egr 0401 code diagnosis 97camry - Toyota Nation Forum : Toyota Car and Truck Forums

alyce · 02-05-2008, 10:23 PM

After weeks of checking for causes of cel 0401 insufficient egr flow I was finally able to locate the culprit. All the recomended diagnostic test in the Haynes Manual and those listed on forums showed no problems. EGR Valve,Vacuum Modulator and VSV were all checked and tested OK. I then drove the car with a vacuum gauge teed in at egr valve. When engine was cold, no vacuum was detected which is normal due to the activation of the VSV. After engine warmed up, vacuum of about 3in. was registered while driving, indicating VSV was no longer activated allowing vacuum to flow to the egr valve. No vacuum was registered at idle which is normal. Understanding the operation of the VSV was the key to solving this mystery. A VERY understanding Toyota Service Advisor did a computer search and gave me a print out on the egr system.The VSV should be turned on(stopping vacuum flow to EGR Valve) by a signal from the ECM under the following conditions: Before engine is warm, during deceleration(throttle valve closed), light engine load, engine idling,engine speed over 4400 rpm, and high engine load. Soon I discovered that when the engine was warm and at idle, I was able to get vacuum flow to the egr valve by applying vacuum to the hose that attaches to the Q port of the vacuum modulator causing the engine to die.This indicates the VSV is open when it should be closed. At this point, I confirmed voltage to the VSV electrical connector with the key turned on using a voltmeter and then reconnected it to the VSV. Even though the bench test for the VSV showed no problem, I realized that the VSV failed to operate once the engine was hot, stuck in the open position. I had read about this part having a high failure rate, but now I was able to confirm its failure when hot. Now I know you will ask why there would be a insufficient flow code if the VSV was stuck open. The ECM monitor manifold absolute pressure with the MAP sensor. The MAP sensor records values with EGR open and compares them to values when the EGR is closed. In order for these changes to be detected, there must be a rapid opening and closing of the EGR valve which requires the activation of the VSV during the conditions listed above. With the VSV stuck open the MAP sensor will not detect changes in manifold absolute pressure and assume the EGR Valve is not functioning properly. If you have a vacuum gauge and monitor vacuum while driving, it should show a rapid on and off flow of vacuum as the VSV is activated and deactivated by the ECM. Replaced the VSV and problem fixed.

dc_98_cam · 02-05-2008, 10:41 PM

There have been previous posts .... which stated that it was not effectively possible to diagnose a defective VSV for EGR .... Just have to replace the part.

bnb1023 · 02-06-2008, 06:31 PM

Stupid question: What is a VSV, what does it stand for, and where is it??

alyce · 02-06-2008, 07:15 PM

Vacuum Switching Valve which is mounted on the back side of engine block under the intake manifold.

Creeps228 · 02-06-2008, 08:18 PM

thats bs i chaged my vsv and cleaned out my egr i still hav ethat code

dc_98_cam · 02-06-2008, 09:17 PM

Quote:

Originally Posted by Creeps228

thats bs i chaged my vsv and cleaned out my egr i still hav ethat code

What he's saying is that there's a little more to the 401 code problem... even though you replaced the VSV for EGR...

This is what is listed in the Gen 4 manual Diagnostic section for the 401 code:
EGR valve stuck closed
Open or short in VSV circuit for EGR
Vacuum or EGR hose disconnected
Manifold absolute pressure sensor
VSV for EGR open or close malfunction
ECM
...
So...
The MAP sensor could be defective ...first remove the electrical connector ...and clean with electrical contact cleaner... replace.
The EGR valve ... even though you cleaned it, could still be sticking.
...
Possibilities:
There is a test procedure for the MAP sensor on p. 53 of the SFI section of the repair manual.... or get a different one from a salvage yard.
Maybe find a different EGR valve to try out...

Creeps228 · 02-06-2008, 10:40 PM

im looking but, like i said before in other threads i dont trust the junk yards around my area, i need to find other ones, i'll take a look at my map sensor if it the connectors are bad adn in need of some cleaning...

alyce · 02-07-2008, 12:50 PM

The purpose of this post was to provide information on checking the VSV after all other parts of the egr system have checked out OK. This way you can confirm whether the VSV is bad or not and don't change a part that is not broken.

dc_98_cam · 02-07-2008, 03:47 PM

Quote:

Originally Posted by alyce

The purpose of this post was to provide information on checking the VSV after all other parts of the egr system have checked out OK. This way you can confirm whether the VSV is bad or not and don't change a part that is not broken.

Your post certainly provides additional information on the operation of the EGR/MAP/VSV component operation.

Then it looks like a major problem occurs when the ECM requires two different measurements ... to be compared ... One measurement when the EGR is closed, and one when open. If the VSV for EGR is not in the required position at the correct instant, then the manifold pressure measurement signals (MAP) to the ECM will not be any different, and will cause a CEL to be turned on.
If the VSV for EGR is not working perfectly, with no sticking or lagging, there could easily be an error in the synchronization of the MAP measurements going to the ECM.
If the EGR has any sort of carbon fouling that would cause its actuator to stick, not move correctly, or obstruct the flow or the exhaust gases, then the resulting manifold pressure could be either insufficient, or out of synchronization with the MAP reading by the ECM.

As you have suggested, an effective diagnostic procedure would be to connect a vacuum gauge between the EGR activation port, and the vacuum outlet port on the VSV for EGR. If at a suitable higher engine RPM, engine warmed up, the vacuum gauge does not show the required high/low switching , then something is wrong with the VSV for EGR. You would have to assume that if you did in fact observe the vacuum gauge switching, then the VSV for EGR was working adequately, but that something was wrong with the EGR valve.

nukem678 · 02-19-2008, 01:45 PM

Hey ive got the same code but my light will turn on one day, go out for a ,and come back on for one day, and so on. Im Stumped?

alyce · 02-19-2008, 05:08 PM

Quote:

Originally Posted by nukem678

Hey ive got the same code but my light will turn on one day, go out for a ,and come back on for one day, and so on. Im Stumped?

The VSV has a history of intermittent failure when engine gets hot. If ECM does not detect a problem during two drive cycles, the code will clear itself. If yours is a 97 4 cylinder,when the cel light comes on after being off, pull the hose off the Q side of the vacuum moduator when the engine is hot and at idle and see if the engine dies when you apply vacuum to the hose. If it dies, the VSV is stuck. Also you can T a vacuum gauge in at the EGR valve hose and drive the car with gauge visible through windshield and see if the gauge always reacts as described in original post or if there is an intermittent failure. Be sure you have checked the operation of the EGR Valve and Vacuum Modulator to rule out any problem with them. Good Luck

nukem678 · 02-19-2008, 06:23 PM

Hi, mines an 01 4 cyl and i just took apart the egr and cleaned it. It seemed clean though when i took it apart. also the part next to the egr seems to work right. Just that light comes on one day and goes of again.