How does the EGR 'sense' insufficient flow (P401 code)?? (1 Viewer)
- Thread starter IdahoDoug
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Mine is a 93. however i failed the smog test due to low EGR flow.
If you pull vacuum on the EGR valve at idle you should stall the engine, if not i would suspect EGR system blockage requiring cleaning.
Mine was flowing just enough to make it run crappy but there was not enough dilution at cruise to bring combustion temps down creating a high NOX condition, FAIL.
I would agree that your system must read temp, possibly temp at vehicle speed?
Remove EGR valve spray in carb cleaner or combustion chamber cleaner.
If you pull vacuum on the EGR valve at idle you should stall the engine, if not i would suspect EGR system blockage requiring cleaning.
Mine was flowing just enough to make it run crappy but there was not enough dilution at cruise to bring combustion temps down creating a high NOX condition, FAIL.
I would agree that your system must read temp, possibly temp at vehicle speed?
Remove EGR valve spray in carb cleaner or combustion chamber cleaner.
Doug this is a pretty simple circuit. Not aimed at you but don't over think it.
Lets assume the engine is up to operating temperature.
Once you press the accelerator the EGR system is activated. By this I mean the VSV is now turned on.
The ECM is pre programed to expect a certain temp reading, within a window, from the sensor for any given situation.
Now if you know the VSV is good and operating properly then there is a limited number of possibilities for the problem.
1. bad temp sensor. It's giving a false signal to what is really happening
2. bad modulator. It's not opening the EGR valve appropriately to the input from the exhaust line.
3. bad EGR valve. not opening or a clogged orifice limiting the amount of exhaust gas.
4. clogged modulator exhaust line. Restricted exhaust flow to modulator diaphragm limiting EGR valve operation.
5. Restricted exhaust feed to EGR valve. Everything is functioning normal but there is limited exhaust available when EGR valve is open to satisfy temp conditions.
One thing I should mention is that all OHM readings should be done at the ECM. The reason is that you want to evaluate the entire circuit not just the component. This is extremely important since the ECM will not know the difference between a high reading from the sensor or that of a bad connection.
Incidentally this is where I had some problems. I was trouble shooting a P0401 for another member and instinctively I went to the connectors at the ECM to evaluate the VSV coil and there were errors in the schematics and FSM. I was a little rushed and didn't get a chance to find the errors but did identify the problem.
Lets assume the engine is up to operating temperature.
Once you press the accelerator the EGR system is activated. By this I mean the VSV is now turned on.
The ECM is pre programed to expect a certain temp reading, within a window, from the sensor for any given situation.
Now if you know the VSV is good and operating properly then there is a limited number of possibilities for the problem.
1. bad temp sensor. It's giving a false signal to what is really happening
2. bad modulator. It's not opening the EGR valve appropriately to the input from the exhaust line.
3. bad EGR valve. not opening or a clogged orifice limiting the amount of exhaust gas.
4. clogged modulator exhaust line. Restricted exhaust flow to modulator diaphragm limiting EGR valve operation.
5. Restricted exhaust feed to EGR valve. Everything is functioning normal but there is limited exhaust available when EGR valve is open to satisfy temp conditions.
One thing I should mention is that all OHM readings should be done at the ECM. The reason is that you want to evaluate the entire circuit not just the component. This is extremely important since the ECM will not know the difference between a high reading from the sensor or that of a bad connection.
Incidentally this is where I had some problems. I was trouble shooting a P0401 for another member and instinctively I went to the connectors at the ECM to evaluate the VSV coil and there were errors in the schematics and FSM. I was a little rushed and didn't get a chance to find the errors but did identify the problem.
Called a mechanic friend and he says, flow sensor (fixed orafice type) in the EGR supply tube.
My understanding is that when the EGR valve is open, exhaust gases flow into the intake. The heat from the exhaust, is detected by the temperature sensor in the intake. If the engine is running at full temperature and and at a certain RPM range, the ECM is expecting to have a minimum temperature reading from the sensor in the intake. If the minimum temperature has not been met, a 401 code is thrown. Likewise, if temperature has been met and the ECM is not expecting it, a 402 code is thrown.
I strongly suspect this as well on the vacuum lines.
I am far from an expert, but my guess would be somewhere in EGR temp sensor. Could the coolant temp sensor tell the ECM that the truck is warmed up and if the exhaust temp is measured low by the EGR temp sensor that is interpreted as low flow? This was first posted by landtank, but I concur.
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Heh, back from BB shooting. One of our pumpkins and a cardboard box paid the ultimate price. Good times had by all.
Excellent input. I probably should have been more patient on waiting for the truck to get home and I appreciate all you folks showing patience and answering a sophomoric question. Great input by all - special mention to Rick. I think this is a solvable issue and I've got a pretty good handle on it now.
My feeling is that the system is fundamentally sound in that all the components of the EGR system are simple and they are durable. On the eve of delving into it, I'm kind of surprised at how carefully the vacuum must be managed to make the system work which is why earlier I said I suspect it will be old hoses allowing a seep or similar.
One of my suspects is carbon building up in our EGR valves, which is causing them not to open fully. Interestingly, I see that one of the tests you can perform involves applying direct vacuum to the EGR valve, which opens it farther than the system normally would. This caught my interest because it's also therefore a way to cycle the EGR valve without removing it (which is a pain to do..) So that's on my menu of tricks to try to provide a bit more EGR flow by jiggling the valve around. I may also put a simple vacuum valve on the EGR that would let me step up the vacuum applied to the EGR slightly.
At any rate, I'll be messing with this a bit later in the week. I am REALLY tired of that dash light so I'm determined to kick this thing's ass for good. Couple of you are gonna get PMs, I expect.
DougM
Excellent input. I probably should have been more patient on waiting for the truck to get home and I appreciate all you folks showing patience and answering a sophomoric question. Great input by all - special mention to Rick. I think this is a solvable issue and I've got a pretty good handle on it now.
My feeling is that the system is fundamentally sound in that all the components of the EGR system are simple and they are durable. On the eve of delving into it, I'm kind of surprised at how carefully the vacuum must be managed to make the system work which is why earlier I said I suspect it will be old hoses allowing a seep or similar.
One of my suspects is carbon building up in our EGR valves, which is causing them not to open fully. Interestingly, I see that one of the tests you can perform involves applying direct vacuum to the EGR valve, which opens it farther than the system normally would. This caught my interest because it's also therefore a way to cycle the EGR valve without removing it (which is a pain to do..) So that's on my menu of tricks to try to provide a bit more EGR flow by jiggling the valve around. I may also put a simple vacuum valve on the EGR that would let me step up the vacuum applied to the EGR slightly.
At any rate, I'll be messing with this a bit later in the week. I am REALLY tired of that dash light so I'm determined to kick this thing's ass for good. Couple of you are gonna get PMs, I expect.
DougM
Ford mechanic, i'll go slap him.
If you plan on pulling vac on the EGR then the line to the modulator will have to be pluged as i believe it may bleed off some of your vac and not open fully. Vince
haha, I was wondering when my old thread would come up with all this p401 code questions, what a bad writeup on my part too though. Read the FSM it makes clear how the egr and trip system works. Also note that the EWD has a gross error, while the FSM has the correct resistance values for the temp sensor. IF you go the resistor route, a 1k ohm is needed not a 1ohm. -- Keep in mind the resistor is a code defeat not a fix, but none the less a solution. Also I've found when the p401 is tripped and the CEL lights, the computer is still in closed loop. I guess Toyota figured it was not a critical issue to put the engine back into open loop.
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Hope this isn't too far off track:
WHO HAS ACTUALLY REPLACED THE TEMP SENSOR IN AN ATTEMPT TO CURE THE "INSUFFICIENT FLOW" ISSUE?
Just a bystander here, but it seems like pretty much everyone either...
a) Assumes it's ok,
b) Pulls, it, cleans a little carbon off, and assumes it's ok, or
c) Bench tests it (good idea but not infallible as Rick points out (test at ECM)) and assumes it's ok.
Just looking for data to help build my conspiracy theory,
Curtis
WHO HAS ACTUALLY REPLACED THE TEMP SENSOR IN AN ATTEMPT TO CURE THE "INSUFFICIENT FLOW" ISSUE?
Just a bystander here, but it seems like pretty much everyone either...
a) Assumes it's ok,
b) Pulls, it, cleans a little carbon off, and assumes it's ok, or
c) Bench tests it (good idea but not infallible as Rick points out (test at ECM)) and assumes it's ok.
Just looking for data to help build my conspiracy theory,
Curtis
I know my 92 3FZ is often belittled for lack of power. . . However, I just soldered the two wires of the EGR temperature sensor together and wrapped tape over the whole lead. No more Check Engine light! I've passed TX (probe in the tailpipe) inspections for 3 years now this way. So maybe the 3FE is not so bad.
Yeah, I reviewed the TSM for my 97 lx and all I can see that there is no way to sense pressure. I could see a couple of things that might prevent the valve from working correctly though. The check valve for the vsv off the evap, and the tubing leaking, or being clogged, and the temp sensor not working. These things, along with a clogged valve, might prevent the temp sensor from working correctly. In all the reading I have done I have not read about anyone who has changed the temp sensor. We should ask Dan if he has ever sold one. I looked elsewhere on the web (search phrase=p401 toyota, http://en.allexperts.com/q/Toyota-Repair-832/f_4498302.htm , http://www.toyotanation.com/forum/showthread.php?threadid=188864 , http://www.toyotanation.com/forum/showthread.php?t=32479 ) on the basis that Toyota likely used the basic idea on all thier cars of the same era. I found that most folks just replaced the vsv and all was ok; however I have read on mud that some have tried that saame solution without success.
Karl
Karl
Rick, so you're saying that the code is thrown when the temp isn't at a certain point? There has to be a second factor. it has to be, when the EGR temp sensor reading is out of spec AND...something else. Either engine temp, RPM's, speed, external wind speed, the temp of my rear in the drivers seat...something!
[''Speed has nothing to do with it! It's a matter of Ratios!'' Sorry, couldn't resist]
What is the magic combination? Is it temp plus air flow...that would make sense. If the temp is in range X, the air flow should be in range Y on a graduated scale.
Doug, Kudos on the BB-gun break! You are an inspiration to us all!
