Lets see....
14 is Coolant temperature sensor signal indicates a temperature of over 275° F for 3 seconds.
32 is The ECM opens each EGR control valve during deceleration and monitors the change in manifold vacuum. The code sets when the change in manifold vacuum is outside the normal range.
35 is Engine idle speed is either 300 RPM above or below the desired idle speed for 50 seconds.
42 is The EST signal did not change when the ECM applied bypass voltage to the ignition module.
Okay first of all these codes indicate your engine overheated, so that opens up territory for one failure (EGR) to cause another (ICM or Ignition control module underneath your coil packs)
I would take your EGR valve off first and perform these bench tests:
TESTS FOR A DIGITAL
EGR VALVE
To check a three-solenoid GM digital EGR
valve, (Refer to Figure C) and measure the
resistance of each solenoid:
• Terminal A to B: 20 ohms
• Terminal A to C: 10 to 17 ohms
• Terminal A to D: 20 to 30 ohms
If any resistance reading is outside the range of
specifications, the unit is defective and needs to
be replaced.
Using a 9-volt battery or sensor tester, run
battery positive (+) current to the “A” terminal,
then ground each of the other terminals (B, C &
D) one at a time (see Figure C). Each solenoid
should click and open its respective pintle valve.
No click or movement would indicate a faulty
solenoid.
With the battery and jumpers disconnected,
spray a small amount of carburetor cleaner into
each orifice (Figure D), being careful not to get
the cleaner in the center hole. If the cleaner seeps
out of any orifice hole into the center, the pintle
is not seating properly and the EGR valve needs
to be replaced.[/size]
See that valve can make your engine run lean and misfire, and that will make it overheat. It does that by adding exhaust to the intake, and thats why you got your bad gas mileage...the ECM was adding fuel to rich out the lean (exhaust filled mixture)
But now you have code 42 which could be a product of all the other codes.
So run your car after you the EGR thing figured out. If it works fine and nothing happens or it doesnt act up and throw codes, sweet, problem solved. If not, your going to have to take off the igntion module (which is under the ignition coils) and have it tested at Murrays (midwest) or Checkers (elsewhere)
and if you don't have those places then your going to have to call around or just get a GM module from the junkyard to see if it works better.