Chevrolet Repair: 1991 Lumina, air fuel mixture, fuel pressure regulator

Question
I have a 1991 Lumina with a 3.1 L V6. The idle fluctuates from very high to very low. Diagnostics showed only a bad EGR valve. It was replaced and there's no change. Any ideas?
I have to drive with one foot on the brake and one on the gas at lights.

Answer
Hi Bob,

Is the check engine light on?  Why would it show a bad EGR valve?  We need to think about the reasons that flag may come up.  

EGR's job is to direct unburned hydrocarbons back to the combustion chamber for reburn.  If the EGR failed, the hydrocarbons will pass the O2.  These hydrocarbons are defined as rich mixture.  The O2 tries to correct this.  It leans the mixture and the car's idle gets low.  Then it corrects that and the idle gets high.  It is a continuous cycle.  The O2 when working properly jumps up and down all the time.  If the O2 felt the fuel was too rich, and it couldn't correct the mixture, the computer may feel as if the EGR is failing.  Unable to fully pinpoint the real problem, it may mislead you.  

What if a bad fuel injector was leaking and making the fuel too rich?  What if a bad plug (or wire) was not completely burning off the air/fuel mixture?  What if a bad fuel pressure regulator was causing too much fuel spray into the cylinders?  Any of these would make the fuel mixture rich, and could be mistaken by the computer as a bad EGR.  

I would look at these possible reasons.  Much can be determined by the condition of the plugs.  Wet, fouled, black are indicators.  

Hope this helps.

C J S