You guys that keep sending people to the FSM and the search features! Sheesh! If we did that, what would we talk about???
Alright....I'll go read it.
Alright....I'll go read it.
How does the EGR 'sense' insufficient flow (P401 code)?? (1 Viewer)
- Thread starter IdahoDoug
- Start date
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Exactly the post I had in mind.
So did anyone else ever decide that that was the way to go?
(Sorry if I'm not up on the latest posts in your thread.
)Curtis
- Thread starter
- #43
I think one of the most fix-resistant aspects of the EGR system is the fact that you can't play around with it and suddenly find the component that's the problem. Unlike say a stereo where you have a blown speaker. With the stereo problem you replace the speaker, turn the stereo on and find the sound is still there. 30 minutes later you have a new stereo in place and turn it on and you've found the problem.
With this issue, you can't 'turn it on and see if it works correctly'. You do something, reset the code and go on with life. A few weeks later the CEL comes on again and you curse the truck and a few days later you mess with the EGR system again and reset the code. In other words, its a long repair cycle for the do it yourselfer who doesn't have an OBD reader and the hours to remove some of this stuff.
Frustrating. Atop that is the stigma of having a check engine light on while you're driving the world's most reliable vehicle. So you can't let it go, yet it is a pain to deal with. Hopefully I'm going to find a way to handle this, because I am NOT going to drive around with that damn light on anymore.
DougM
With this issue, you can't 'turn it on and see if it works correctly'. You do something, reset the code and go on with life. A few weeks later the CEL comes on again and you curse the truck and a few days later you mess with the EGR system again and reset the code. In other words, its a long repair cycle for the do it yourselfer who doesn't have an OBD reader and the hours to remove some of this stuff.
Frustrating. Atop that is the stigma of having a check engine light on while you're driving the world's most reliable vehicle. So you can't let it go, yet it is a pain to deal with. Hopefully I'm going to find a way to handle this, because I am NOT going to drive around with that damn light on anymore.
DougM
You mean this section? This actually very clearly describes what we have been asking. The introduction also has a great section on testing for electrical problems. Testing all the components in the EGR system is not an easy or 15 minute task either. That's probably why the problem is so persistent. Lots of effort without seeing a huge pay off. We put in the effort and the light goes out. So....do we see a jump in MPG? Longer Cat life? Longer life for O2 sensors? Or, does the light just go off? I pass emissions with the light on.
I keep pulling the fuse. I hate seeing that dubious orange glow when I'm driving at night. It's been there since I got the vehicle. And I don't have the money to change everything that could be wrong.
I didn't see Doug's post before I finished mine...that's funny.
My resistor trick might be you're answer. Less than $5 and 5 min.
DING DING DING!!
If you read more about the system, I think you'll conclude like I did that even with a p401 the computer stays in closed loop. With the system in closed loop, the engine should be runing normal and so also during emissions testing. If they stick the sniffer in the tail pipe at idle there is NO EGR fuction. The engine is under light load and according to the FSM there is no EGR function. Now if they somehow have a two roller setup for a full time 4x4, you will fail when the EGR should be flowing.
Yes, I have the '95 version. This is a huge section with lots of details. I imagine if I actually read the FSM I wouldn't have to ask so many questions in tech.
''Ding! Ding!''
But back to my previous question, what are we buying by fixing this? I know - we all like our 80's to be like new and the light is embarrassing, but are we wearing anything out or losing significant MPG's? or, is it just a light?
''Ding! Ding!''
But back to my previous question, what are we buying by fixing this? I know - we all like our 80's to be like new and the light is embarrassing, but are we wearing anything out or losing significant MPG's? or, is it just a light?
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The EGR is a US emissions requirement, most other markets don't require it. You're helping the enviornment by having a fully functioning system. However with my 11-12mpg beast it would be an oxymoron trying to have an eco-friendly vehicle.
Again, the system is in closed loop so all you really have with a p401 is an annoying light in the dash.
of note when I put the new exhaust manifold on my 1fzfe this past summer it didn't come with a grind in the #6 flange to pass exhasut from the #6 exhaust port to the egr port in the head. I took a die grinder and made a shallow 1/8" high by 3/16" wide ditch (The oe flange has a port 1/2" wide by 1/4" deep) in the flange to serve as this connection figuring it likely was going to be insufficient but that was all the hot gas I would allow into the intake and if it gave me a code I would give it a resistor. Well my truck hasn't thrown a code with this little passage. I dont know what to make of this
Doug couple things,
nothing in the system is ever actually exposed to true intake manifold vacuum, the vacuum source is from small ports near the throttle plate that create a variable vacuum signal that changes at the throttle plate passes over these ports, the modulator take these and exhaust manifold pressure and figures out load and passes a vacuum to the EGR, resulting signal looks like this
Idle, no vacuum,
Moderate throttle to WOT, no vacuum
Regular acceleration ~5 inches of mercury
Cruise, lowest pressure, on a slight downhill as low as 12 or 13 inches of mercury on steep down hills the pressure would increase, and uphill it would also increase
The VSV actually shuts down the system by opening and passing ambient pressure air to the EGR
Flow insufficient/excessive is a little misleading naming, it is not a fine measurement, it is a little more of a on/off thing, excessive would be more appropriately names EGR on when is should be off, insufficient would be off when it should be on,
what is on or off is just is the sensor "hot" (finger tip searing) or "cold" not a fine measure of temperature.
actually you can instantly see feedback if your repair has worked by watching the vacuum signal to the EGR valve
I wish more people would try my EGR test method, I really feel it is the best way to tackle it, but as of yet it has not been popular,
https://forum.ih8mud.com/showpost.php?p=1535318&postcount=10
my VSV passed the FSM tests but my own vacuum test found that in actual use it was intermittent causing a P0401
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- Thread starter
- #55
Raven,
Thanks - I'll read that thread tomorrow.
Quick question or clarification on the VSV. It looks to me like when it opens (sensor is hot), it passes a 'low vacuum' to the EGR valve, which lets the EGR valve open more easily/farther. And when the sensor is cold, the VSV remains closed which means the EGR gets ambient pressure. The low vacuum seems to be provided by the tiny ports in the intake upstream of the throttle plate. Gimme your thoughts on that because I want to make sure I have the operation of this system down and your post above says the VSV opens to shut down the EGR - opposite what I just typed.
I find it interesting that you also noted there's no direct intake vacuum operating the system, instead using this 'low vacuum' from the 4 tiny ports. I think the system would have been more robust to minor leaks had Toyota designed it to operate at higher and more leak-tolerant vacuum levels that have more muscle to move the vacuum valves and modulator. It seems to me that at these low levels of vacuum even a hardened hose will throw the system off by reducing already weak "power" to a level that a speck of carbon can resist. Tying in the EVAP system also using this low vacuum and sharing a hose is just asking for trouble by doubling the number of potential leaks. Makes me want to replumb it, ya know??
DougM
Thanks - I'll read that thread tomorrow.
Quick question or clarification on the VSV. It looks to me like when it opens (sensor is hot), it passes a 'low vacuum' to the EGR valve, which lets the EGR valve open more easily/farther. And when the sensor is cold, the VSV remains closed which means the EGR gets ambient pressure. The low vacuum seems to be provided by the tiny ports in the intake upstream of the throttle plate. Gimme your thoughts on that because I want to make sure I have the operation of this system down and your post above says the VSV opens to shut down the EGR - opposite what I just typed.
I find it interesting that you also noted there's no direct intake vacuum operating the system, instead using this 'low vacuum' from the 4 tiny ports. I think the system would have been more robust to minor leaks had Toyota designed it to operate at higher and more leak-tolerant vacuum levels that have more muscle to move the vacuum valves and modulator. It seems to me that at these low levels of vacuum even a hardened hose will throw the system off by reducing already weak "power" to a level that a speck of carbon can resist. Tying in the EVAP system also using this low vacuum and sharing a hose is just asking for trouble by doubling the number of potential leaks. Makes me want to replumb it, ya know??
DougM
When the VSV opens there is no EGR flow. It opens and ports ambient pressure to the EGR valve defeating the vacuum signal from the modulator, interestingly enough also from the TB it is from a drilling in the TB that leads to the idle air port but upstream of the idle air control valve so it is an ambient pressure.
You can actually plug the drivers side port on the EGR valve (that leads through the intake to the VSV) with a cap and the EGR system will work as it is suppose to, only difference is the EGR would operate during warm up (it is the job of the VSV to prevent EGR operation until the engine has warmed up some) and throw a P0402 "EGR flow excessive" (EGR on when it should be off)
I was thinking about this last night after posting, without any true unlimited vacuum source the system cannot be activated at idle no matter what failure happens in the vac system with the exception of a leaking egr valve itself, this means no stalling at idle witch could be a accident hazard, nobody want to go for a hole shot and wind up stalling in front of oncoming traffic.
Comment from Raventai's post that explains one of the disadvantages to leaving the P0401 alone:
"The leaking exhaust gasses would have eventually fouled the whole system with carbon witch would have added other problems on top of the existing problems making troubleshooting even more difficult, but I caught it early enough where the fouling was limited to just the modulator and its filter."
carbon buildup = bad mojo.
"The leaking exhaust gasses would have eventually fouled the whole system with carbon witch would have added other problems on top of the existing problems making troubleshooting even more difficult, but I caught it early enough where the fouling was limited to just the modulator and its filter."
carbon buildup = bad mojo.
So raven,
I just read your thread, thanks for all the research and sharing it with the rest of us. I noticed you did not test the VSV. Were you able to perform all the other tests without removing the intake manifold? I've been there done that and what a pain. I'm thinking I'll try your tests along with the tests in the FSM and see if I can solve my code issues, but if I have to remove the intake I'll wait till the weather warms up. I'm a sissy without a garage. Thanks.
I just read your thread, thanks for all the research and sharing it with the rest of us. I noticed you did not test the VSV. Were you able to perform all the other tests without removing the intake manifold? I've been there done that and what a pain. I'm thinking I'll try your tests along with the tests in the FSM and see if I can solve my code issues, but if I have to remove the intake I'll wait till the weather warms up. I'm a sissy without a garage. Thanks.
I'll just throw in that at 90k my sister's '95 kicked on the p401. Dealer cleaned the intake runner for the temp sensor and replaced the modulator. Now at 230k she has never had another p401.
My damn '97 kicks the p401 on every now and then, some times after a few thousand miles and some times after less than a hundred miles. My light came on around 80k and I had the new style modulator that was replaced by the older model. I started with the modulator, cleaned the intake runner, cleaned the temp sensor. There was no carbon in the vacum lines, vsv tested fine, temp sensor tested fine. So I have the resistor in and I'm done with it.
My conclusions points to what others here have noticed. '97 ECM's have an increased sensitivity for the egr program and the p401 is a constant annoyance.
My damn '97 kicks the p401 on every now and then, some times after a few thousand miles and some times after less than a hundred miles. My light came on around 80k and I had the new style modulator that was replaced by the older model. I started with the modulator, cleaned the intake runner, cleaned the temp sensor. There was no carbon in the vacum lines, vsv tested fine, temp sensor tested fine. So I have the resistor in and I'm done with it.
My conclusions points to what others here have noticed. '97 ECM's have an increased sensitivity for the egr program and the p401 is a constant annoyance.
I bought my truck with 95k on it with the CEL. Replaced the modulator, vsv, and a bunch of vacuum lines. I now have 155k miles and the light hasn't come back.
