A couple of days after driving on semi-desert dirt roads with "corrugations" (6 inches apart and 1 to 2 inches deep) which caused extreme vibrations (I thought I might need Loctite to hold the car in one piece :), my idle speed in neutral has mysteriously increased suddenly from about 1000 to 2000 rpm in neutral. If I turn aircon on it drops to 1500 rpm and if I put it in drive it drops to about 1000 rpm. Excluding this problem the car drives well and fuel consumption is OK. Before going to the desert I set idle to run smoothly at about 1000 rpm in neutral and about 800 rpm in drive.

My car is K-Jetronic with no engine or idle management computer(s) - only electronics is transistorized ignition. Idle speed is set by large black plastic "needle valve" idle screw adjacent to throttle body.

Before this trouble did the following as part of general maintenance and to get OK idling:
* cleaned throttle body
* set butterfly valve fully closed at idle (as per Brickboard)
* replaced throttle body gasket
* replaced idle screw O-ring
* checked all hoses for cracks & splits
* checked aircon idle-up valve
* replaced fuel injector O-rings (large & small)
* replaced inlet manifold gasket with Volvo OEM gasket(could not use torque wrench to tighten as had to use combination spanners)
* replaced air-flow-meter/fuel-distributor support bushes
* used an unlit propane torch to check for leaks after all this work

I cant see anything fallen off or cracked after my trip. Since I've now got a new air leak how could this be caused? Can new gaskets shrink or manifolds vibrate loose? Can the auxiliary air valve vibrate to fail fully open - even if its electric heater fails, engine heat should keep it closed? Vibration could affect the resting position of the 3 inch venturi plate in the air-flow-meter but this should only affect idle mixture - not get more air into engine?

sutherml - damaged(?) in the desert at 520,000 km