Significant Success, TBI Idle Hunting and Idle Miss

[Mr Bear]

In case it may help others. I've taken a deliberative approach as the 87 Jimmy (350/700R4, TBI, 94K original miles) is driven 700 miles weekly as a daily driver. Exhaust is true dual, 2.5" with no cat and with two irritatingly loud magnaflows.

All original and good mechanical condition but with warm (restart only, and disappears as soon as you drive a few yards) idle hunting and an occasional warm idle miss. Figured I'd start with cooling and move through all the systems since they are 30 years old. No reason not to freshen it up using all AC parts for reliability.

Initial fuel mileage on 87 Octane was about 14 mpg.

Since the radiator was leaking, new rad, coolant, coolant temp sensor and temp sending unit and new stant 195 thermostat. No change in idle issues.

Plugs were rusty and worn variously from 50 thou to 90 thou, wires new so new NGK V power gapped 45 thou, new coil, new brass cap, new rotor and reset warm timing, wire disconnected, from 10 After to 0. Much more power, No change in idle issues.

Rebuilt TBI with new Standard Motor injectors, 14 lb regulator spring, 1/4 injector spacer, new air cleaner, new IAC, new AC Fuel filter, new tank, new pump. Fuel mileage now averaging 15.9 mpg and no change in idle issues.

New AC Professional EGR and checked solenoid, no issues. But, no change in idle issues.

Checked for vacuum leaks. None. Checked intake manifold and TBI, no leaks (Vacuum or Fuel). Replaced every inch of vacuum hose. Subsequently checked vacuum at every TBI port.....20" steady with a slow and even wander from 19" to 20" on those occasions where the idle would wander slightly after a warm restart. No change in idle issues.

Looked at the tag on the radiator support.....plug gap is listed as 35 thou. Remembering I had a new AC factory off cap coil, decided to give it a try as I also remembered the original AC plugs were worn from wide to very wide. Pulled the new plugs, all had the proper toast brown steaks and the temp line was on the flat just before the turn of the electrode. No wear and still sitting at a snug 45 thou gap. So, they looked great after 2000 miles of driving. Regapped them all to 35 thou and reinstalled.

Driving yesterday and today.....the occasional warm idle miss is completely gone. The warm restart wander is significantly reduced. I suspect I'll continue to update the old sensors anyway, after all they are 30. Next on the list will be egr solenoid and MAP sensor (both checked good though old). Then the O2 sensor, TPS, ESC and finally a new dizzy and ignition module.

Progress.