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.
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.
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
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.
The car has a new filter so that is out. I will pull the EGR and check
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.
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?
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
*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.
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.
Here's a person with a similar problem:
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.
This one looks like it fits your problem quite closely:
Looks like you are going to have to do some probing with a voltmeter.
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.
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
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.
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