Short version.
If you don’t have the time to read this post, it was the vacuum modulator.
Long version.
Background.
1997 1FZ-FE, 80K miles, MIL = P0401, EGR gas underflow detected.
History.
Have suffered the little idiot light for about 6 months.
Observations.
The most common fault reported among Cruiser owners - and their mechanics - is failure of the vacuum switching valve (VSV for EGR), occasionally carbon build up in the EGR pipe, intake ports, or elsewhere (though most commonly on higher milage rigs), and even less often, the EGR temperature sensor, or possibly (but rarely) the vacuum modulator. All of these preclude the more obvious disconnected or cracked and leaking vacuum line(s).
Troubleshooting.
Working from the easiest, to the most obvious...
1. The vacuum lines are all connected, not brittle or cracked, nor plugged.
2. The EGR temperature sensor measures within tolerance on an ohm meter.
Digression. In my haste to solve this problem last December, I misread the FSM as to the allowable tolerance, and happily announced my solution. Even an over exuberant Gumby went dashing out “into the cold Chicago night” to measure the resistance on his temperature sensor, only to find that some fool out West had misread the book and had sent him out there for nothing. In his carpet slippers, no doubt. ( I still don’t feel particularly bad about it though, as it was snowing here at the time...)
3. Replaced the VSV. As soon as the valve came out, I realized this was not it because the resistance of the coil was within the published tolerance. The new one went on anyway, because this is the most common cause of this particular fault, and I did not want to mess with this particular PITA a second time.
For those who would like to know, the swap was done with intake manifold on - removing the valve bracket from below with a very long extension. It took two of us a little over 2 hrs, with some horsing around... and the MIL returned almost immediately, as expected.
4. Decided to pass on the EGR valve/pipe because with such low milage, I did not expect carbon build up to be an issue. In fact, when the temperature sensor came out, there was almost no carbon present, confirming this observation. The difficultly in removing the EGR valve/pipe also helped sway the decision to pass on this step.
5. Pulled the vacuum modulator (simply slips out of its holder after removing the vacuum lines). Removed the filter in the top, blew it out and replaced. Blew through the valve (as described in the FSM), and though extremely subjective, it appeared to be ok.
This initially appeard to have solved the problem... but after about 200 miles, the MIL returned with another P0401 from the OBD.
Final Solution.
Ironically, the solution - in theory anyway - comes from Gumby. Here is an observation he made last month:
I'm taking an advanced FI course for Fords this week and today we talked about EGR for a bit. Fords have terrible EGR systems too, so I learned a lot. Some problems they are having with low flow include small cracks in the EGR tube, Their version of the VSV, and their pressure monitoring system. They no longer use the temp sensor, but use a MAP type piezio crystal sensor. Their VSV gets plugged on the vent that releives the vacuum to the EGR when the valve shuts. I assume that's what the modulator does on the Toyota system. That would account for high flow, though. I wonder if a leaky modulator would not let the EGR open enough?
“A leaky modulator would not let the EGR open enough?” = EGR underflow = P0401
1. Again, remove the vacuum modulator.
2. Blow out all passages really good. What I mean by this, is firmly plug two of the four ports with the tips of your fingers, then blow about 20-25 psi through one of the open ports in a couple short, quick bursts, then from the other side, blow back in the other direction. Proceed to block each of the ports, and force air through the open pair in both directions, making sure you have accounted for every possible combination of blocked/open ports.
You will be able to hear the internal valve popping open and closed, in some cases letting the air pass, and in others blocking it completely. My suspicion was that the valve was not completely opening (or closing/seating) - perhaps from dislodged carbon, or whatever - and allowing the modulator to “leak” when it should have been sealed.
3. Reinstall the modulator, and clear the MIL.
It is now 1000+ miles later - up and down 4K-10K foot grades in the Sierra Nevada, more drive cycles than I can count - and no MIL / engine check light. No more P0401 codes. No more “soft” codes from OBD either, for that matter.
It was a “leaky” vacuum modulator. Thanks Gumby. That will learn us to bone up on theory... and make damn sure your students know I said so.
Cheers,
R -
If you don’t have the time to read this post, it was the vacuum modulator.
Long version.
Background.
1997 1FZ-FE, 80K miles, MIL = P0401, EGR gas underflow detected.
History.
Have suffered the little idiot light for about 6 months.
Observations.
The most common fault reported among Cruiser owners - and their mechanics - is failure of the vacuum switching valve (VSV for EGR), occasionally carbon build up in the EGR pipe, intake ports, or elsewhere (though most commonly on higher milage rigs), and even less often, the EGR temperature sensor, or possibly (but rarely) the vacuum modulator. All of these preclude the more obvious disconnected or cracked and leaking vacuum line(s).
Troubleshooting.
Working from the easiest, to the most obvious...
1. The vacuum lines are all connected, not brittle or cracked, nor plugged.
2. The EGR temperature sensor measures within tolerance on an ohm meter.
Digression. In my haste to solve this problem last December, I misread the FSM as to the allowable tolerance, and happily announced my solution. Even an over exuberant Gumby went dashing out “into the cold Chicago night” to measure the resistance on his temperature sensor, only to find that some fool out West had misread the book and had sent him out there for nothing. In his carpet slippers, no doubt. ( I still don’t feel particularly bad about it though, as it was snowing here at the time...)
3. Replaced the VSV. As soon as the valve came out, I realized this was not it because the resistance of the coil was within the published tolerance. The new one went on anyway, because this is the most common cause of this particular fault, and I did not want to mess with this particular PITA a second time.
For those who would like to know, the swap was done with intake manifold on - removing the valve bracket from below with a very long extension. It took two of us a little over 2 hrs, with some horsing around... and the MIL returned almost immediately, as expected.
4. Decided to pass on the EGR valve/pipe because with such low milage, I did not expect carbon build up to be an issue. In fact, when the temperature sensor came out, there was almost no carbon present, confirming this observation. The difficultly in removing the EGR valve/pipe also helped sway the decision to pass on this step.
5. Pulled the vacuum modulator (simply slips out of its holder after removing the vacuum lines). Removed the filter in the top, blew it out and replaced. Blew through the valve (as described in the FSM), and though extremely subjective, it appeared to be ok.
This initially appeard to have solved the problem... but after about 200 miles, the MIL returned with another P0401 from the OBD.
Final Solution.
Ironically, the solution - in theory anyway - comes from Gumby. Here is an observation he made last month:
I'm taking an advanced FI course for Fords this week and today we talked about EGR for a bit. Fords have terrible EGR systems too, so I learned a lot. Some problems they are having with low flow include small cracks in the EGR tube, Their version of the VSV, and their pressure monitoring system. They no longer use the temp sensor, but use a MAP type piezio crystal sensor. Their VSV gets plugged on the vent that releives the vacuum to the EGR when the valve shuts. I assume that's what the modulator does on the Toyota system. That would account for high flow, though. I wonder if a leaky modulator would not let the EGR open enough?
“A leaky modulator would not let the EGR open enough?” = EGR underflow = P0401
1. Again, remove the vacuum modulator.
2. Blow out all passages really good. What I mean by this, is firmly plug two of the four ports with the tips of your fingers, then blow about 20-25 psi through one of the open ports in a couple short, quick bursts, then from the other side, blow back in the other direction. Proceed to block each of the ports, and force air through the open pair in both directions, making sure you have accounted for every possible combination of blocked/open ports.
You will be able to hear the internal valve popping open and closed, in some cases letting the air pass, and in others blocking it completely. My suspicion was that the valve was not completely opening (or closing/seating) - perhaps from dislodged carbon, or whatever - and allowing the modulator to “leak” when it should have been sealed.
3. Reinstall the modulator, and clear the MIL.
It is now 1000+ miles later - up and down 4K-10K foot grades in the Sierra Nevada, more drive cycles than I can count - and no MIL / engine check light. No more P0401 codes. No more “soft” codes from OBD either, for that matter.
It was a “leaky” vacuum modulator. Thanks Gumby. That will learn us to bone up on theory... and make damn sure your students know I said so.
Cheers,
R -
except Woody hasn't given us back our smilies yet.
