I failed my emission test yesterday on my 94 LT1 z28.

NOX was at 640 vs 480 limit. HC and CO were fine. According to what I have read, high NOX can be caused by high engine temps, EGR malfunction or too much timing. My engine run pretty cool (at about the 1/8 mark while cruising and at the 1/4 mark during stop and go in the summer) so I think I am ok there.

EGR System
Any ideas on how to test the egr system? I took it out after the test and sprayed it with some carb cleaner. It didnt appear to be *that* dirty. I tried pushing down on the plunger part and it moved up and down but there was a bit of resistance - should these things be really easy to move?

Timing
I have a mail order tune and it has the timing advanced... Do you think returning it to stock will be enough to get the NOX down 160 ppm NOX???

Any other suggestions would be greatly appreciated.

Most late model EGR systems use vacuum and positive exhaust pressure to activate the EGR system at different times, so yes, a little resistance is good, and make sure you can pull a vacuum on the vacuum canister for the EGR.

Advancing the spark can cause more NOX. Not sure if it could be responsible for the readings you have, but it can affect NOX. If you can easily return it to stock, I'd do that. LT1 motors, in mostly stock trim, though, are pretty clean cars. Make sure you have no vacuum leaks in the intake system. You may also want to pull and clean the throttle body, they can get pretty dirty, and can plug up passages there. When they do, though, the car usually idles poorly. Also, make sure the MAF is clean. If you are using a K&N-like air filter, oil from that can get on the MAF wires and cause the settings to be off. A good spray down with carb cleaner will help there.

Pulling the timing back and getting it nuclear hot before the test will probably get it to pass. A bottle of the RXT or whatever the emissions additive is called would probably help.