Chevrolet Repair: no power, excessive fuel use, throttle position sensor, oxygen sensor

Question
I have a 1990 K2500 pickup with a 5.7L V8. My problem started when my service engine light came on, code reader said EGR failure so I replaced it. Then a couple of days later it began to run eratic so I found out the new EGR valve stuck open  so I replaced it and also replaced EGR vacuum selenoid. Since that time it has seemed to be flooding, no power, excessive fuel consumption went from 14 MPG to 7 MPG  computer code read Map valve rich, replaced it cleared codes, didnt help problem, during this time I also replaced the electronic spark control, knock sensor, spark plugs, spark plug wires, distributor cap, rotor button. Then the code read oxygen sensor rich, I replaced it also,cleared codes but still didnt help problem. At this time it is reading no codes found but it still has no power and still uses excessive amounts of fuel. So, now I replaced the engine control computer because I figured it was all that was left but it still is doing the same thing. It idles rough no power at low to mid RPM but in 3RD gear about 35MPH it feels like something releases and it will have better power until you go into 4rth gear and then no power again. Also, I can hook my trailor behind it and cant get up hills at above 25MPH in second gear, where as it used to cruise over with no problem in high gear.  

Answer
Hello Danny,
First, you gotta stop just guessing.
The oxygen sensor said rich....hey!! it was rich. The oxygen sensor was reading correctly, and told you so. Don't kill the messenger.

The EGR valve can, and should be cleaned. But something is causing it to stay open. Possibly readings from some sensor to the ECM. Or it could be just getting some carbon knocked loose and sticking in the seat.

Here are a couple things to check.
I would want to check the fuel pressure.
It should stay at 13 PSI when running. Then check the throttle position sensor. Check it with an ohm meter, or a scan tool, but you want a nice even change, all the way through its travel, and consistent.

Look for a bad gasket or seal in the throttle body. They are easy to replace, but if they leak, will just dump gas into the manifold.

Check or clean the MAF sensor. Do you happen to be running a K&N air filter? See this link.
http://www.fordscorpio.co.uk/cleanmaf.htm
It is Ford, but they work the same.

Check the temperature sensor. See this forum topic.
http://www.chevytalk.org/threads/showthreaded.php?Cat=0&Number=1366279

While you are there, join that forum, and do some browsing. It is a great site.

Check your injectors...they should be spraying clean cone shaped fuel patterns, not dribbling.

And check for vacuum leaks, not forgetting the brake master cylinder.

Van