EGR Valve

Hi

I just wanted to know what exactly an EGR valve does, and if there is
one that is not working properly in a car, what damage is possible.
This is a civic engine...non VTEC. Anybody??

 

What year Civic?

The EGR valve is an emission control device for the purpose of lowering
oxides of nitrogen (NO, NOx). EGR has no role to play other than emissions.

The EGR valve (and its associated control hardware and gas passages)
provides the metering of small amounts of spent exhaust gases back into the
combustion chamber in order to lower combustion temperatures and thus
oxides of nitrogen.

If this device fails or clogs shut, your NO readings will increase and you
may fail an emissions test. If it sticks open, you can suffer poor
driveability and stumbling at part-throttle.

If the device fails shut, it is possible for combustion temperatures to
increase to the point that damaging detonation occurs, since the engine
control computer is expecting that the EGR is functional. This is less
likely on post-'95 cars, which have knock sensors.

If your car is a 1996 or later, EGR problems will turn on the MIL (Check
Engine light) with an OBD-II code of P04xx, P14xx, or P24xx. The Check
Engine light on earlier cars will not necessarily illuminate when the EGR
malfunctions.

 

Hey..thakns a lot for the info, its really appreciated. I have a 96
civic and my engine check light does come on. I have had the problem
identified by a mechanic...it seems that the valve itself is fine but
the electronic sensor attached to it has malfunctioned. So I was
wondering wether to get the whole thing replaced...is the engine check
light the only indicator that tells me if my EGR is working, or is
there some other way to monitor it?

 

my 98 civic (non vtec though) does not have an EGR valve.

 

EGR is only needed if your Honda is an automatic. Stick cars
don't need it because they don't waste a lot of fuel running
the transmission during idling.

 

Typically, the EGR valve is controlled closed-loop by the ECU. The ECU
applies a signal to the EGR valve and then reads the feedback from the
sensor to see how far open the valve is. The ECU then modifies the drive
signal until it reads back the correct valve-opening (for the current
operating conditions) from the sensor. So with a broken sensor, even
tough the valve can still open and close, the EGR system is not working
properly. Unlike the second O2 sensor, which only function is to turn on
the check engine light, the EGR valve sensor is part of the emissions
control loop.

 

Honda has used EGR valves on both automatics and stick shifts. The 1994
Civic VX manual had an EGR valve, the 1994 Civic EX Automatic did not.

 

A bad/failing O2 sensor will cost you several MPG.
BTDT
And try passing a rigouris emission test without one.

The EGR is to manage NOX emmisions.
Terry

 

What does your mechanic say? More importantly, what was the OBD-II code
number?

Sometimes the valve itself is fine, but the exhaust gases carbon-up the
passages and sensors, causing problems.

There are various tests that can be applied to test the parts of the EGR
system. For example, the EGR valve is vacuum-controlled. If you apply
vacuum directly to the EGR valve while the engine is idling, the engine
should stumble badly or stall. If the idle does not change, then the valve
is carboned up.

What's that error code? Should be something like Px4xx (where x is a
number)

 

wrote in

Yeah, about 1-2 mpg per year if you drive a lot.

Terry, I wish you would turn quoting back on.

Randolph was referring to the *after-cat* O2 sensor, not the pre-cat one.
Very different function.

 

What are the respective purposes of the pre and
post cat O2 Sensors? My little brothers pre cat
o2 sensor was unplugged on his 93 Nissan
Altima. How would this affect engine operation
and economy? (I did plug it back in, hopefully
it was unplugged for no good reason) (Also, it
only had 1 wire.)

Thanks
t

 

Pre-cat is used by the engine computer to adjust fuel mixture so as to be
as close to stoichiometric (correct) as possible. This is the device that
enables "closed-loop feedback" operation.

The effect is roughly equivalent to the role played by your ears when you
talk: As you talk, you hear yourself talk, and how you sound to yourself
results in you adjusting how you say your next words. Put earplugs in, and
you go into "open-loop mode", with resulting distortion.

The post-cat one only measures the efficiency of the catalytic converter.
The computer does not use its signals to adjust anything. A post-cat sensor
is used on 1995/96 and up (ODB-II cars).

If it was unplugged, the engine would have remained in "open loop" mode, as
though the engine was never warmed up, or was continually being operated at
full-throttle.

Unplugging the cat will result in excessive fuel consumption, and runs the
risk of meltdown or other damage to catalytic converter's core, requiring
replacement if you have to pass emissions tests.

A single-wire O2 sensor would be late-'80s/early-'90s. Around 1992 or 1993,
a second wire was added. This wire was for a heater to bring the element up
to operating temperature faster than the exhaust could on its own. Later
sensors have more heaters and more wires.

 

so the only way to test if the cat is fried is
to do an emissions test? Wouldn't this cause
the ECU to throw a code and set off the CEL?
(cuz CEL never came on)

Just wondering if there was a possible way we
might have been able to dodge the bullet of
cooking the catalyst....

t

 

In a pre-OBD-II car (with no downstream O2 sensor), yes.

Only in an OBD-II car. Unless the ECU receives an incorrect signal fom some
sort of sensor, it has no way of knowing how good the cat is.

The downstream sensor is supposed to flatline, showing a constant high
voltage. As the cat starts to degrade over time, it leaves unconsumed
oxygen in the exhaust. This oxygen will cause the downstream sensor's
voltage to begin fluctuating. Once the fluctuations cross a certain
threshhold, the MIL will illuminate. The MIL will also illuminate if the
downstream sensor itself malfunctions.

If you don't have to pass an emissions test, the car runs OK, and no error
codes are stored, then I wouldn't worry about it.

If you do have to get the cat replaced, aftermarket ones are available for
as little as $150 installed, depending on your region. Honda OEM cats are
horrendously, shockingly expensive.

 

First of all, I am posting about a Nissan in the Honda group, I know
I know.... but those guys over there are jerks and never respond, us
honda guys are much friendlier.... lol, and I have a 98 civic. I just
like helping people, and not everyone has the privelege to drive a
honda....

93 Altima has a downstream sensor. The downstream one has more than
1 wire too, the pre-cat one has only 1 wire (and thats the one i plugged
back in). You can get pics of either the pre or post cat on this
website:

http://tinyurl.com/5myvs

or

https://www.automedicsupply.com/catalog4.php?PHPSESSID=1482b8faa8f46369df0938447b18c929#

Since we are in Manitoba, no biggie, and I will probably leave it. But
if he ever moves to another province, ie BC or ONT, then we'll probably
have to address it then. Does Alberta have Clean Air legislation?

I found a Converter on Maximumautoparts.com for $135 US. Not to bad,
looks like it should be OK to install too, as long as I have access to
impact. but we will leave it for now, as long
as running with a possibly bad Cat does not
damage the engine.

Thanks Tegger, you are very helpful.

T

 

Sorry about the quoting thing, I am posting directly via Google beta
and it dosen't give you a lot of control. And qx.net, my ISP, only
supports
outlook.

And sorry aboutmy confussion about pre and post O2 senosors.
I haven't had a car new enough to have a pre and post O2 sensor.

Terry

 

I believe that only means that your ISP will assist its clients in using
only Outlook (Express), but no other newsreader programs. You should be
able to use any newsreader program with your ISP's news server.

 

wrote in

I did a search in Google, and found this:
"I hadn't realised when I used google that the "reply"
button wouldn't linked the original message in properly or include a
quote (which has been achieved in THIS response by using the "options"
link instead which then offers up a "reply" option that DOES include
quotes - go figure!)"

A newsreader is a newsreader is a newsreader. If a program can connect to
your ISP's news server via port 119 and understands what to do with the
stuff it gets from there, then it will work.

Your ISP is simply refusing to help you set up or troubleshoot problems
with any program other than Outlook, that's all. It's a strategy that makes
it easy to find hired help.

 

I don't know how Nissan does things. it's entirely possible they went to a
2-sensor system earlier than Honda did, and even before they went OBD-II.

Sounds like your pre-cat was unheated and the post-cat heated, which seems
a bit odd. Perhaps it was an attempt at compensating for the heat loss in
the exhaust as it moves further away from the engine.

That's a slotted O2 sensor socket.

Doesn't look like it. Curly knows, I'm sure.

Every little bit helps!

 

Sorry, that link sucked. Go to Oxygensensors.com
and enter the make and model, and it shows a pic
of both the pre and post cat sensors.

t