EGR P0401 - Exhaust Gas Recirculation Flow Insufficient

Hi,
I have a 1996 Chevy Lumna sedan with 130.000 miles. The engine light is on and my scanner reades P0401. All othere checks are ok.
The car runs fine. Some causes could be excessive backpressure. How
do I check for that? Any way I can check the EGR valve incase that is the problem? A new one cost $160 and it is too expensive to take the shotgun approach. Any ideas would be appreciated. The car is due for it's state inspection in August.
Thanks, Ed
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Your MAP sensor is also a suspect for your code. A reduction of manifold pressure as sensed by the MAP sensor is how the computer figures out there is sufficient EGR flow when it runs the test.

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I noticed that if I view the data while the car is running, the scanner says EGR passed. Could the MAP sensor be triggering the P0401 code, but not trigger it's own fail code of P0106 code?
Ed
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On Thu, 07 Jul 2005 13:56:30 -0700, Ed wrote:

Yes I believe they work together. See Silver Surfer's reply.
Before you replace the EGR, remove it and clean out the passages in the manifold with carb cleaner. Check to see if the pintel moves freely on the EGR valve when you press it (be careful!). Also check the vacc hoses, etc.
Some EGR's can be tested by applying vacuum, others cannot. Some are negative pressure and others positive pressure valves and it's not always consistant which one is in which model car because GM changes things. They can tell by the numbers on the valve which one you have and make certain you get the correct type.
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My EGR has no vacuum lines . It is electronic. I will be checking it all this weekend. I noticed there is a vacuum line on the MAP valve.
thanks
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On Thu, 07 Jul 2005 15:23:45 -0700, Ed wrote:

Ok. I wasn't certain what kind of EGR you have.
The Map sensor is essentially a device that determines engine load by generating a differential voltage based upon engine vacuum vs outside barometric pressure. The computer uses this information to determine the timing (along with other sensors) and how much load is on the engine. IOW it measures engine vacuum and generates a voltage.
They are fairly sensitive and tend to produce codes at the slightest provocation.
I would say check hoses,clean egr passages of carbon maybe a new air filter like the other person said and basically do everything that is easy and cheap first before springing for an expensive EGR valve.
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Thanks,
The car has a new filter so that is out. I will pull the EGR and check that.
The vacuum on the car is running about 11-12. Is there a way to see if the EGR is closing all the way? I read somewhere that if it doesn't close all the way it could cause a low vacuum.
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On Fri, 08 Jul 2005 06:03:50 -0700, Ed wrote:

If the EGR isn't closing all the way at idle, which it should, the most common symptom you would see is that you will have a very poor idle and more than likely will be stalling out when you come to stop lights etc. The usual cause of this is carbon build up on either the pintel or the passage under the EGR when you take it off.
Is the idle smooth when the car is just parked, warmed up and idling?
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Yes,
The car starts fine. At idle the car runs smooth. Reving the engine appears to be ok too.
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On Fri, 08 Jul 2005 16:58:06 -0700, Ed wrote:

Then the egr is closed because most cars idle terrible when it sticks open at idle.
I would start with the cleaning and see if the code goes away.
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OK,
I will do that tomorrow and let you know the results.
Thanks
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Here is what I tried today;
*Remove EGR clean exhaust pipe to manifold and clean inside valve. It didn't look bad. Very little carbon. *Ran car at idle was fine. Took it down the road up to about 35 mph and light came back on P0401 *Car at operating temp. *Removed Linear EGR valve, but left wired. *Started car. Idled a little rough until I plugged the intake manafold port. *I reved the engine while observing the EGR valve. It never opened up even after I plugged the intake port.
I know on the old vacuum ones that they opened when reving the engine.
My manual does not have any electrical checks to do on the EGR. Does this mean PCM or MAP could be the problem. MAP monitors the vacuum.
Thanks
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On Sat, 09 Jul 2005 09:23:20 -0700, Ed wrote:

I am familiar with the standard, I suppose old fashioned vaccuum types which DO open when the engine is revved. Was the engine up to normal operating temperature when did the test? The PCM might ignore the EGR until a certain temperature is reached, IOW the car is warmed up.
Check out this link. I believe your model has the digital version, which they say is independent of engine vacuum.
http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/0e/cf/f2/0900823d800ecff2.jsp
Here's a person with a similar problem:
http://www.obdii.com/ubb/Forum1/HTML/002411.html
Maybe your throttle body needs some cleaning?
Here is one for a Ford, but maybe your EGR has a similar backpressure sensor unless it uses the MAP for that function.
http://www.automotiveforums.com/vbulletin/t425799.html
This one looks like it fits your problem quite closely:
http://www.obd-codes.com/trouble_codes/generic/p0401-insufficient-exhaust-gas-recirculation-flow-egr.php
Looks like you are going to have to do some probing with a voltmeter.
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It will open when engine is at operating temperature and engine speed is above idle. If the EGR was not opening when the PCM commanded it to, you should get a P0406 as well as a P0401.
Now, for the 1996 model year W car with the 3100 engine there is a TSB (87-65-22, August 1998) where the MIL comes on because the EGR flow test fails because the expected change in manifold pressure falls outside the calibrated window during deceleration.
The correction to this is having the PROM reprogrammed by the dealer.
Steve

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Is there ma way to check the eprom date with a scanner?
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If you can check the "Calibration Number", the new ones are as follows:
09365714 - All W with 3.33 axle - Unleaded Export/Canadian 09365704 - All W with 3.33 axle - USA Nationwide 09365694 - Chev W with 3.33 axle - USA Nationwide Police Package 09365724 - Chev W with 3.33 axle - Unleaded Export/Canadian Police Package
Steve

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I may have figured it out. The passageway from the EGR to the intake manafold was plugged and little air would pass. I pulled off the EGR valve and the engine idled good. I put my thumb over the opening and there was very little vacuum.
After using gumout and compressed air I managed to clean out the opening. Now if I start the car with the EGR removed, the car barely runs as should be. There is a lot more vacuum at the opening. I buttoned it up and cleared the error code. We will give it 4 days and see what happens.
Ed
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Car is fixed. No more P0401 code
It was worth all the work. I learned a lot more about sensors and have a better idea how to go about checking them now..
Thanks for all your help.
Ed
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Ed wrote:

My wife's Impala gave that code and it turned out to just be a dirty air filter.
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