I have a 99 LS with 130k on the 4.3L. The car runs good for around town trips of less than 40 or 50 miles. Here is what it does. I drove it on the interstate for about 40 miles, as I slowed to go thru a toll booth ( not stopping, 25 -35 mph), everything was fine. As I stepped on the gas to resume driving there was no response, I looked at the dash, the tachometer showed the engine had stalled. I pulled over and tried to start it again, no luck. It would crank but not start. After about 30 min with the hood up it finally started again. I traded cars and the wife brought the car back home, again stalling after slowing from highway speeds. The car also stalled two more times around town before returning home. The key, letting the car sit and cool seems to help it start again.
Repair history
No codes showing in the computer, check engine light does not come on. There is some "mode 6" data codes showing failures over the life of the car. Codes are: $OC, $OD, $20. In Feb 08 it had a #5 misfire: I changed the plugs, wires, cap & rotor, PCV. Everything works fine. In Mar/Apr 08 the fuel pump went out. Changed the pump and sending assembly with a NAPA unit, replaced the fuel filter. Everything works great. May 08 replaced water pump and radiator, reason: fan hub separated from the w/pump shaft. In mid May this stalling problem started, for the first time.
The last time it happened was 2 weeks ago, I drove 40 miles around town, stop and go, starting and stopping. Brought the car home it sat for 20 min, the wife took it drove 10 miles and it quit when she stopped at a friends house.
What I have done: Checked it each time with a diagnostic scanner ( not a code reader). No codes. Removed and cleaned the EGR valve, plunger moves like new. Appeared to have normal carbon build. Removed and tested the ICM (tested 3-4 times) passed.
Any ideas? I have read several of the forum issues and have had no success with there fixes. Thanks for any help

 

I am leaning towards either an ICM heat related failure or an ignition switch failure (due to the communication codes). The comm codes that you listed mean nothing to me though... They should be Cxxxx or Bxxxx codes.

 

Ignition sw??? explain? Why after driving at highway speed for 30 mins? I would think an ignition sw would be at starting most of the time. Any other time should be vibration related maybe???

 

Looking at the other symptoms is what brought to mind the ignition switch. I am just throwing ideas out there for you.

The ICM should be tested at least 10 times for heat related problems. When testing it outside the hot confines found underhood, you really have to test it repeatedly to get it as hot as it normally is. I am really leaning towards a failing ICM.