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Discussion Starter · #1 ·
I have a 1996 Corolla 1.6 Eng. with 241,300. miles on it. I have been chasing a P0401 code EGR Flow Insufficient problem some time now without any solution, so I'm posting here to see if you folks may have any suggestions.

History: Months ago I had a check engine light that was for a P0401 code. To date either I or my local mechanic have done the following:

*Checked (verified faulty) and replaced the VSV (vacuum switching valve).

*Checked the EGR work fine, call stalls when vacuum is applied, also check passageways for carbon but they were clean.

*Broken diaphragm in vacuum modulator (replaced modulator)

* Inspected and replaced any marginal vacuum hoses

* Inspected and cleaned all throttle body vacuum ports.

At this point I'm at a loss as to what might be causing the problem or what to try next. Any suggestions would be greatly appreciated at this point as I have run out of options.
 

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1994 Corolla DX
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Every time I see a post like this I get very thankful that my 94 Corolla doesn't have an EGR system :)

Use the "Search this forum" functionality on the upper right hand side, and put in your P0401 code. You will see that this is something numerous users are struggling with, and chasing. Read up on those posts to see what might help...
 

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1997 Corolla
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How does the computer detect insufficient EGR flow? Is there some sort of temperature sensor on the air passage that may not be working?
 

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1997 Corolla
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There should be a temperature sensor build into the EGR valve. Is there a wiring connector leading to it. You can test the resistance. I'm not sure what the values should be, but they shouldn't be 0 or infinite, and they should change cold versus after driving around for a while (under condition where the EGR valve opens).
 

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Discussion Starter · #7 ·
There is no temp sensor on the EGR valve itself. Both the upstream and downstream O2 sensors have been monitored and they seem to be working within specs as well as the temp sensor. That's the frustrating part, each component when checked appears to be within specs. Can't seem to find anything that's even marginal???
 

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1997 Corolla
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The Chilton manual shows an EGR gas temp sensor on the wiring diagram for 96-97, and it says to disconnect the wiring for it in the EGR valve removal instructions. Are you sure it's not hidden somewhere underneath?

Maybe someone with your year engine or a Toyota factory manual can verify if there is a temp sensor somewhere.
 

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1997 Corolla
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I guess Chilton is wrong (not unusual). Perhaps only California cars have the sensor, but Chilton shows it and lets the reader figure out if they have one or not.

In this thread others said they don't have the sensor:
http://www.toyotanation.com/forum/1...3-1997/391165-egr-gas-temperature-sensor.html

But I'm still curious how the computer knows the EGR gas flow is insufficient without a temp sensor. Does it use other sensor data to surmise it?

How long have you owned the car? Did it not have this code for a period while you owned it, then it appeared, or has it had this problem the whole time you've had it? Just wondering if a previous owned may have modified something.
 

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Count me in as one with a P0401 code, been driving the car with the code and CEL on for probably 2 years now. I've replaced the vac modulator and the VSV, plus cleaned out the EGR valve itself with no joy yet. My next step will be cleaning out the actual IM and the EGR port itself as it may be blocked up. I'm happy to see that the IM comes apart in halves, rather than having to remove the entire IM from the engine head. I've done that before on my Integra and it's a royal PITA, especially getting that old gasket off of the head.

There is a series of articles on Motor.com from awhile back that goes into great detail about diagnosing and repairing problems with Toyota EGR systems, including the P0401 Insufficient Flow code. Someone linked them in another thread here on TN. I probably need to read through them all fairly closely, although some of the stuff in the articles gets to be really technical.
 

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The sensor is how the ECM determines there's insufficient EGR flow. It's on the intake-side of the system and if not enough exhaust-gases flow to heat up the sensor, it throws that code.
What sensor? Neither the EGR valve nor the vacuum modulator on the OBD-II 7A-FE have any electrical connections. Only the EGR VSV has a connection, a solenoid use to vent off vacuum.
 

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Porsche
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These two articles deal specifically with the vacuum-modulated type of EGR systems on the 7A-FE:

http://www.motor.com/article_pdf_download.asp?article_ID=1503
http://www.motor.com/article_pdf_download.asp?article_ID=1515
Those articles are irrelevant because it's for a 2000 Camry with 5S-FE. Neither the word "Corolla" nor "7A-FE" show up anywhere in either article. That's also the next generation compared to the 7th-gen Corolla. Even within the same 7th-gen Corolla, the '93-95 EGR system works in the opposite way as the '96-97 cars. So one generation later with a completely different model car and engine doesn't relate.

Also researching the other "EGR" threads and reading the OP seem to indicate that the 1.6L 4AFE doesn't have EGR temp-sensor.

What sensor? Neither the EGR valve nor the vacuum modulator on the OBD-II 7A-FE have any electrical connections. Only the EGR VSV has a connection, a solenoid use to vent off vacuum.
Where are you getting this information? Take a look at this 7AFE ECM-pinout. What do you see on the right-hand column just beneath the TPS???



Then look up the actual schematic and trace the black/red THG line from the ECM and what is it connected to at the other end of the wire?



Let's have a little background info on feedback loops. The ECM is blind, it doesn't have the same senses as humans and it relies on feedback of its actions. When it sends out an output signal on one of its wires, it's very helpful to get back some feedback on an input line to see if things are working as planned. Such as when an injector signal is sent out, the O2-sensor sends back data that the ECM can use to verify that the injectors indeed squirted sufficient fuel for a 14.7:1 mixture. If not, the ECM can then readjust the next fuel-squirt to get on target.

Same thing with ignition. When the ECM sends out an IG pulse to dump the coil, the ignitor sends back and IGF pulse to confirm it indeed do that.

On the EGR system, the ECM sends out a duty-cycle signal to the VSV that controls the actuator. Then it gets back an EGR-temp signal from the EGR-temp sensor. The temperature tells the ECM how much exhaust gas is flowing through the EGR valve. Simple.

Here's a picture of where the EGR temp-sensor is located.


Now, read this entire Autozone 1996 Corolla Repair Guide - Exhaust Gas Recirculation (EGR) System. You will notice that the very beginning of the article refers to the "EGR Temperature Sensor" and the very end of the article shows how to test it.
 

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94-95 7A-FE EGR valves have a EGR temp sensor. 96-97 7A-FE EGR valves do not, apparently. I do not see one anywhere in the vicinity of the EGR valve on my 96 1.8L.

But those motor.com articles still deal with the same type of EGR system that is in 96-97 Corollas. The article is about Toyota EGR systems in general, but they happen to be looking at a 2000 5S-FE engine.
 

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1997 Corolla
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Looks like maybe you could drill and tap the newer part for the temp sensor, in a pinch?
 

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Porsche
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This may be a Cali vs. Fed emissions difference. The ECM pinout was from '96, the wiring-schematic from an '97 and the Autozone repair-procedure was for an '96. Without the sensor, I wonder how the ECM knows how much exhaust is flowing?
 

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Well I cracked open the manifold on my wife's car today, and man it was NASTY inside! Sure enough, EGR pipe was gunked and clogged up solid. I cleaned the insides out the best I could, including the clogged EGR pipe. I cleared out the trouble codes, then drove it around 2 separate times this evening for a total of 15 miles, and I think it's FINALLY fixed!

It would have cost me nothing, except for tearing one of the original hoses between the valve cover and IM, and also breaking the PCV valve in the process. So about $12 for some new hoses, new clamps, and new PCV valve.
 
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