'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,