EGR valve opens just off-idle, closes at wfo

1992 Blazer 1500
5.7L TBI Recent O2 sensor, Recent ECU.


My EGR is opening most of the time. The only time it closes is at idle, and wide open. I know this because I connected a test light to the wire driving the EGR solonoid valve. (It stays closed until ECU goes into closed loop mode when engine gets warm)

The EGR valve/solonoid seems to be functioning properly. The vaccuum is controlled by a solenoid, which is controlled by the ECU.

So the truck idles great, which suggests the EGR is not dirty. As soon as the throttle is cracked open, the EGR opens, and it starts sputtering.

I replaced the ECU for another reason, but I still have the EGR problem.

For now I have the EGR disconnected and the truck runs fine. I do get the SES Service Engine Soon light after a while, and throw the EGR error code.

I have an ALDL scanner, so I am able to monitor all sensors, error codes. I can't see anything obvious.

So what is causing my ECU to think it is time to open the EGR when I crack the throttle?

Have you checked the TPS?

TPS looks good. But watch the O2 sensor bounce around.

http://files.automotiveforums.com/gallery/watermark.php?file=/503/309561ALDL-med.JPG

Do you have an oscilliscope w/ voltage scale (or is there a voltage corresponding to the 0-250 scale on the right?)? At operating temp, the O2 sensor should toggle from 200mV to 800mV and back at least 8 times per second at idle.

Could it be possible that you have an exhaust restriction that is forcing exhaust gas into the EGR?

how do you go about dissconecting the egr just undo all the eletrical and vacumm lines? ive got lobing and now sputtering during acceleration ive replaced the fuel filter and found 1 broken vac line did that it ran great for 1 night and then did it all over again the next day i coded it and it checked out fine if you dissconect the egr does it stay closed? if it does then ill do that and next replace the fuel pump