I see sig lines with F code and S code engines. What does that mean? Is there an RPO code to indicate which I have? I didn't see any description of engine codes in the FAQ page.

Thanks,
Sixto
97 C2500 burb

It is indicated by the 8th digit of your VIN.
S - More restrictive intake, has a EGR
F - Different intake, no EGR

There are more differences, I'm sure someone else will chime in with them, but I can't, I'm already up past my bedtime

Scope out the internet search on gmc rpo, here's one website.

http://www.gm-trucks.com/home/content/view/10/30/

A trip to our FAQ Sticky also has majority of RPO codes, as well as other info on the S to F truck information, anytime you have a question that is a good place to start looking for answers.

In addition to 8th digit of VIN, from RPO codes a S engine is L56 option, and F engine is a L65.

In addition to EGR equipment the L56 programming has lowered max fuel request 56mm vs 63mm of the L65 F engine.

Later model S in addidtion to EGR has MAF sensor and "smart feedback" I'll call it, EGR monitoring via vacuum flow return that complicates elimination of EGR system for troubleshooting.