Just got a 91 Z28 5.7 to add to my "used car lot". Drove it home with zero problems after replacing the fuel pump and a quick oil change and a check of the other fluids (it had been sitting for almost 18 months). The temp never got above 160 (figure a 160 degree t-stat). Parked it in the driveway and let it get up to what would be normal operating temp and it started idling like crap. Then it would idle fine. The temp was still normal. Jumpered A to B on the ALDL and found out it is jumping back and forth between open loop and closed loop operation and had a code 51. I verified this with a diagnostic tool. Everything else seems normal. Any ideas as to the problem? Already read the Chilton and Haynes. Reseated the EPROM. Could the EPROM be bad? The car is bone stock with only 47,000 miles. Any suggestions would be greatly appreciated.

 

Zig,

It isn't uncommon for a car at idle to switch between open- and closed-loop modes. As the oxygen sensor cools down, the signal from it can become erratic, forcing the mode change. It probably doesn't help that you have a colder thermostat, but even a stock 195°F stat will do that.

The DTC 51 means that the ECM cannot get a good checksum verification from the EPROM, but obviously the CALPAK is there or the engine wouldn't run at all. I'd suggest removing the ECM and CALPAK, and make sure teh EPOM itself is firmly seated in tehCALPAK module. You can see the EPROM through the cutouts in the top of the module, and press it firmly n place via the same holes. Listen carefully for a slight crackling moise when you do this, indicating the pins are seating further into the sockets. Thermal creep of the connection pins is not uncommon on socketed devices. Reseat the CALPAK module a couple of times to assure a good connection (you might want to inspect the connection pins closely when you do), and try it again. Chances are you have a connection issue. It is possible that the EPROM has been affected by a voltage spike or surge, but not likely.

You might also want to remove and reseat the wiring harness connections on the ECM itself a couple of times to clean them as well. Do the easy things first.