1987 R10 Silverado L03 305 TBI with a 42 code

[mcgarijs]

Engine stumbles randomly at highway speeds and at Idle. A code 42 is stored. When the codes are clear the engine runs smooth. But will randomly every 5 to 10 min stumble. if at idle it may actually die and has to be restarted. I don't have an oscilloscope to check square waves as I read in a troubleshooting guide. The previous owner may have been chasing the same problem. I have receipts from the dealer Capital Chevrolet in Austin Tx. they took it to and  replaced both fuel tank pumps, selector valve, tank selector switch both TBI injectors,distributor,ignition coil, TStat, Alternator, IAC valve. Will start and run just fine but will randomly stumble has me puzzled.

[VileZambonie]

Most likely you have a faulty ignition control module and a poor engine ground. You do not need an oscilloscope, just follow the diagnostic strategy attached.

[mcgarijs]

give yourself a cold beer put it on my tab! replaced the distributor Ignition module and it cleared up the stumbling. the distributor was a new Jegs and a piece of crap it is. When pulling the wires off the top of the cap one of the brass electrodes that go into the distributor and the rotor fires to, pulled out with the wire, then looking at the rotor it was shiney around the center looks to have been getting hot. Replaced the Cap, Rotor and ignition module. The code hasn't come back after about 20 miles of test driving so I think its fixed . for now anyway.

[Shifty]

I carry a spare in the glove compartment....