'98 S-10 4.3 surging at low rpm

'98 S-10, 4.3, auto, 4X4 131,000, has a "surge" between 1500 and 1900, and then again between 2200 and 2500 RPM. Only seems to be present when warm, when engine is not under load or coasting. Slight increase or decrease in throttle position seems to help the situation. Range of surge is 200 - 300 RMP, cyclic, about 1 cycle per second, slowly dampens after about 10 - 15 cycles (possible change in engine load?). There is also a noticeable "buzz" in the same ranges.

So far the distributor cap and rotor have been replaced, fuel filter replaced, throttle body cleaned. The battery terminals have been cleaned, the engine ground has been cleaned, the positive lead at the master fuse and relay box has been cleaned. The surge still remains. OBDII shows no codes. I'm looking for suggestions as to where to look next.

Thanks in advance,

Steve Sharp snipped-for-privacy@comcast.net

You didnt say anything about plugs and wires being changed. Are they due? Is it a jerky surge or a slow lose of power surge, like a "bog" ? If it is jerky it is most likely ignition, probably a plug or wire. If it is a "bog" type surge it is most likely fuel and you should check the fuel pressure, could be the pump going out. How old is the filter? Please dont put a pump in it just for fun, because it isn't :)

Checked it sans the A/C - no change

More ideas?

Steve Sharp

Putting a pump in an S-10 is a violation of the Geneva Convention.

Big Chris

do it again though.