Truck Repair: Engine runs ruff AFTER it warms up, vacuum leaks, washington emission test

Question
Hi Van - My 92 Chevy Silverado 5.7L 4X4 starts and runs great until it warms up.(about 5 minutes).  Then it will sound like it wants to quit.  The idle will go down to around two to four hundred rpms then go back to a normal idle.  When I start out at a light it will cough, sputter and then decide to run.  We have done a basic tune up, plugs, distributor, roter, fuel filter ect.  It passed the Washington emission test.  The Catalytic Converter is fine. What do you think.  Thanks

Answer
Hi Mark,
I would sure want to check the fuel pressure. Make sure it is a good 13 PSI.
Also check for vacuum leaks. make sure to check the EGR valve. Make sure it isn't sticking open, and make sure it isn't getting vacuum to open it while at idle.

Van