I have a check engine light that comes on only when cruising at a steady 75-80 mph (1700-1800 rpm) for about 5 minutes or more. I checked for codes and get 28 and 32, anyone have an idea what that would be on a 93 Z-28 M6.

Thanks

 

DTC 32 = EGR system

DTC 28 = Quad Driver #3 in 93 only

Is your EGR system still connected? Odd for that code to show up in OBD-I, even if EGR is deleted.

The "Quad Driver" is still a mystery to me after 10 years of playing with these cars.... it is unique to the 3rd Gens, and the 93. There is supposedly a high amp driver module in the PCM to handle the heavy loads like solenoids, and it has 4 (quad) drivers in the module. In the 94 and later cars, that code is "Transmission Range (TR) pressure switch assembly fault", but since the 93 doesn't have an electronic tranny, that probably does not apply.

 

Yes, the EGR is still connected, my manual referenced an Electronic Vacuum Regulator Valve (EVRV) associated with code 32, is that the little black box on the passengers side of the intake with the connector and vacuum lines?
It's strange to me that it only occurs when I'm at a steady state (6th) cruise at that particular speed (rpm).
That Quad-Driver code sounds bizarre, do you think that the EVRV would be considered a "solenoid" that it could be monitoring? I think the two codes seem to occur simultaneously.

 

I'm with Injuneer on the mystery of the Quad drivers. I think the EGR solenoid is actually controlled by Quad #1 along with the evap purge solenoid. I'm still trying to figure out exactly which solenoids are driven by which drivers. One thing for sure is that we are talking about coil loads so that would narrow it down a little. The fault usually lies in the driven device and not the driver but sometimes is the driver and the PCM would need to be replaced.

 

The "little black box" on the passenger side of the intake is NOT the EGR solenoid.... that is the EEC canister purge solenoid. The EGR vacuum solenoid (at least on my 94) is on the drivers side of the intake, right near injector #7.