Chevrolet 3.1L (3100) EGR headaches...

I am posting this article in hopes someone else with the same engine may find this information useful; for I spent quite a long time figuring this one out.
First of all, let me just say that if you are ever in the market for a new car and a chevrolet with a GM 3.1L or 2.8L engine (V6 from hell) seems like a good deal, there is a good reason for it. Once you encounter this beast, run from the car as fast as you can and never look back. I have owned three
GM cars previously and NEVERMORE. This is the last american car I will ever buy.
Let me get down to it now... the "Service Engine Soon" light would come up every time I drove on the highway whenever the car was in fourth gear (OD)
(automatic transaxle) , going above 55 miles/hour, OR third gear (D)
(automatic transaxle) going above 45 miles/hour AND the throttle angle was
0% (coasting, foot off gas pedal). The code reported was 77. Here is a description of the code,

"Digital EGR solenoid 3 failed 3 consecutive times.
Engine speed was between 1400 and 2000 RPM when throttle angle was 0% and the MAP sensor showed high manifold vacuum."

Now, from the error description one would think it is the EGR selenoid that is bad.... wrong! It turns out that the 3.1L (3100) MFI engine has an annoying tendency to build carbon deposits on the EGR valve passage leading to the plenum right passed the throttle body. The error comes from the fact that the solenoid does open, but since the passage leading to the plenum is blocked there is absolutely no drop in the manifold vacuum. The ECM assumes this is an error related to the selonoid it just tried to open. Well, guess what? that is just not the case. The problem is that your passage is blocked with A WHOLE LOT of carbon deposits. So next time you go to the shop and they try to charge you $400 for a brand new digital EGR valve, make sure you take the plenum (intake manifold) and clean it off, make sure the passage from the valve to the intake is not restricted. I hope this post saves someone some trouble. It was a pain in the ass to diagnose for me;
specially considering that there are other codes to express no EGR flow.
Well, that is all for now folks, I hope this helps someone out there. Also if you are wondering, the car I bought is a Chevrolet Beretta 1994 with the
3.1L (3100) V6 from pandemonium engine.

It's called "that great GM feeling". If it runs fine and only the light being on bothers you, pull the bulb and drive it.