after a 1300 mile drvie recently my 87 5.7 L98 has developed a tendency to ping badly during the first few minutes after cold startup, then later seems to go away. the idle is also high and stays high after warmup... when stopped at a light with gear in drive it's at a steady 750-800 rpm. i can also hear a popping in the exhaust like it's misfiring.
so far i've checked the plug wires and replaced the plugs. the plugs all looked similar and normal, ie were not fouled, although there were some white deposits indicating pinging.
checked the base timing and it's exactly where i set it 2 years ago at 6 deg BTDC.
there's no trouble codes regsitering on the ECM.

here's a list of other stuff i've checked.
- distributor rotor, cap, ignition module connections, all look normal
- thorough check for vaccum leaks, vaccum at idle is normal
- fuel pressure is exactly where it should be
- battery and sensor connections, and ground straps
- checked for fried, severed or melted wires in the engine bay.
- visual inspection of MAF

i think it's an electrical problem because every now and then the car drops down to a normal idle speed of 600 rpm but it won't stay there for long.
or maybe it's carbon buildup or possibly clogged injectors.
maybe the O2 sensor ??

anybody seen this problem before ??

 

for anyone reading this thread just thought i'd let you know the problem was eventually solved... it was the ECM. the PROM and MEMCAL were fine... it was something wrong in the ECM itself.