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MIL P0401- EGR Low Flow Detected

(1999 Toyota Camry)
in Mead, WA on April 08, 2010
I just changed the engine in my '99 Camry with 231K mi, went and runs smooth as silk. But on the test drive the check engine light illuminated. Using my scan tool revaled code 401- Insufficient EGR flow. I cleared the code and began troubleshooting the EGR system. First i verified the EGR valve was clean, so i removed it and cleaned all the passages. Verified the diaphram actuates and will hold vacuum. reinstalled EGR valve, applied vacuum at idle to it and verified the engine ran rough and died.
Next i tested the vacuum modulator, cleaned the filter, all checked good. went to local napa and bougt another and installed it (takes about 60 seconds). During test drive MIL illuminated and same code 401. Reinstalled the old vacuum modulator and returned the new one (was $61.99+tax).
Removed the EGR VSV (vacuum switching valve) and tested. Passed flow tests properly, resistance was exactly in the middle of the range and had no continuity with the outer housing and the electrical pins.
Tested actual functionig of the EGR valve with the engine warm and running, I put my finger under the diaphram to feel it and raced the engine, as it revved up the EGR actuated open, as the rpm's climbed the egr actuated back to shut as it should over 4200 rpm.
The only thing that i can find that would cause a MIL p0401 at this point is the sensor sending an incorrect reading of the EGR's correct function, which should be a EGR temperature sensor (thermistor). But I cannot find any such sensor on the manifold, or in any repair diagrams. I have found parts listed for the 3.0L V6 engine but not the 2.2L 4 cyl.
So my question is, what reports EGR flow to the ECM on the 2.2l engine.
Displaying 3 answers & 4 comments
Popular Answer
on April 08, 2010
See this article I composed for P0401 on Toyota. I do Emissions work in N Calif.

As for what reports to the PCM well, there is more than 1 component and there is more than 1 phase. Toyota uses at least 2 phases for their EGR Monitor.
Firstly, when you put Vacuum on the valve , it MUST DIE IMMEDIATELY, not just run rough and eventually die. This is critical. Also no more than 5-7 inches of vacuum should do the trick for maximum opening. If you have to use say 12 inches, that means the EGR valve spring has hardened and that is a fail.

Also, ALWAYS USE RED AND WHITE BOX TOYOTA PARTS or you will tear your hair out, believe me when I say this. The Toyota specs are very, very tight for their Emissions devices. AFTER MARKET WILL NOT CUT IT, in many cases, and then your are fighting more than 1 gremlin i.e. less than proper spec parts and the original problem which = nightmare of confusion.

Now as to what reports to the EGR there are 2 things primarily.
When you are at high way speed, and you do a coast down with no throttle applied, the EGR valve is lifted to its maximum opening and then the MAP Sensor looks at the rise in manifold pressure. That is number 1. there should be a minimum 3 " change. You can do this with 2 vacuum gauges and run the engine at 3000 rpm in a service bay open the valve with a vacuum pump and there should be at LEAST a 3" change in vacuum. This is a test I do for the State on a frequent basis. ( I have Smog Failure cars sent to me by the State of CA to Diagnose )
Number 2 is short term fuel trim which the amount the front oxygen sensor changes( dips actually) when the EGR is applied during city driving cruise speeds.

Now, did you verify that the there is a good vacuum source to the solenoid?
Did you actively test the solenoid with a vacuum gauge teed into the EGR System while you r are driving. I have seen the VSV be very intermittent, or they will not work correctly as they warm up. Just because VSV ohms are good does not mean it works correctly in the heat of battle. Also, is the Computer giving the correct signal to the VSV at the right time? How is the exhaust source? Was the engine a complete match, because if it is from say, Japan, then the Cam timing may not be the same and the EGR will never work correctly.
I wish I could be there because there are lots of things still to check. The EGR system on your Camry is very subtle and I have spent years learning to deal with them. I do lots of EGR Diagnosis for other shops that get stumped. This can be a bear.
on April 09, 2010
Thanks LOTS! That was an excellent response and tells me more than I had already found out this morning at my local shop.
First, the mechanic tested the vacuum pressure at the egr while driving (after first checking the system in the shop), and found only 2" of vacuum when the vsv cycled open. Figures that there is a restriction in the vsv itself. Since this was my only part left to be changed in the egr system, and probably this vsv has about 231k mi on it (if origional), i will pony up and buy a vsv from the local Toyota dealer.
Second, since i replaced the engine with a 2.0L 3SFE-G engine from japan ('87 camry engine), the tech who checked my car has concerns about the MAP reading being lower than the ECM is lookig for since the displacement is %10 less.
I will change the VSV and report back.
on April 09, 2010
Ok... or not! I replaced the EGR VSV (about $85 with tax). drove the car, no light. Let it cool down to cold, and went for another drive. Check engine light came on after about 5 miles.
So the question is, is there a problem because the engine is a conversion engine and 2.0L rather than 2.2L? Heard of the possibility that because of the displacement diffrence the MAP sensor will never give the ECM a signal within its programmed parameters.
Doesnt the vacuum in both engines reach the same pressure since that is really affected by the volumemetric efficency of the intake tract, and with the throttle plate at various settings, which should have almost no measureable effect? Loss of efficency due to wear could also cause a similar effect to the MAP reading too, couldnt it?
on December 31, 2010
All I know is I had the same problem. Ohm out the VCV valves!! If the ohm's are way above 40 ohms replace it!
on November 17, 2016
I have same problem with mine camry ....code 0401 I need buy vcv valve for fix? I ready spending 400 $$%
on December 31, 2010
Every part of my EGR system was working when I tested it but I still got the low flow MIL light randomly. I finally bought a new egr valve, filter and a new vacuum switch valve and replaced one by one. When I replaced the vacuum valve - my problem went away and stayed away. I tested the old valve in the house connected to a 12Volt power supply and that valve did work - but it didn't open as far as it needed to - resulting in a low flow to the EGR valve. This was a 1999 Camry - Japan built and it has the 4 cylinder engine. If the EGR valve can be moved by hand, and it stalls the engine when moved manually by hand (by pressing the diaphram by hand or using a vacuum hand pump - then the vacuum valve would be the next likely problem. To get to the valve, I had to remove the passenger side tire, then reach straight in at the upper back side of the engine. The valve has two hoses connected to it (mark one so you don't cross them later). I had to relocate the valve because I just couldn't get my hand back in far enough to get the bolt started while the valve with hoses was trying to move in the opposite direction - but the new valve is now up on top of the engine - easy to get to - and I've had several months with no MIL errors of any kind. Maybe it's carbon build-up that eventually reduces flow in that vacuum valve - I really don't know why it had reduced flow - but I am positive that is what finally fixed that problem. I used only geniune Toyota parts - this engine is already notorious for barely passing smog - I didn't want to introduce another issue with an aftermarket part that may not have been quite the same tolerances. So - I guess the moral of this repair was that even with that valve "apparently" working - it wasn't working enough to give the engine what it needed.
on October 15, 2014
in 27 years working on Toyotas I've never seen a bad e.g.r. valve, that said it doesn't mean there cant be one, just that its unlikely, on the Camerys with the 5sfe in line 4 about 85% of the time its the vsv,(under the intake), there is no vacuum solenoid, but he is right about the vsv check, its useless, and if its not this you will be better off taking it to a professional.
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