Chevrolet Repair: follow up to EGR question, transducer system, engine vacuum

Question
Subject   1994 Suburban EGR valve
Question   The EGR valve opens at about 1500rpm. It doeswn't matter if the engine is loaded or in nuetral. It then stays open until the engine chuggs down so much there isn't enough vacuum to pull the valve in, when the engine stabalizes again, the valve is reopened. The valve has been replaced along with the control solenoid, the MAP sensor, the TPS, and finally the ECM. The condition still persists. The sol. is being commanded on at the above mention RPM. This was determeined by the signal at the sol., not the output of the ECM. I am leaning toward a wire getting grounded out.(I suppose it is possible that the driver in the "new" ECM is bad showing the same symptoms as the previous one, buty that seems unlikely.) Any tips or guidance you have would be greatly appreciated.  
Answer   Hi Keith,
A grounded wire would probably stay grounded, which would keep it energized all the time I believe.
There are a couple EGR systems, and I don't know which yours has.
But if it has a backpressure transducer system, it uses vacuum, but also uses exhaust system backpressure.
If you have a restricted exhaust system, that could be generating the backpressure.
That chugging down till it stabilizes....any chance it is chugging down due to a restriction?

Van


That is a good point. I forgot to mention in the  above discription that the base engine vacuum is upewards of 21 inches of merc. That should eliminate exhaust backpressure as a contributor. I will look for port vacuum, and off idle vacuum to see if teh cat is breaking down. The short I referred to would be more intermitten that constant, but it does beg the question of what could be causing something to short out at a specific RPM. Engine movement unloaded (or loaded for that matter) should be minimal, but I have seen stanger things. Any further tips you might have would be welcome.

Keith

Answer
Hi Keith,
Reading up in my manual, which is two years older than your truck, I see an item called an electronic vacuum control, which according to the book looks like it is wired parallel to the egr valve. I don't know if it could be shorted.
I would be tempted to remove the wire from the ecm connector, seems like it is A11, Gray wire,  and see if the egr still opens. That is engines with a 4L80E transmission. All other engines, it is still a gray wire, but connector a3 or a4.

There was a mention of RPM's. It says the ECM checks for EGR operation at 1600 RPM's.

If it doesn't open with that wire disconnected, then I would say the ecm is doing it. Doesn't mean the ecm is bad though. Could be recieving inputs that are wrong.