Don -

With a 300 rpm surge, I think you are either getting a faulty reading from one of the sources that determine what the idle relay sends to the valve, or a transient vacuum leak.

- temp sensor at rear of engine. Will increase rpm when engine is cold - drops it down at about 17C

- AC. increases rpm when compressor comes on

- tach (from ignition control module). sends low signal and relay tries to increase RPM

The wiring from these sources could also be the problem.

- vacuum leak. False air (not monitored) will increase RPM until relay can slow it down.


This is going to be tough to track down.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe