240D: temporary idling like a racehorse

[300SDog]

1979 240D, 179k miles...... Its my parts car, been sitting undriven 6 weeks.

Took it on 70 mile spin yesterday and the idle was high for the first 1/2 of the trip. It idled so high that I had to slip the clutch with my other foot on the brake at red lights to slow it down....... No such high idle situation had ever occurred before it was parked in January.

But then it miraculously calmed down and resumed idling at normal speed for the return trip!

There is no alda, weird vacuum/electric regulating device for fuel/air and no crazy oil separator breathing apparatus on this '79 240D. There's no cruise control and nothing looked sticky with the throttle linkage or idle adjust.

Fuel is ordinary truck stop #2 diesel with about 4 ounces of Diesel Kleen additive used at every fill up. And the vehicle has no maintenance issues re/ filters, valve clearances etc...... What caused this *temporary* high idle situation??