OBDII EGR duty cycle

I have an OBDII system with the oem EGR system in place. Could some one school me on conditions when the PCM opens the valve and when it closes the valve? Any instructional links on EGR duty cycle and system details? I understand that it is either open (100%) or closed (0%) correct?

I am trying to determine if my EGR system is working properly. Why I am doing this is a long story. I was observing my scanner while driving today and noticed that at highway speeds at 2k rpms or less the scanner showed that the EGR was at 100%!? If the rpms were higher than 2k rpms at speed it was at 0%. This would occur even if I depressed the throttle to accel or lifted to deccel. WHen shifting to 6th gear the EGR would go to 100% if the RPMS were below 2k even while accelerating. The % didn't seem to change. The cylce seems to be affected by the RPM level. At idle at a stop wthe cycle was 0%.

Is my experience and readings considered normal? I thought the EGR would open upon deccel only and be closed at all accel conditions (i.e. including tip in of throttle at high way speeds).

Also can a LT1 PCM editor change the duty cycle, like to keep it closed (0%) most of the time?

Thanks for any help.

Don, not sure about OBD2, but on mine (OBD1) it wasn't just 0 or 100%, there were also numbers between (12.5, 25, 37.5 etc., so multiple's of 12.5).
As for the operation - stock setting for EGR in '94 Camaro was EGR enable betwen 900 and 2200 rpm with minimum MAP pressure at 36.2 Kpa, so it would open only under those conditions.
But what's strange: after setting the RPM range out of reach for my car DataMaster still shows activity of 12.5% EGR from time to time, I really don't know why.

Thanks, Greg, that was helpful.

I wonder if there is some special programming of my PCM that causes the EGR duty cycle readings to be either 0 or 100? I have a Hypertech program with a canned add on module program for a hot cam and headers or something like that. It seems to work o.k. according to my scanner readings.

Perhaps my scanner, a Hypertech programmer w/scan module, is not capable of detailed readings of the EGR. However, I doubt this because of the scanners ability to provide considerable detail on other functions/data (i.e. fuel trim, ECT, etc)

So I can get custom programming to include a change of the EGR rpm operating window to reduce or stop the EGR valve from opening? That would be nice.

Why you would want to stop it from opening is beyond me, but if you truly want to stop it, you should be able to unplug the power to the EGR solenoid then it shouldn't be able to get vacuum to the valve to open it.

I think I remember reading that the EGR system is either on or off (0% or 100%) but it cycles back and forth to get an overall duty cycle. This might be an issue of Datamaster showing information differently than Autotap or whatever other scanning software you might be using. Fuel trims are like this too (shown differently, that is).

LT1 Edit will allow you to adjust some of the EGR system's parameters to control when it comes into play, it will also allow you to disable it.

Lee, I am trying to track down the cause of a frequent P300 and I am trying to rule out the EGR system. Determining the cause of the P300 will be made easier if I can rule out the EGR. If I know the EGR is functioning properly or completely out of the picture, it will help track down the culprit.

Right now the P300 SES appearance seems to be related to the open cyle of the EGR...at least I think so. See the info in my orginal post above.

I suspect that my P300 is coming from a light weight flywheel (-8lbs) and from my mild cam, but I am not sure. Then again if my EGR system is malfunctioning that would definitely casue a P300.

Guys, thanks for the informative posts. :)