I have a '94 isuzu p'up 2.3l with 184,400 miles. Recently replaced oil pump, fuel pump, timing belt, spark plugs, wires, dist. Cap, rotor, O2 sensor, fuel filter, and map sensor. When first starting engine idles at 900rpm, after warming up will idle at 1500rpm. When letting off the gas and pressing in clutch/ putting in neutral. Will idle at 1500rpm, then drop to 900 and go back to 1500. Usually does this 3-4 times then levels out at 1500rpm and stays there. I have looked for vacuum leaks, but have been unable to find any. Earlier today the check engine light came on, and got a code 32, which would be EGR sensor/circuit. Started engine later and light did not come back on. Perhaps just sticky diaphragm. I am aware that this vehicle is equipped with a feedback carb, but don't know how to approach it for testing. Haynes and Chilton manuals are very little help on this truck. I thought about simply adjusting idle speed on carb, but not sure if that would help.
Ok, today I found the timing light in the garage and thought I would give it a try. There are 4 marks on the timing belt cover 0, 4, 8, and 12 degrees. Well turns out the timing was off the scale, if I had to guess, 24 degrees, judging by the distance past the 12 degree mark. Brought it down to 6 degrees, just like it's supposed to be and the idle is now 950-1000. The up/down idle is no longer an issue. Will keep an eye on it see how it goes over the next few days.