My 93' drives fine in the city but whenever I go on trips or drive for 10 minutes or more over 75 mph I get the "service engine soon" light. I checked the codes and they are:

23,28,46,63,64.

Why would the car throw codes on the highway and not on the street? Does anyone elses modded Z do this? Here is a list of mods just in case it dosent show up in sig.

CAI, Hooker long tubes, heated 02s, Hypertech chip, no cat, stock muffler.

 

Mine throws the egr code once in a while when I am on the hiway. I can't remember if it still does it even when I haven't cleared it.

 

Back to the top, anyone else have this problem...I would hate to be the only one

 

Mine too did this, I had it checked at Autozone and it turned out to be the EGR.

 

Yup, two friends 97's did the same thing...both were EGR.

 

Not sure why the EGR would cause 23 (IAT sensor), 28 (quad driver), 46 (PASS-key), 63 (Rgt O2 open circuit) and 64 (Rgt O2 lean).... maybe someone could explain that to me.

Sounds like it might be a PCM problem, with excessive heat affecting the part that controls all these items. Have you verified that all of the codes return EVERY time the SES light goes on?

 

Didn't know if the codes are consistent, so can't really address that. As far as the symptom of sustained cruising at highway speeds setting codes, I've never heard of anything other than EGR causing it.

Best bet is to clear the codes and check them again as soon as it sets...