
least 35°C (95°F) above ambient air temperature.
P0401: Insufficient Flow
When the EGR valve is open, the ECM compares EGR temperature to
intake air temperature. If the temperature does not rise a specified
amount over ambient temperature, the ECM assumes there is a problem
in the system, and this information is stored in the ECM. If the problem
occurs on the second trip, DTC P0401 will set and the MIL will turn on.
P0402: Excessive Flow
When the EGR is off, the ECM measures EGR temperature. If the EGR is
open, the EGR temperature will be higher than expected. If the problem
occurs on the second trip, DTC P0402 will set and the MIL will turn on.
2. EGR Valve Position /
Temperature Detection Method
This method uses a temperature sensor for insufficient EGR flow and an
EGR Valve Position sensor for excessive EGR flow on the Constant
Vacuum EGR system.
P0401: Insufficient Flow
When the EGR valve is open, the ECM compares EGR temperature to
intake air temperature. If the temperature does not rise a specified
amount over ambient temperature, the ECM assumes there is a problem
in the system, and this information is stored in the ECM. If the problem
occurs on the second trip, DTC P0401 will set and the MIL will turn on.
P0402: Excessive Flow
The ECM uses the EGR valve height position sensor to detect excessive
flow. When the EGR is off and the sensor signal is greater than the
specification stored in the ECM, the ECM assumes the EGR valve did not
close. If the problem occurs on the second trip, DTC P0402 will set and
will turn the MIL on.
1. EGR Temperature Detection Method
This method is used on the VSV Cut-off Control EGR System. An EGR
temperature sensor is installed in the EGR passageway. During normal
EGR flow, the temperature of the EGR temperature sensor will rise at
least 35°C (95°F) above ambient air temperature.
P0401: Insufficient Flow
When the EGR valve is open, the ECM compares EGR temperature to
intake air temperature. If the temperature does not rise a specified
amount over ambient temperature, the ECM assumes there is a problem
in the system, and this information is stored in the ECM. If the problem
occurs on the second trip, DTC P0401 will set and the MIL will turn on.
P0402: Excessive Flow
When the EGR is off, the ECM measures EGR temperature. If the EGR is
open, the EGR temperature will be higher than expected. If the problem
occurs on the second trip, DTC P0402 will set and the MIL will turn on.
2. EGR Valve Position /
Temperature Detection Method
This method uses a temperature sensor for insufficient EGR flow and an
EGR Valve Position sensor for excessive EGR flow on the Constant
Vacuum EGR system.
P0401: Insufficient Flow
When the EGR valve is open, the ECM compares EGR temperature to
intake air temperature. If the temperature does not rise a specified
amount over ambient temperature, the ECM assumes there is a problem
in the system, and this information is stored in the ECM. If the problem
occurs on the second trip, DTC P0401 will set and the MIL will turn on.
P0402: Excessive Flow
The ECM uses the EGR valve height position sensor to detect excessive
flow. When the EGR is off and the sensor signal is greater than the
specification stored in the ECM, the ECM assumes the EGR valve did not
close. If the problem occurs on the second trip, DTC P0402 will set and
will turn the MIL on.
Of the 2 systems described above, can we safely assume US 80s are equipped with system #1? Does anyone know if we have an "EGR valve height position sensor" as part of the EGR system?
BTW, kup99- any word?
BTW, kup99- any word?
According to the info. Ebag333 posted,
The wording seems to use "ambient air temperature" and "intake air temperature" interchangeably.
- Using intake air temp, if we assume worst-case intake air temp of 150F (any comments on if this is realistic for worst-case?), then EGR temp when open should be at least 250F to prevent P0401.
- Assuming a worst-case minimum intake air temp of 80F (any comments if this is realistic?) , assuming the EGR system is using approx. the same expected temp. delta as in (1), we might think a P0402 will trip if the temp sensor reads above 180F. However, note that 95F is the minimum expected temp rise with EGR open, so let's assume the ECU gives it some margin up to at most 250F, so the EGR temp when closed should be at most 250F to prevent P0402.
- From these assumptions, I believe the temp sensor should be somewhere between 180F and 250F to prevent a P0401 and P0402. Using these barely-educated-guesses as assumptions, I will try resistance values that put the sensor somewhere around 250F. From my measurements using a heat gun on the temp sensor, this would be an approx. 10kohm resistor.
The wording seems to use "ambient air temperature" and "intake air temperature" interchangeably.
- Using intake air temp, if we assume worst-case intake air temp of 150F (any comments on if this is realistic for worst-case?), then EGR temp when open should be at least 250F to prevent P0401.
- Assuming a worst-case minimum intake air temp of 80F (any comments if this is realistic?) , assuming the EGR system is using approx. the same expected temp. delta as in (1), we might think a P0402 will trip if the temp sensor reads above 180F. However, note that 95F is the minimum expected temp rise with EGR open, so let's assume the ECU gives it some margin up to at most 250F, so the EGR temp when closed should be at most 250F to prevent P0402.
- From these assumptions, I believe the temp sensor should be somewhere between 180F and 250F to prevent a P0401 and P0402. Using these barely-educated-guesses as assumptions, I will try resistance values that put the sensor somewhere around 250F. From my measurements using a heat gun on the temp sensor, this would be an approx. 10kohm resistor.
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- Thread starter
- #156
Look at page 3. The air temp must be between 10* and 60* C (14* - 140* F) in order to even test the EGRsystem.
So in your worst case scenario, the EGR system would never even be tested and a code would never be thrown.
So that puts the operating range at 45* - 95* C (109* - 235* F).
Based on that operating range, one would think that a 1k or 2k resistor would be way too much. I would be leaning more toward the 10k-20k range.....
/me edits
Upon further reflection, this would make sense that Mr. T would give the too low resistor numbers, just right, and too high numbers. But obviously the ECU isn't picky as people are running 1k resistors without problems.
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4.7K resistor .... approx. 150 miles... not CEL!
The first thing I am confused about is intake air temp vs. ambient temp- the manual seems to use the two interchangeably as they do here: "When the EGR valve is open, the ECM compares EGR temperature to intake air temperature. If the temperature does not rise a specified amount over ambient temperature, the ECM assumes there is a problem in the system"
So let me look at this again (thinking aloud and I'd be interested in your comments):
- When the EGR is supposed to be open, the ECU looks for AT LEAST a 95F increase in temperature over ambient/intake to verify the EGR is open, so assuming we're using ambient temp, then, yes, from your numbers it is looking for 109-235F to prevent a P0401. However, a temp greater than this is ok, so let's say worst-case we need T>235F to prevent a P0401 under cold or hot ambient temp.
- When the EGR is supposed to be closed, the ECU looks for an UNSPECIFIED change in temperature over ambient/intake to verify the EGR is closed, so assuming we're using ambient temp, from Mr. T's own wording we would expect to see AT LEAST a 95F increase over ambient with the EGR open, which (using worst case 14F ambient) takes us to 109F. However, this is a MINIMUM expected temp increase, so we should include a generous margin on this number since they likely don't want false alarms on this code. Using a 50% margin, let's say worst case, we need T<157F to prevent a P0402 under cold ambient temps.
- Even using a 50% margin on the max temp, it looks like we might just see a P0402 if temps are cold enough. It would be nice to eliminate this possibility under all conditions.
- If we could modify/replace the temp sensor thermistor so that it produces the resistances we're looking for under the conditions above, we could solve this problem. But we won't know if the system is sensitive enough to require this without testing this mod in <=14F temps.
Ebag333- again, I've seen no evidence of people running the 1k resistors without problems in the OBD2 trucks we're trying to figure out here. Please do point me to some if I'm wrong- I'd love to make this simpler!
I'm going to try a 10K resistor (approx. 250F) and report back.
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kup99- what are your ambient temps?
{edit:}
kup- I see you're in Calgary- perfect! If you can check your mod (do a few start stop cycles and some mixed driving) when the temps get to around -10C, then we'll have something closer to an answer. With a 4.7k resistor, you're looking at around a 280F EGR temp. I would think that would produce a:
"P0402: Excessive Flow
When the EGR is off, the ECM measures EGR temperature. If the EGR is
open, the EGR temperature will be higher than expected. If the problem
occurs on the second trip, DTC P0402 will set and the MIL will turn on."
but hopefully I'm wrong.
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I have no way to measure the temp of the egr. what can I use? how do I do it?
I drive my 80 everyday!! I drive it about 10 min at 80 km/hour, and a few minutes before and after at 50Km/hour. I have driven this everyday for about a week and a half with no CEL. Does any of this help?
